http://www.biogeosciences.net/Biogeosciences Latest Articlesenhttp://www.biogeosciences.net/10/4037/2013/<b>Sea–air CO₂ fluxes in the Southern Ocean for the period 1990&ndash;2009</b><br /><br />Biogeosciences, 10, 4037-4054, 2013<br /><br />Author(s): A. Lenton, B. Tilbrook, R. M. Law, D. Bakker, S. C. Doney, N. Gruber, M. Ishii, M. Hoppema, N. S. Lovenduski, R. J. Matear, B. I. McNeil, N. Metzl, S. E. Mikaloff Fletcher, P. M. S. Monteiro, C. Rödenbeck, C. Sweeney, and T. Takahashi<br /><br />The Southern Ocean (44–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea–air CO₂ fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea–air CO₂ fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea–air CO₂ flux of −0.42 ± 0.07 Pg C yr⁻¹ for the 44–75° S region, is consistent with the −0.27 ± 0.13 Pg C yr⁻¹ calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr⁻¹ and observations: +0.04 ± 0.02 Pg C yr⁻¹), with most of the net annual flux located in the 44 to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr⁻¹ and observations: −0.35 ± 0.09 Pg C yr⁻¹). Seasonally, in the 44–58° S region, the median of 5 ocean biogeochemical models captures the observed sea–air CO₂ flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea–air CO₂ flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO₂ fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux, with 25% of this interannual variability attributed to the region south of 58° S. Resolving long-term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 1990–2009 ocean biogeochemical models do show increasing oceanic uptake consistent with the expected increase of −0.05 Pg C yr⁻¹ decade⁻¹. In contrast, atmospheric inversions suggest little change in the strength of the CO₂ sink broadly consistent with the results of Le Quéré et al. (2007).Wed, 19 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/4009/2013/<b>Diversity, distribution and spatial structure of the cold-water coral fauna of the Azores (NE Atlantic)</b><br /><br />Biogeosciences, 10, 4009-4036, 2013<br /><br />Author(s): A. Braga-Henriques, F. M. Porteiro, P. A. Ribeiro, V. de Matos, &Iacute;. Sampaio, O. Ocaña, and R. S. Santos<br /><br />Cold-water corals are widely considered as important structural components of benthic habitats, potentially enhancing local abundance in a variety of fish and invertebrate species. Yet, current knowledge of the taxonomic diversity and distribution patterns of these vulnerable, slow-growing organisms is scarce and fragmented, limiting the effectiveness of spatial management and conservation measures. We have conducted an exhaustive compilation of records of alcyonaceans, antipatharians, scleractinians and stylasterids available through present day to assess the diversity, distribution and spatial structure of coral assemblages in the Azores exclusive economic zone (EEZ). The resulting database comprises 2501 entries concerning historical oceanographic expeditions and other published sources, as well as unpublished data from bottom longline by-catch. Our taxonomic inventory appears to be fairly complete for the explored habitats, accounting for 164 species (79 alcyonaceans, 58 scleractinians, 18 antipatharians and 9 stylasterids), nine of which were documented for the first time. The Azores EEZ harbours a mixed coral fauna with several zoogeographic origins, showing the closest affinity with the Lusitanian–Mediterranean region. Very few apparent endemics were found (14%), and only in part supported by consistent sampling. Coral diversity is particularly high between 300 and 900 m depths, in areas recognized as traditional fishing grounds or exploitable fish habitat within the 100-mile limit of the EEZ. The composition of coral assemblages shows significant geographical structure among longitudinal sections of the study area at comparable depths (100–1500 m). There is no evidence of a possible role of the Mid-Atlantic Ridge or latitudinal effects underlying this pattern, which suggests that it may instead reflect assemblage variability among features. Stronger changes in species composition were found along the bathymetric gradient. Notwithstanding the mix of partially overlapping steno- and eurybathic species that characterize the vertical distribution of corals, there is a distinct transition from shallow (100–600 m) to intermediate (600–1000 m) depths. The analysis presented here constitutes a valuable contribution for efficient conservation policies of coral-associated vulnerable marine ecosystems and their sustainable use as fishing areas.Wed, 19 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3997/2013/<b>Photosynthate translocation increases in response to low seawater pH in a coral–dinoflagellate symbiosis</b><br /><br />Biogeosciences, 10, 3997-4007, 2013<br /><br />Author(s): P. Tremblay, M. Fine, J. F. Maguer, R. Grover, and C. Ferrier-Pagès<br /><br />This study has examined the effect of low seawater pH values (induced by an increased CO₂ partial pressure) on the rates of photosynthesis, as well as on the carbon budget and carbon translocation in the scleractinian coral species <i>Stylophora pistillata</i>, using a new model based on ¹³C labelling of the photosynthetic products. Symbiont photosynthesis contributes to a large part of the carbon acquisition in tropical coral species, and it is thus important to know how environmental changes affect this carbon acquisition and allocation. For this purpose, nubbins of <i>S. pistillata</i> were maintained for six months at two pH_<i>T</i>s (8.1 and 7.2, by bubbling seawater with CO₂). The lowest pH value was used to tackle how seawater pH impacts the carbon budget of a scleractinian coral. Rates of photosynthesis and respiration of the symbiotic association and of isolated symbionts were assessed at each pH. The fate of ¹³C photosynthates was then followed in the symbionts and the coral host for 48 h. Nubbins maintained at pH_<i>T</i> 7.2 presented a lower areal symbiont concentration, and lower areal rates of gross photosynthesis and carbon incorporation compared to nubbins maintained at pH_<i>T</i> 8.1. The total carbon acquisition was thus lower under low pH. However, the total percentage of carbon translocated to the host as well as the amount of carbon translocated per symbiont cell were significantly higher under pH_<i>T</i> 7.2 than under pH_<i>T</i> 8.1 (70% at pH_<i>T</i> 7.2 vs. 60% at pH_<i>T</i> 8.1), such that the total amount of photosynthetic carbon received by the coral host was equivalent under both pHs (5.5 to 6.1 μg C cm⁻² h⁻¹). Although the carbon budget of the host was unchanged, symbionts acquired less carbon for their own needs (0.6 compared to 1.8 μg C cm⁻² h⁻¹), explaining the overall decrease in symbiont concentration at low pH. In the long term, such decrease in symbiont concentration might severely affect the carbon budget of the symbiotic association.Tue, 18 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3983/2013/<b>The role of microorganisms at different stages of ecosystem development for soil formation</b><br /><br />Biogeosciences, 10, 3983-3996, 2013<br /><br />Author(s): S. Schulz, R. Brankatschk, A. Dümig, I. Kögel-Knabner, M. Schloter, and J. Zeyer<br /><br />Soil formation is the result of a complex network of biological as well as chemical and physical processes. The role of soil microbes is of high interest, since they are responsible for most biological transformations and drive the development of stable and labile pools of carbon (C), nitrogen (N) and other nutrients, which facilitate the subsequent establishment of plant communities. Forefields of receding glaciers provide unique chronosequences of different soil development stages and are ideal ecosystems to study the interaction of bacteria, fungi and archaea with their abiotic environment. In this review we give insights into the role of microbes for soil development. The results presented are based on studies performed within the Collaborative Research Program DFG SFB/TRR 38 (<a href="http://www.tu-cottbus.de/ecosystem"_target="blank">http://www.tu-cottbus.de/ecosystem</a> ) and are supplemented by data from other studies. The review focusses on the microbiology of major steps of soil formation. Special attention is given to the development of nutrient cycles on the formation of biological soil crusts (BSCs) and on the establishment of plant–microbe interactions.Tue, 18 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3963/2013/<b>Responses of soil respiration and its temperature/moisture sensitivity to precipitation in three subtropical forests in southern China</b><br /><br />Biogeosciences, 10, 3963-3982, 2013<br /><br />Author(s): H. Jiang, Q. Deng, G. Zhou, D. Hui, D. Zhang, S. Liu, G. Chu, and J. Li<br /><br />Both long-term observation data and model simulations suggest an increasing chance of serious drought in the dry season and extreme flood in the wet season in southern China, yet little is known about how changes in precipitation pattern will affect soil respiration in the region. We conducted a field experiment to study the responses of soil respiration to precipitation manipulations – precipitation exclusion to mimic drought, double precipitation to simulate flood, and ambient precipitation as control (abbr. EP, DP and AP, respectively) – in three subtropical forests in southern China. The three forest sites include Masson pine forest (PF), coniferous and broad-leaved mixed forest (MF) and monsoon evergreen broad-leaved forest (BF). Our observations showed that altered precipitation strongly influenced soil respiration, not only through the well-known direct effects of soil moisture on plant and microbial activities, but also by modification of both moisture and temperature sensitivity of soil respiration. In the dry season, soil respiration and its temperature sensitivity, as well as fine root and soil microbial biomass, showed rising trends with precipitation increases in the three forest sites. Contrarily, the moisture sensitivity of soil respiration decreased with precipitation increases. In the wet season, different treatments showed different effects in three forest sites. The EP treatment decreased fine root biomass, soil microbial biomass, soil respiration and its temperature sensitivity, but enhanced soil moisture sensitivity in all three forest sites. The DP treatment significantly increased soil respiration, fine root and soil microbial biomass in the PF only, and no significant change was found for the soil temperature sensitivity. However, the DP treatment in the MF and BF reduced soil temperature sensitivity significantly in the wet season. Our results indicated that soil respiration would decrease in the three subtropical forests if soil moisture continues to decrease in the future. More rainfall in the wet season could have limited effect on the response of soil respiration to the rising of temperature in the BF and MF.Tue, 18 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3943/2013/<b>Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf &ndash; is there any recovery after eutrophication?</b><br /><br />Biogeosciences, 10, 3943-3962, 2013<br /><br />Author(s): A. Capet, J.-M. Beckers, and M. Grégoire<br /><br />The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from counterbalancing the large consumption of oxygen due to respiration in the bottom waters and in the sediments, and sets the stage for the development of seasonal hypoxia. <br><br> A three-dimensional (3-D) coupled physical–biogeochemical model is used to investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at seasonal and then at interannual scales (1981–2009), and to differentiate its driving factors (climatic versus eutrophication). <br><br> Model skills are evaluated by a quantitative comparison of the model results to 14 123 in situ oxygen measurements available in the NOAA World Ocean and the Black Sea Commission databases, using different error metrics. This validation exercise shows that the model is able to represent the seasonal and interannual variability of the oxygen concentration and of the occurrence of hypoxia, as well as the spatial distribution of oxygen-depleted waters. <br><br> During the period 1981–2009, each year exhibits seasonal bottom hypoxia at the end of summer. This phenomenon essentially covers the northern part of the NWS – which receives large inputs of nutrients from the Danube, Dniester and Dnieper rivers – and extends, during the years of severe hypoxia, towards the Romanian bay of Constanta. <br><br> An index <i>H</i> which merges the aspects of the spatial and temporal extension of the hypoxic event is proposed to quantify, for each year, the intensity of hypoxia as an environmental stressor. <br><br> In order to explain the interannual variability of <i>H</i> and to disentangle its drivers, we analyze the long time series of model results by means of a stepwise multiple linear regression. This statistical model gives a general relationship that links the intensity of hypoxia to eutrophication and climate-related variables. <br><br> A total of 82% of the interannual variability of <i>H</i> is explained by the combination of four predictors: the annual riverine nitrate load (<i>N</i>), the sea surface temperature in the month preceding stratification (<i>T</i>_s), the amount of semi-labile organic matter accumulated in the sediments (<i>C</i>) and the sea surface temperature during late summer (<i>T</i>_f). Partial regression indicates that the climatic impact on hypoxia is almost as important as that of eutrophication. <br><br> Accumulation of organic matter in the sediments introduces an important inertia in the recovery process after eutrophication, with a typical timescale of 9.3 yr. <br><br> Seasonal fluctuations and the heterogeneous spatial distribution complicate the monitoring of bottom hypoxia, leading to contradictory conclusions when the interpretation is done from different sets of data. In particular, it appears that the recovery reported in the literature after 1995 was overestimated due to the use of observations concentrated in areas and months not typically affected by hypoxia. This stresses the urgent need for a dedicated monitoring effort in the Black Sea NWS focused on the areas and months concerned by recurrent hypoxic events.Tue, 18 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3931/2013/<b>Long-term nitrogen addition decreases carbon leaching in a nitrogen-rich forest ecosystem</b><br /><br />Biogeosciences, 10, 3931-3941, 2013<br /><br />Author(s): X. Lu, F. S. Gilliam, G. Yu, L. Li, Q. Mao, H. Chen, and J. Mo<br /><br />Dissolved organic carbon (DOC) plays a critical role in the carbon (C) cycle of forest soils, and has been recently connected with global increases in nitrogen (N) deposition. Most studies on effects of elevated N deposition on DOC have been carried out in N-limited temperate regions, with far fewer data available from N-rich ecosystems, especially in the context of chronically elevated N deposition. Furthermore, mechanisms for excess N-induced changes of DOC dynamics have been suggested to be different between the two kinds of ecosystems, because of the different ecosystem N status. The purpose of this study was to experimentally examine how long-term N addition affects DOC dynamics below the primary rooting zones (the upper 20 cm soils) in typically N-rich lowland tropical forests. We have a primary assumption that long-term continuous N addition minimally affects DOC concentrations and effluxes in N-rich tropical forests. Experimental N addition was administered at the following levels: 0, 50, 100 and 150 kg N ha⁻¹ yr⁻¹, respectively. Results showed that seven years of N addition significantly decreased DOC concentrations in soil solution, and chemo-physical controls (solution acidity change and soil sorption) rather than biological controls may mainly account for the decreases, in contrast to other forests. We further found that N addition greatly decreased annual DOC effluxes from the primary rooting zone and increased water-extractable DOC in soils. Our results suggest that long-term N deposition could increase soil C sequestration in the upper soils by decreasing DOC efflux from that layer in N-rich ecosystems, a novel mechanism for continued accumulation of soil C in old-growth forests.Tue, 18 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3917/2013/<b>Detection of large above-ground biomass variability in lowland forest ecosystems by airborne LiDAR</b><br /><br />Biogeosciences, 10, 3917-3930, 2013<br /><br />Author(s): J. Jubanski, U. Ballhorn, K. Kronseder, J Franke, and F. Siegert<br /><br />Quantification of tropical forest above-ground biomass (AGB) over large areas as input for Reduced Emissions from Deforestation and forest Degradation (REDD+) projects and climate change models is challenging. This is the first study which attempts to estimate AGB and its variability across large areas of tropical lowland forests in Central Kalimantan (Indonesia) through correlating airborne light detection and ranging (LiDAR) to forest inventory data. Two LiDAR height metrics were analysed, and regression models could be improved through the use of LiDAR point densities as input (<i>R</i>² = 0.88; <i>n</i> = 52). Surveying with a LiDAR point density per square metre of about 4 resulted in the best cost / benefit ratio. We estimated AGB for 600 km of LiDAR tracks and showed that there exists a considerable variability of up to 140% within the same forest type due to varying environmental conditions. Impact from logging operations and the associated AGB losses dating back more than 10 yr could be assessed by LiDAR but not by multispectral satellite imagery. Comparison with a Landsat classification for a 1 million ha study area where AGB values were based on site-specific field inventory data, regional literature estimates, and default values by the Intergovernmental Panel on Climate Change (IPCC) showed an overestimation of 43%, 102%, and 137%, respectively. The results show that AGB overestimation may lead to wrong greenhouse gas (GHG) emission estimates due to deforestation in climate models. For REDD+ projects this leads to inaccurate carbon stock estimates and consequently to significantly wrong REDD+ based compensation payments.Mon, 17 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3901/2013/<b>Phosphorus recycling in sediments of the central Baltic Sea</b><br /><br />Biogeosciences, 10, 3901-3916, 2013<br /><br />Author(s): L. Viktorsson, N. Ekeroth, M. Nilsson, M. Kononets, and P. O. J. Hall<br /><br />Benthic fluxes of phosphorus (P) were measured in situ in the Eastern Gotland Basin (EGB), central Baltic Sea, using benthic landers. A total of 40 flux measurements of dissolved inorganic P (DIP) on 13 stations at water depths ranging 30–210 m and under different oxygen regimes were carried out on three cruises during three consecutive years (2008–2010) in August–September. Our study is the first to report in situ DIP fluxes in the Baltic proper, and it provides the most comprehensive dataset of benthic fluxes of DIP and dissolved organic P (DOP) in the Baltic proper existing to date. DIP fluxes increased with increasing water depth and with decreasing bottom water oxygen concentration. Average DIP fluxes were calculated for oxic bottom water conditions (&minus; 0.003 &pm; 0.040 mmol m^&minus;2 d^&minus;1), hypoxic conditions (0.027&pm; 0.067 mmol m^&minus;2 d^&minus;1) and anoxic conditions (0.376 &pm; 0.214 mmol m^&minus;2 d^&minus;1). The mean DIP flux at anoxic bottoms was higher than previous estimates based on ex situ measurements of pore water gradients. The DIP flux was positively correlated with the organic carbon inventory of sediment, and the benthic flux of dissolved inorganic carbon (DIC) at anoxic stations, but these variables were uncorrelated at oxic stations. The positive correlation between DIP and DIC fluxes suggests that the benthic DIP efflux from anoxic bottoms in the Baltic Proper is mainly controlled by rates of deposition and degradation of organic matter. The flux from anoxic sediment was very P rich in relation to both C and nitrogen (N). The average C : P ratio in fluxes at anoxic accumulation bottoms was 69 &pm; 15, which is well below the Redfield C : P ratio of 106 : 1. At oxic stations, however, the C : P flux ratio was much higher than the Redfield ratio, consistent with well-known P retention mechanisms associated with iron and bacteria in oxidised sediment. Using a benthic mass balance approach, a burial efficiency estimate of 0.2–12% was calculated for the anoxic part of the EGB, which suggests that anoxic Baltic sediments are very efficient in recycling deposited P. Based on the measured fluxes and the average areal extent of anoxic bottoms during years 1999–2006, an internal DIP load of 152 kton yr⁻¹ was calculated. This is almost 9 times higher than the average external total phosphorus (TP) supply to the Baltic proper during the same period. This comparison clearly highlights the dominance of internally regenerated P as a DIP source in the Baltic Sea.Mon, 17 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3889/2013/<b>Seasonality of CO₂ in coastal oceans altered by increasing anthropogenic nutrient delivery from large rivers: evidence from the Changjiang–East China Sea system</b><br /><br />Biogeosciences, 10, 3889-3899, 2013<br /><br />Author(s): W.-C. Chou, G.-C. Gong, W.-J. Cai, and C.-M. Tseng<br /><br />Model studies suggested that human-induced increase in nutrient load may have stimulated primary production and thus enhanced the CO₂ uptake capacity in the coastal ocean. In this study, we investigated the seasonal variations of the surface water's partial pressure of CO₂ (<i>p</i>CO₂^sw) in the highly human-impacted Changjiang–East China Sea system between 2008 and 2011. The seasonality of <i>p</i>CO₂^sw has large spatial variations, with the largest extreme of 170 ± 75 μatm on the inner shelf near the Changjiang Estuary (from 271 ± 55 μatm in summer to 441 ± 51 μatm in autumn) and the weakest extreme of 53 ± 20 μatm on the outer shelf (from 328 ± 9 μatm in winter to 381 ± 18 μatm in summer). During the summer period, stronger stratification and biological production driven by the eutrophic Changjiang plume results in a very low dissolved inorganic carbon (DIC) in surface waters and a very high DIC in bottom waters of the inner shelf, with the latter returning high DIC to the surface water during the mixed period. Interestingly, a comparison with historical data shows that the average <i>p</i>CO₂^sw on the inner shelf near the Changjiang Estuary has decreased notably during summer, but has increased during autumn and winter from the 1990s to the 2000s. We suggest that this decadal change is associated with recently increased eutrophication. This would increase both the photosynthetic removal of DIC in surface waters and the respiratory release of DIC in bottom waters during summertime, thereby returning more DIC to the surface during the subsequent mixing seasons and/or episodic extreme weather events (e.g., typhoons). Our finding demonstrates that increasing anthropogenic nutrient delivery from a large river may enhance the sequestration capacity of CO₂ in summer but may reduce it in autumn and winter. Consequently, the coastal ocean may not necessarily take up more atmospheric CO₂ in response to increasing eutrophication, and the net effect largely depends on the relative timescale of air–sea gas exchange and offshore transport of the shelf water. Finally, the case we report for the Changjiang system may have general ramifications for other eutrophic coastal oceans.Mon, 17 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3869/2013/<b>Insights into mechanisms governing forest carbon response to nitrogen deposition: a model&ndash;data comparison using observed responses to nitrogen addition</b><br /><br />Biogeosciences, 10, 3869-3887, 2013<br /><br />Author(s): R. Q. Thomas, G. B. Bonan, and C. L. Goodale<br /><br />In many forest ecosystems, nitrogen (N) deposition enhances plant uptake of carbon dioxide, thus reducing climate warming from fossil fuel emissions. Therefore, accurately modeling how forest carbon (C) sequestration responds to N deposition is critical for understanding how future changes in N availability will influence climate. Here, we use observations of forest C response to N inputs along N deposition gradients and at five temperate forest sites with fertilization experiments to test and improve a global biogeochemical model (CLM-CN 4.0). We show that the CLM-CN plant C growth response to N deposition was smaller than observed and the modeled response to N fertilization was larger than observed. A set of modifications to the CLM-CN improved the correspondence between model predictions and observational data (1) by increasing the aboveground C storage in response to historical N deposition (1850–2004) from 14 to 34 kg C per additional kg N added through deposition and (2) by decreasing the aboveground net primary productivity response to N fertilization experiments from 91 to 57 g C m⁻² yr⁻¹. Modeled growth response to N deposition was most sensitive to altering the processes that control plant N uptake and the pathways of N loss. The response to N deposition also increased with a more closed N cycle (reduced N fixation and N gas loss) and decreased when prioritizing microbial over plant uptake of soil inorganic N. The net effect of all the modifications to the CLM-CN resulted in greater retention of N deposition and a greater role of synergy between N deposition and rising atmospheric CO₂ as a mechanism governing increases in temperate forest primary production over the 20th century. Overall, testing models with both the response to gradual increases in N inputs over decades (N deposition) and N pulse additions of N over multiple years (N fertilization) allows for greater understanding of the mechanisms governing C–N coupling.Mon, 17 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3849/2013/<b>Riparian zone control on base cation concentration in boreal streams</b><br /><br />Biogeosciences, 10, 3849-3868, 2013<br /><br />Author(s): J. L. J. Ledesma, T. Grabs, M. N. Futter, K. H. Bishop, H. Laudon, and S. J. Köhler<br /><br />Riparian zones (RZ) are a major factor controlling water chemistry in forest streams. Base cations' (BC) concentrations, fluxes, and cycling in the RZ merit attention because a changing climate and increased forest harvesting could have negative consequences, including re-acidification, for boreal surface waters. We present a two-year study of BC and silica (Si) flow-weighted concentrations from 13 RZ and 14 streams in different landscape elements of a boreal catchment in northern Sweden. The spatial variation in BC and Si dynamics in both RZ and streams was explained by differences in landscape element type, with highest concentrations in silty sediments and lowest concentrations in peat-dominated wetland areas. Temporal stability in BC and Si concentrations in riparian soil water, remarkably stable Mg/Ca ratios, and homogeneous mineralogy suggest that patterns found in the RZ are a result of a distinct mineralogical upslope signal in groundwater. Stream water Mg/Ca ratios indicate that the signal is subsequently maintained in the streams. Flow-weighted concentrations of Ca, Mg, and Na in headwater streams were represented by the corresponding concentrations in the RZ, which were estimated using the Riparian Flow-Concentration Integration Model (RIM) approach. Stream and RZ flow-weighted concentrations differed for K and Si, suggesting a stronger biogeochemical influence on these elements, including K recirculation by vegetation and retention of Si within the RZ. Potential increases in groundwater levels linked to forest harvesting or changes in precipitation regimes would tend to reduce BC concentrations from RZ to streams, potentially leading to episodic acidification.Fri, 14 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3839/2013/<b>Iodine-129 concentration in seawater near Fukushima before and after the accident at the Fukushima Daiichi Nuclear Power Plant</b><br /><br />Biogeosciences, 10, 3839-3847, 2013<br /><br />Author(s): T. Suzuki, S. Otosaka, J. Kuwabara, H. Kawamura, and T. Kobayashi<br /><br />Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived ¹²⁹I, the ¹²⁹I concentrations in seawater before and after the accident were compared. <br><br> Before the accident (2008–2009), the ¹²⁹I concentrations in the western margin of the North Pacific between 32&deg; N and 44&deg; N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average ¹²⁹I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of ¹²⁹I in surface seawater, the accident-derived ¹²⁹I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively. <br><br> As of October 2011, depth profiles of ¹²⁹I revealed that ¹²⁹I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived ¹²⁹I were estimated to be (1.6–9.6) × 10¹² atoms m⁻² in this study area. <br><br> On the basis of the ¹²⁹I data in the seawater near Fukushima, the effective dose of ¹²⁹I from seafood ingestion was much smaller than the annual dose limit.Fri, 14 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3817/2013/<b>DNA from lake sediments reveals the long-term dynamics and diversity of <i>Synechococcus</i> assemblages</b><br /><br />Biogeosciences, 10, 3817-3838, 2013<br /><br />Author(s): I. Domaizon, O. Savichtcheva, D. Debroas, F. Arnaud, C. Villar, C. Pignol, B. Alric, and M. E. Perga<br /><br />While picocyanobacteria (PC) are important actors in carbon and nutrient cycles in aquatic systems, factors controlling their interannual dynamics and diversity are poorly known due to the general lack of long-term monitoring surveys. This study intended to fill this gap by applying a DNA-based paleolimnological approach to sediment records from a deep subalpine lake that has experienced dramatic changes in environmental conditions during the last century (eutrophication, re-oligotrophication and large-scale climate changes). In particular, we investigated the long-term (100 yr) diversity and dynamics of <i>Synechococcus,</i>, PC that have presumably been affected by both the lake trophic status changes and global warming. The lake's morphological and environmental conditions provided the ideal conditions for DNA preservation in the sediment archives. Generalised additive models applied to quantitative PCR (qPCR; quantitative Polymerase Chain Reaction) results highlighted that an increase in summer temperature could have a significant positive impact on the relative abundance of <i>Synechococcus,</i> (fraction of <i>Synechococcus,</i> in total cyanobacteria). The diversity of <i>Synechococcus,</i> in Lake Bourget was studied by phylogenetic analyses of the 16S rRNA gene and the following internally transcribed spacer (ITS). Up to 23 different OTUs (based on 16S rRNA), which fell into various cosmopolitan or endemic clusters, were identified in samples from the past 100 yr. Moreover, the study of ITS revealed a higher diversity within the major 16S rRNA-defined OTUs. Changes in PC diversity were related to the lake's trophic status. Overall, qPCR and sequencing results showed that environmental changes (in temperature and phosphorus concentration) affected <i>Synechococcus,</i> community dynamics and structure, translating into changes in genotype composition. These results also helped to re-evaluate the geographical distribution of some <i>Synechococcus,</i> clusters. Providing such novel insights into the long-term history of an important group of primary producers, this study illustrates the promising approach that consists in coupling molecular tools and paleolimnology to reconstruct a lake's biodiversity history.Fri, 14 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3809/2013/<b>Natural and Fukushima-derived radioactivity in macroalgae and mussels along the Japanese shoreline</b><br /><br />Biogeosciences, 10, 3809-3815, 2013<br /><br />Author(s): Z. Baumann, N. Casacuberta, H. Baumann, P. Masqué, and N. S. Fisher<br /><br />Following the failure of the nuclear power plant in Fukushima Prefecture in March 2011, peer-reviewed publications describing radioactivity levels in organisms inhabiting coastal environments are scarce. This paper reports on elevated levels of ¹³⁴Cs and ¹³⁷Cs in macroalgae and mussels (up to ~ 800 Bq kg⁻¹ dry wt.) in June 2011. Cs concentrations in biota sampled in early June 2011 were higher in areas south of Fukushima than sampled in the last third of the month north of Fukushima. Activity concentrations from ^134+137Cs in organisms south of Fukushima were comparable to or lower than those from the naturally occurring ⁴⁰K in the same samples. While ²¹⁰Pb and ²¹⁰Po concentrations were generally lower than these other radionuclides, ²¹⁰Po as an &alpha;-emitter is more significant from a radiological viewpoint than &gamma;-emitters as it can inflict greater biological damage. By applying known bioconcentration factors of Cs in biota, measured biota concentrations of Cs were also used to estimate Cs concentrations in coastal seawater to be in the range of 10²–10³ Bq m⁻³. These estimates show that, 3 months after the accident and maximal release of radioactive Cs, levels of Cs persisted in coastal waters, although at levels that were two orders of magnitude lower than at the time of release. These June coastal seawater Cs levels were four orders of magnitude above Cs concentrations off Japan prior to the Fukushima disaster.Thu, 13 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3793/2013/<b>Mercury dynamics in the Rocky Mountain, Colorado, snowpack</b><br /><br />Biogeosciences, 10, 3793-3807, 2013<br /><br />Author(s): X. Faïn, D. Helmig, J. Hueber, D. Obrist, and M. W. Williams<br /><br />Gaseous elemental mercury (GEM) was monitored at the Niwot Ridge (NWT) Long-Term Ecological Research (LTER) site (Colorado, USA, 40&deg; N) from interstitial air extracted from the snowpack at depths ranging from the snow surface to 10 cm above the soil. A highly dynamic cycling of mercury (Hg) in this mid-latitude snowpack was observed. Patterns were driven by both GEM production in surface snow and GEM destruction in the deeper snowpack layers. Thorough mixing and vertical transport processes were observed through the snowpack. GEM was photochemically produced near the snow-air interface throughout the entire winter, leading to enhanced GEM levels in interstitial air of surface snow of up to 8 ng m⁻³. During low-wind periods, GEM in surface snow layers remained significantly above ambient air levels at night as well, which may indicate a potential weak GEM production overnight. Analyses of vertical GEM gradients in the snowpack show that surface GEM enhancements efficiently propagated down the snowpack, with a temporal lag in peak GEM levels observed with increasing depth. Downward diffusion was responsible for much of these patterns, although vertical advection also contributed to vertical redistribution. Destruction of GEM in the lower snowpack layers was attributed to dark oxidation of GEM. Analysis of vertical GEM / CO₂ flux ratios indicated that this GEM destruction occurred in the snow and not in the underlying soil. The strong, diurnal patterns of photochemical GEM production at the surface ultimately lead to re-emission losses of deposited Hg back to the atmosphere. The NWT data show that highest GEM surface production and re-emissions occur shortly after fresh snowfall, which possibly resupplies photoreducible Hg to the snowpack, and that photochemical GEM reduction is not radiation-limited as it is strong even on cloudy days.Thu, 13 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3767/2013/<b>NW European shelf under climate warming: implications for open ocean &ndash; shelf exchange, primary production, and carbon absorption</b><br /><br />Biogeosciences, 10, 3767-3792, 2013<br /><br />Author(s): M. Gröger, E. Maier-Reimer, U. Mikolajewicz, A. Moll, and D. Sein<br /><br />Shelves have been estimated to account for more than one-fifth of the global marine primary production. It has been also conjectured that shelves strongly influence the oceanic absorption of anthropogenic CO₂ (carbon shelf pump). Owing to their coarse resolution, currently applied global climate models are inappropriate to investigate the impact of climate change on shelves and regional models do not account for the complex interaction with the adjacent open ocean. In this study, a global ocean general circulation model and biogeochemistry model were set up with a distorted grid providing a maximal resolution for the NW European shelf and the adjacent northeast Atlantic. <br><br> Using model climate projections we found that already a~moderate warming of about 2.0 K of the sea surface is linked with a reduction by ~ 30% of the biological production on the NW European shelf. If we consider the decline of anthropogenic riverine eutrophication since the 1990s, the reduction of biological production amounts is even larger. The relative decline of NW European shelf productivity is twice as strong as the decline in the open ocean (~ 15%). The underlying mechanism is a spatially well confined stratification feedback along the continental shelf break. This feedback reduces the nutrient supply from the deep Atlantic to about 50%. In turn, the reduced productivity draws down CO₂ absorption in the North Sea by ~ 34% at the end of the 21st century compared to the end of the 20th century implying a strong weakening of shelf carbon pumping. Sensitivity experiments with diagnostic tracers indicate that not more than 20% of the carbon absorbed in the North Sea contributes to the long-term carbon uptake of the world ocean. The rest remains within the ocean's mixed layer where it is exposed to the atmosphere. <br><br> The predicted decline in biological productivity, and decrease of phytoplankton concentration (in the North Sea by averaged 25%) due to reduced nutrient imports from the deeper Atlantic will probably affect the local fish stock negatively and therefore fisheries in the North Sea.Wed, 12 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3749/2013/<b>Field intercomparison of four methane gas analyzers suitable for eddy covariance flux measurements</b><br /><br />Biogeosciences, 10, 3749-3765, 2013<br /><br />Author(s): O. Peltola, I. Mammarella, S. Haapanala, G. Burba, and T. Vesala<br /><br />Performances of four methane gas analyzers suitable for eddy covariance measurements are assessed. The assessment and comparison was performed by analyzing eddy covariance data obtained during summer 2010 (1 April to 26 October) at a pristine fen, Siikaneva, Southern Finland. High methane fluxes with pronounced seasonality have been measured at this fen. The four participating methane gas analyzers are commercially available closed-path units TGA-100A (Campbell Scientific Inc., USA), RMT-200 (Los Gatos Research, USA), G1301-f (Picarro Inc., USA) and an early prototype open-path unit Prototype-7700 (LI-COR Biosciences, USA). The RMT-200 functioned most reliably throughout the measurement campaign, during low and high flux periods. Methane fluxes from RMT-200 and G1301-f had the smallest random errors and the fluxes agree remarkably well throughout the measurement campaign. Cospectra and power spectra calculated from RMT-200 and G1301-f data agree well with corresponding temperature spectra during a high flux period. None of the gas analyzers showed statistically significant diurnal variation for methane flux. Prototype-7700 functioned only for a short period of time, over one month, in the beginning of the measurement campaign during low flux period, and thus, its overall accuracy and season-long performance were not assessed. The open-path gas analyzer is a practical choice for measurement sites in remote locations due to its low power demand, whereas for G1301-f methane measurements interference from water vapor is straightforward to correct since the instrument measures both gases simultaneously. In any case, if only the performance in this intercomparison is considered, RMT-200 performed the best and is the recommended choice if a new fast response methane gas analyzer is needed.Tue, 11 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3731/2013/<b>Spectrally resolved efficiencies of carbon monoxide (CO) photoproduction in the western Canadian Arctic: particles versus solutes</b><br /><br />Biogeosciences, 10, 3731-3748, 2013<br /><br />Author(s): G. Song, H. Xie, S. Bélanger, E. Leymarie, and M. Babin<br /><br />Spectrally resolved efficiency (i.e. apparent quantum yield, AQY) of carbon monoxide (CO) photoproduction is a useful indicator of substrate photoreactivity and a crucial parameter for modeling CO photoproduction rates in the water column. Recent evidence has suggested that CO photoproduction from particles in marine waters is significant compared to the well-known CO production from chromophoric dissolved organic matter (CDOM) photodegradation. Although CDOM-based CO AQY spectra have been extensively determined, little is known of this information on the particulate phase. Using water samples collected from the Mackenzie estuary, shelf, and Canada Basin in the southeastern Beaufort Sea, the present study for the first time quantified the AQY spectra of particle-based CO photoproduction and compared them with the concomitantly determined CDOM-based CO AQY spectra. CO AQYs of both particles and CDOM decreased with wavelength but the spectral shape of the particulate AQY was flatter in the visible regime. This feature resulted in a disproportionally higher visible light-driven CO production by particles, thereby increasing the ratio of particle- to CDOM-based CO photoproduction with depth in the euphotic zone. In terms of depth-integrated production in the euphotic zone, CO formation from CDOM was dominated by the ultraviolet (UV, 290–400 nm) radiation whereas UV and visible light played roughly equal roles in CO production from particles. Spatially, CO AQY of bulk particulate matter (i.e. the sum of organics and inorganics) augmented from the estuary and shelf to the basin while CO AQY of CDOM trended inversely. Water from the deep chlorophyll maximum layer revealed higher CO AQYs than did surface water for both particles and CDOM. CO AQY of bulk particulate matter exceeded that of CDOM on the shelf and in the basin, but the sequence reversed in the estuary. Without consideration of the potential role of metal oxides (e.g. iron oxides) in particle photochemistry, mineral absorption-corrected CO AQY of particulate organic matter (POM) could, however, surpass its CDOM counterpart in all three sub-regions and displayed magnitudes in the estuary that overtook those in shelf and offshore waters. In terms of CO photoproduction, POM was thus more photoreactive than CDOM, irrespective of the organic matter's origins (i.e. terrigenous or marine). Riverine CDOM exhibited higher photoreactivity than marine CDOM and land-derived POM appeared more photoreactive than marine POM. AQY-based modeling indicates that CO photoproduction in the study area is underestimated by 12–32% if the particulate term is ignored.Wed, 05 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3715/2013/<b>The ocean response to volcanic iron fertilisation after the eruption of Kasatochi volcano: a regional-scale biogeochemical ocean model study</b><br /><br />Biogeosciences, 10, 3715-3729, 2013<br /><br />Author(s): A. Lindenthal, B. Langmann, J. Pätsch, I. Lorkowski, and M. Hort<br /><br />In high-nutrient–low-chlorophyll regions, phytoplankton growth is limited by the availability of water-soluble iron. The eruption of Kasatochi volcano in August 2008 led to ash deposition into the iron-limited NE Pacific Ocean. Volcanic ash released iron upon contact with seawater and generated a massive phytoplankton bloom. Here we investigate this event with a one-dimensional ocean biogeochemical column model to illuminate the ocean response to iron fertilisation by volcanic ash. The results indicate that the added iron triggered a phytoplankton bloom in the summer of 2008. Associated with this bloom, macronutrient concentrations such as nitrate and silicate decline and zooplankton biomass is enhanced in the ocean mixed layer. The simulated development of the drawdown of carbon dioxide and increase of pH in surface seawater is in good agreement with available observations. Sensitivity studies with different supply dates of iron to the ocean emphasise the favourable oceanic conditions in the NE Pacific to generate massive phytoplankton blooms in particular during July and August in comparison to other months. By varying the amount of volcanic ash and associated bio-available iron supplied to the ocean, model results demonstrate that the NE Pacific Ocean has higher, but limited capabilities to consume CO₂ after iron fertilisation than those observed after the volcanic eruption of Kasatochi.Wed, 05 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3705/2013/<b>Macrofaunal community inside and outside of the Darwin Mounds Special Area of Conservation, NE Atlantic</b><br /><br />Biogeosciences, 10, 3705-3714, 2013<br /><br />Author(s): N. Serpetti, E. Gontikaki, B. E. Narayanaswamy, and U. Witte<br /><br />Spatial distribution and patchiness of deep sea macrofaunal communities were studied from samples collected in the Rockall Trough, NE Atlantic. In June 2011, two areas, located outside and within the Darwin Mound Special Area of Conservation (SAC), were sampled. Three megacores were deployed in each area at approximately 900 m depth. The two areas, ~ 18 km apart, did not differ in terms of sediment organic matter and percentage of mud content, but small significant differences were found in sediment median grain size and depth. Macrofaunal communities were found to differ significantly, with the difference mostly driven by changes in the abundance of polychaetes, crustaceans and nematodes whilst no significant differences were seen for the other phyla. Whereas overall macrofaunal abundance was higher outside the SAC compared to within, this pattern varies considerably between phyla. Diversity indices showed no significant differences between protected and unprotected sites. Deep-water trawling regularly take place outside the Darwin Mounds SAC whilst the area inside the SAC has been closed to bottom trawling since 2004, and the above distribution patterns are discussed in the context of both environmental and anthropogenic causes.Wed, 05 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3691/2013/<b>A meta-analysis on the impacts of partial cutting on forest structure and carbon storage</b><br /><br />Biogeosciences, 10, 3691-3703, 2013<br /><br />Author(s): D. Zhou, S. Q. Zhao, S. Liu, and J. Oeding<br /><br />Partial cutting, which removes some individual trees from a forest, is one of the major and widespread forest management practices that can significantly alter both forest structure and carbon (C) storage. Using 748 observations from 81 studies published between 1973 and 2011, we synthesized the impacts of partial cutting on three variables associated with forest structure (mean annual growth of diameter at breast height (DBH), stand basal area, and volume) and four variables related to various C stock components (aboveground biomass C (AGBC), understory C, forest floor C, and mineral soil C). Results show that the growth of DBH increased by 111.9% after partial cutting, compared to the uncut control, with a 95% bootstrapped confidence interval ranging from 92.2 to 135.9%, while stand basal area and volume decreased immediately by 34.2% ([−37.4%, −31.2%]) and 28.4% ([−32.0%, −25.1%]), respectively. On average, partial cutting reduced AGBC by 43.4% ([−47.7%, −39.3%]), increased understory C storage by 391.5% ([220.0%, 603.8%]), but did not show significant effects on C stocks on forest floor and in mineral soil. All the effects, if significant (i.e., on DBH growth, stand basal area, volume, and AGBC), intensified linearly with cutting intensity and decreased linearly over time. Overall, cutting intensity had more strong impacts than the length of recovery time on the responses of those variables to partial cutting. Besides the significant influence of cutting intensity and recovery time, other factors such as climate zone and forest type also affected forest responses to partial cutting. For example, a large fraction of the changes in DBH growth remains unexplained, suggesting the factors not included in the analysis may play a major role. The data assembled in this synthesis were not sufficient to determine how long it would take for a complete recovery after cutting because long-term experiments were scarce. Future efforts should be tailored to increase the duration of the experiments and balance geographic locations of field studies.Wed, 05 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3679/2013/<b>Response of bacterioplankton community structure to an artificial gradient of <i>p</i>CO₂ in the Arctic Ocean</b><br /><br />Biogeosciences, 10, 3679-3689, 2013<br /><br />Author(s): R. Zhang, X. Xia, S. C. K. Lau, C. Motegi, M. G. Weinbauer, and N. Jiao<br /><br />In order to test the influences of ocean acidification on the ocean pelagic ecosystem, so far the largest CO₂ manipulation mesocosm study (European Project on Ocean Acidification, EPOCA) was performed in Kings Bay (Kongsfjorden), Spitsbergen. During a 30 day incubation, bacterial diversity was investigated using DNA fingerprinting and clone library analysis of bacterioplankton samples. Terminal restriction fragment length polymorphism (T-RFLP) analysis of the PCR amplicons of the 16S rRNA genes revealed that general bacterial diversity, taxonomic richness and community structure were influenced by the variation of productivity during the time of incubation, but not the degree of ocean acidification. A BIOENV analysis suggested a complex control of bacterial community structure by various biological and chemical environmental parameters. The maximum apparent diversity of bacterioplankton (i.e., the number of T-RFs) in high and low <i>p</i>CO₂ treatments differed significantly. A negative relationship between the relative abundance of <i>Bacteroidetes</i> and <i>p</i>CO₂ levels was observed for samples at the end of the experiment by the combination of T-RFLP and clone library analysis. Our study suggests that ocean acidification affects the development of bacterial assemblages and potentially impacts the ecological function of the bacterioplankton in the marine ecosystem.Tue, 04 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3661/2013/<b>The fate of riverine nutrients on Arctic shelves</b><br /><br />Biogeosciences, 10, 3661-3677, 2013<br /><br />Author(s): V. Le Fouest, M. Babin, and J.-É. Tremblay<br /><br />Present and future levels of primary production (PP) in the Arctic Ocean (AO) depend on nutrient inputs to the photic zone via vertical mixing, upwelling and external sources. In this regard, the importance of horizontal river supply relative to oceanic processes is poorly constrained at the pan-Arctic scale. We compiled extensive historical (1954–2012) data on discharge and nutrient concentrations to estimate fluxes of nitrate, soluble reactive phosphate (SRP), silicate, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), particulate organic nitrogen (PON) and particulate organic carbon (POC) from 9 large Arctic rivers and assess their potential impact on the biogeochemistry of shelf waters. Several key points can be emphasized from this analysis. The contribution of riverine nitrate to new PP (PP_new) is very small at the regional scale (< 1% to 6.7%) and negligible at the pan-Arctic scale (< 0.83%), in agreement with recent studies. By consuming all this nitrate, oceanic phytoplankton would be able to use only 14.3% and 8.7–24.5% of the river supply of silicate at the pan-Arctic and regional scales, respectively. Corresponding figures for SRP are 28.9% and 18.6–46%. On the Beaufort and Bering shelves, riverine SRP cannot fulfil phytoplankton requirements. On a seasonal basis, the removal of riverine nitrate, silicate and SRP would be the highest in spring and not in summer when AO shelf waters are nitrogen-limited. Riverine DON is potentially an important nitrogen source for the planktonic ecosystem in summer, when ammonium supplied through the photoammonification of refractory DON (3.9 × 10⁹ mol N) may exceed the combined riverine supply of nitrate and ammonium (3.4 × 10⁹ mol N). Nevertheless, overall nitrogen limitation of AO phytoplankton is expected to persist even when projected increases of riverine DON and nitrate supply are taken into account. This analysis underscores the need to better contrast oceanic nutrient supply processes with the composition and fate of changing riverine nutrient deliveries in future scenarios of plankton community structure, function and production in the coastal AO.Tue, 04 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3649/2013/<b>⁹⁰Sr and ⁸⁹Sr in seawater off Japan as a consequence of the Fukushima Dai-ichi nuclear accident</b><br /><br />Biogeosciences, 10, 3649-3659, 2013<br /><br />Author(s): N. Casacuberta, P. Masqué, J. Garcia-Orellana, R. Garcia-Tenorio, and K.O. Buesseler<br /><br />The impact of the earthquake and tsunami on the east coast of Japan on 11 March 2011 caused a loss of power at the Fukushima Dai-ichi nuclear power plant (NPP) that resulted in one of the most important releases of artificial radioactivity into the environment. Although several works were devoted to evaluating the atmospheric dispersion of radionuclides, the impact of the discharges to the ocean has been less investigated. Here we evaluate the distribution of Fukushima-derived ⁹⁰Sr (<i>n</i> = 57) and ⁸⁹Sr (<i>n</i> = 19) throughout waters 30–600 km offshore in June 2011. Concentrations of ⁹⁰Sr and ⁸⁹Sr in both surface waters and shallow profiles ranged from 0.8 ± 0.2 to 85 ± 3 Bq m⁻³ and from 19 ± 6 to 265 ± 74 Bq m⁻³, respectively. Because of its short half-life, all measured ⁸⁹Sr was due to the accident, while the ⁹⁰Sr concentrations can be compared to the background levels in the Pacific Ocean of about 1.2 Bq m⁻³. Fukushima-derived radiostrontium was mainly detected north of Kuroshio Current, as this was acting as a southern boundary for transport. The highest activities were associated with near-shore eddies, and larger inventories were found in the closest stations to Fukushima NPP. The data evidence a major influence of direct liquid discharges of radiostrontium compared to the atmospheric deposition. Existing ¹³⁷Cs data reported from the same samples allowed us to establish a ⁹⁰Sr / ¹³⁷Cs ratio of 0.0256 ± 0.0006 in seawater off Fukushima, being significantly different than that of the global atmospheric fallout (i.e., 0.63) and may be used in future studies to track waters coming from the east coast of Japan. Liquid discharges of ⁹⁰Sr to the ocean were estimated, resulting in an inventory of 53 ± 1 TBq of ⁹⁰Sr in the inshore study area in June 2011 and total releases of ⁹⁰Sr ranging from 90 to 900 TBq, depending upon the reported estimates of ¹³⁷Cs releases that are considered.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3635/2013/<b>The role of wind in hydrochorous mangrove propagule dispersal</b><br /><br />Biogeosciences, 10, 3635-3647, 2013<br /><br />Author(s): T. Van der Stocken, D. J. R. De Ryck, T. Balke, T. J. Bouma, F. Dahdouh-Guebas, and N. Koedam<br /><br />Although wind has been recognized to be an important factor in the dispersal of hydrochorous mangrove propagules, and hence in the quantification of (meta)population dynamics, the species-specific sensitivity to wind effects has not been studied. We combined observations from a controlled experiment (flume tank) and in situ experiments to understand wind and water current contributions to dispersal potential as well as to estimate real dispersal ranges due to immediate response to tidal currents (two outgoing tides). This was done for 4 species with propagules differing in morphological and buoyancy properties (i.e. <i>Rhizophora mucronata, Ceriops tagal, Heritiera littoralis</i> and <i>Xylocarpus granatum</i>). The flume experiments revealed that the influence of wind depends on the density of a propagule (and hence its buoyancy characteristics) and that typical morphological characteristics of the dispersal unit are additionally important. <i>H. littoralis</i> propagules were influenced most, because on the one hand their low density (613.58 g L⁻¹; <i>n</i> =10) enables them to float on top of the water surface, and on the other hand their "sailboat-like" structure provides a relatively large surface area. The <i>X. granatum</i> fruits appeared to be the least influenced by ambient wind conditions, explained by the smooth surface and spherical shape of which, because of the fruit's high density (890.05 g L⁻¹; <i>n</i> = 1), only a small part sticks above the water surface. Although the seeds of <i>X. granatum</i> are of a similar size class than <i>H. littoralis</i> propagules, they are (like the <i>X. granatum</i> fruits) largely submerged due to their high density (870.66 g L⁻¹; <i>n</i> = 8), hence catching less wind than <i>H. littoralis</i> propagules. The influence of wind on the dispersal of the horizontally floating <i>C. tagal</i> and <i>R. mucronata</i> dispersal units was strong, comparable to that of <i>H. littoralis</i> propagules. A differential effect of wind was found within elongated propagules, which directly follows from the floating orientation of the propagules. While the dispersal path of vertically floating propagules was influenced by the strength and direction of the water currents and to a lesser extent by ambient wind conditions, the dispersal path of horizontally floating propagules was influenced by both strength and direction of the water currents and prevailing wind forces. To validate the flume results, propagules of <i>C. tagal</i> and <i>R. mucronata</i> were released during outgoing tide in a tidal creek in Gazi Bay (Kenya), followed by observation of their dispersal distance and direction, while knowing the actual dominant wind direction. In line with the flume results, this study showed that wind plays an important role in the dispersal distance of the propagules. The present study provides important mechanistic insight into the effect of wind on hydrochorous mangrove propagule dispersal, thereby yielding an essential step towards the construction and optimization of (particle-based) hydrodynamic dispersal models.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3615/2013/<b>Emission of atmospherically significant halocarbons by naturally occurring and farmed tropical macroalgae</b><br /><br />Biogeosciences, 10, 3615-3633, 2013<br /><br />Author(s): E. C. Leedham, C. Hughes, F. S. L. Keng, S.-M. Phang, G. Malin, and W. T. Sturges<br /><br />Current estimates of global halocarbon emissions highlight the tropical coastal environment as an important source of very short-lived (VSL) biogenic halocarbons to the troposphere and stratosphere, due to a combination of assumed high primary productivity in tropical coastal waters and the prevalence of deep convective transport, potentially capable of rapidly lifting surface emissions to the upper troposphere/lower stratosphere. However, despite this perceived importance, direct measurements of tropical coastal biogenic halocarbon emissions, notably from macroalgae (seaweeds), have not been made. In light of this, we provide the first dedicated study of halocarbon production by a range of 15 common tropical macroalgal species and compare these results to those from previous studies of polar and temperate macroalgae. Variation between species was substantial; CHBr₃ production rates, measured at the end of a 24 h incubation, varied from 1.4 to 1129 pmol g FW⁻¹ h⁻¹ (FW = fresh weight of sample). We used our laboratory-determined emission rates to estimate emissions of CHBr₃ and CH₂Br₂ (the two dominant VSL precursors of stratospheric bromine) from the coastlines of Malaysia and elsewhere in South East Asia (SEA). We compare these values to previous top-down model estimates of emissions from these regions and, by using several emission scenarios, we calculate an annual CHBr₃ emission of 40 (6–224 Mmol Br⁻¹ yr), a value that is lower than previous estimates. The contribution of tropical aquaculture to current emission budgets is also considered. Whilst the current aquaculture contribution to halocarbon emissions in this regional is small, the potential exists for substantial increases in aquaculture to make a significant contribution to regional halocarbon budgets.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3605/2013/<b>Taking nature into lab: biomineralization by heavy metal-resistant streptomycetes in soil</b><br /><br />Biogeosciences, 10, 3605-3614, 2013<br /><br />Author(s): E. Schütze, A. Weist, M. Klose, T. Wach, M. Schumann, S. Nietzsche, D. Merten, J. Baumert, J. Majzlan, and E. Kothe<br /><br />Biomineralization by heavy metal-resistant streptomycetes was tested to evaluate the potential influence on metal mobilities in soil. Thus, we designed an experiment adopting conditions from classical laboratory methods to natural conditions prevailing in metal-rich soils with media spiked with heavy metals, soil agar, and nutrient-enriched or unamended soil incubated with the bacteria. As a result, all strains were able to form struvite minerals (MgNH₄PO₄&bull; 6H₂O) on tryptic soy broth (TSB)-media supplemented with AlCl₃, MnCl₂ and CuSO₄, as well as on soil agar. Some strains additionally formed struvite on nutrient-enriched contaminated and control soil, as well as on metal contaminated soil without addition of media components. In contrast, switzerite (Mn₃(PO₄)₂&bull; 7H₂O) was exclusively formed on minimal media spiked with MnCl₂ by four heavy metal-resistant strains, and on nutrient-enriched control soil by one strain. Hydrated nickel hydrogen phosphate was only crystallized on complex media supplemented with NiSO₄ by most strains. Thus, mineralization is a dominant property of streptomycetes, with different processes likely to occur under laboratory conditions and sub-natural to natural conditions. This new understanding might have implications for our understanding of biological metal resistance mechanisms. We assume that biogeochemical cycles, nutrient storage and metal resistance might be affected by formation and re-solubilization of minerals like struvite in soil at microscale.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3593/2013/<b>Impact of CO₂ and climate on Last Glacial maximum vegetation &ndash; a factor separation</b><br /><br />Biogeosciences, 10, 3593-3604, 2013<br /><br />Author(s): M. Claussen, K. Selent, V. Brovkin, T. Raddatz, and V. Gayler<br /><br />The factor separation of Stein and Alpert (1993) is applied to simulations with the MPI Earth system model to determine the factors which cause the differences between vegetation patterns in glacial and pre-industrial climate. The factors firstly include differences in the climate, caused by a strong increase in ice masses and the radiative effect of lower greenhouse gas concentrations; secondly, differences in the ecophysiological effect of lower glacial atmospheric CO₂ concentrations; and thirdly, the synergy between the pure climate effect and the pure effect of changing physiologically available CO₂. It is has been shown that the synergy can be interpreted as a measure of the sensitivity of ecophysiological CO₂ effect to climate. The pure climate effect mainly leads to a contraction or a shift in vegetation patterns when comparing simulated glacial and pre-industrial vegetation patterns. Raingreen shrubs benefit from the colder and drier climate. The pure ecophysiological effect of CO₂ appears to be stronger than the pure climate effect for many plant functional types – in line with previous simulations. The pure ecophysiological effect of lower CO₂ mainly yields a reduction in fractional coverage, a thinning of vegetation and a strong reduction in net primary production. The synergy appears to be as strong as each of the pure contributions locally, but weak on global average for most plant functional types. For tropical evergreen trees, however, the synergy is strong on global average. It diminishes the difference between glacial and pre-industrial coverage of tropical evergreen trees, due to the pure climate effect and the pure ecophysiological CO₂ effect, by approximately 50 per cent.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3559/2013/<b>Coupled physical/biogeochemical modeling including O₂-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela</b><br /><br />Biogeosciences, 10, 3559-3591, 2013<br /><br />Author(s): E. Gutknecht, I. Dadou, B. Le Vu, G. Cambon, J. Sudre, V. Garçon, E. Machu, T. Rixen, A. Kock, A. Flohr, A. Paulmier, and G. Lavik<br /><br />The Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of the global catches in the ocean. Often associated with Oxygen Minimum Zones (OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle. Important bioavailable N loss due to denitrification and anammox processes as well as greenhouse gas emissions (e.g, N₂O) occur also in these EBUS. However, their dynamics are currently crudely represented in global models. In the climate change context, improving our capability to properly represent these areas is crucial due to anticipated changes in the winds, productivity, and oxygen content. <br><br> We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated OMZs. We implemented this model in a 3-D realistic coupled physical/biogeochemical configuration in the Namibian upwelling system (northern Benguela) using the high-resolution hydrodynamic ROMS model. We present here a validation using in situ and satellite data as well as diagnostic metrics and sensitivity analyses of key parameters and N₂O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N₂O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate, and chlorophyll <i>a</i> concentrations, and the rates of microbial processes (e.g, NH₄⁺ and NO₂^&minus; oxidation, NO₃^&minus; reduction, and anammox) as well. Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking, and nitrification play a key role for the low-oxygen water content, N loss, and N₂O concentrations in the OMZ. Moreover, the explicit parameterization of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is necessary to improve the representation of microbial activity linked with the OMZ. The simulated minimum oxygen concentrations are driven by the poleward meridional advection of oxygen-depleted waters offshore of a 300 m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. <br><br> In the OMZ off Namibia, the magnitude of N₂O outgassing and of N loss is comparable. Anammox contributes to about 20% of total N loss, an estimate lower than currently assumed (up to 50%) for the global ocean.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3547/2013/<b>Tracing atmospheric nitrate in groundwater using triple oxygen isotopes: evaluation based on bottled drinking water</b><br /><br />Biogeosciences, 10, 3547-3558, 2013<br /><br />Author(s): F. Nakagawa, A. Suzuki, S. Daita, T. Ohyama, D. D. Komatsu, and U. Tsunogai<br /><br />The stable isotopic compositions of nitrate dissolved in 49 brands of bottled drinking water collected worldwide were measured, to trace the fate of atmospheric nitrate (NO_{3^&minus; atm}) that had been deposited into subaerial ecosystems, using the ¹⁷O anomalies (&Delta;¹⁷O) of nitrate as tracers. The use of bottled water enables collection of groundwater recharged at natural, background watersheds. The nitrate in groundwater had small &Delta;¹⁷O values ranging from −0.2&permil; to +4.5&permil; <i>n</i> = 49). The average &Delta;¹⁷O value and average mixing ratio of atmospheric nitrate to total nitrate in the groundwater samples were estimated to be 0.8&permil; and 3.1%, respectively. These findings indicated that the majority of atmospheric nitrate had undergone biological processing before being exported from the surface ecosystem to the groundwater. Moreover, the concentrations of atmospheric nitrate were estimated to range from less than 0.1 &mu;mol L^&minus;1 to 8.5 &mu;mol L^&minus;1 with higher NO_3^&minus;atm concentrations being obtained for those recharged in rocky, arid or elevated areas with little vegetation and lower NO_3^&minus;atm concentrations being obtained for those recharged in forested areas with high levels of vegetation. Additionally, many of the NO_3^&minus;atm-depleted samples were characterized by elevated &delta;¹⁵N values of more than +10&permil;. Uptake by plants and/or microbes in forested soils subsequent to deposition and the progress of denitrification within groundwater likely plays a significant role in the removal of NO_3^&minus;atm.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3535/2013/<b>Changes in polychaete standing stock and diversity on the northern side of Senghor Seamount (NE Atlantic)</b><br /><br />Biogeosciences, 10, 3535-3546, 2013<br /><br />Author(s): A. J. Chivers, B. E. Narayanaswamy, P. A. Lamont, A. Dale, and R. Turnewitsch<br /><br />Interest in seamount research has gathered momentum over the past five years in an effort to understand the physical, geochemical and biological characteristics as well as the interconnectedness of seamount ecosystems. The majority of biological seamount research has concentrated upon the rich and diverse suspension feeding organisms that dominate the megafauna, such as gorgonians and antipatharian corals; by comparison there have been few studies that have investigated the no less enigmatic, but possibly just as important infauna. To help fill this knowledge gap, the macrofaunal community was sampled from a total of five stations along a northerly transect (capturing water depths from &sim;130 m to &sim;3300 m), on Senghor Seamount (NE Atlantic). The focus of this study is on the polychaete communities. Polychaete abundance peaked at the summit and a mid-slope station (&sim;1500 m), a pattern mirrored by the biomass values. The polychaete community along the transect appeared to be particularly diverse, with 135 species nominally identified to putative species from a total of 954 individuals. A diversity maximum was identified on the upper slope at &sim;800 m depth, with species diversity, richness and evenness also all peaking at this station. Depth is likely to be a significant factor in determining levels of similarity between stations.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3525/2013/<b>Concentration and vertical flux of Fukushima-derived radiocesium in sinking particles from two sites in the Northwestern Pacific Ocean</b><br /><br />Biogeosciences, 10, 3525-3534, 2013<br /><br />Author(s): M. C. Honda, H. Kawakami, S. Watanabe, and T. Saino<br /><br />At two stations in the western North Pacific, K2 in the subarctic gyre and S1 in the subtropical gyre, time-series sediment traps were collecting sinking particles when the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident occurred on 11 March 2011. Radiocesium (¹³⁴Cs and ¹³⁷Cs) derived from the FNPP1 accident was detected in sinking particles collected at 500 m in late March 2011 and at 4810 m in early April 2011 at both stations. The sinking velocity of ¹³⁴Cs and ¹³⁷Cs was estimated to be 22 to 71 m day⁻¹ between the surface and 500 m and >180 m day⁻¹ between 500 m and 4810 m. ¹³⁷Cs concentrations varied from 0.14 to 0.25 Bq g⁻¹ dry weight. These values are higher than those of surface seawater, suspended particles, and zooplankton collected in April 2011. Although the radiocesium may have been adsorbed onto or incorporated into clay minerals, correlations between ¹³⁴Cs and lithogenic material were not always significant; therefore, the form of the cesium associated with the sinking particles is still an open question. The total ¹³⁷Cs inventory by late June at K2 and by late July at S1 was 0.5 to 1.7 Bq m⁻² at both depths. Compared with ¹³⁷Cs input from both stations by April 2011, estimated from the surface ¹³⁷Cs concentration and mixed-layer depth and by assuming that the observed ¹³⁷Cs flux was constant throughout the year, the estimated removal rate of ¹³⁷Cs from the upper layer (residence time in the upper layer) was 0.3 to 1.5% yr⁻¹ (68 to 312 yr). The estimated removal rates and residence times are comparable to previously reported values after the Chernobyl accident (removal rate: 0.2–1%, residence time: 130–390 yr).Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3507/2013/<b>Organic carbon and total nitrogen stocks in soils of the Lena River Delta</b><br /><br />Biogeosciences, 10, 3507-3524, 2013<br /><br />Author(s): S. Zubrzycki, L. Kutzbach, G. Grosse, A. Desyatkin, and E.-M. Pfeiffer<br /><br />The Lena River Delta, which is the largest delta in the Arctic, extends over an area of 32 000 km² and likely holds more than half of the entire soil organic carbon (SOC) mass stored in the seven major deltas in the northern permafrost regions. The geomorphic units of the Lena River Delta which were formed by true deltaic sedimentation processes are a Holocene river terrace and the active floodplains. Their mean SOC stocks for the upper 1 m of soils were estimated at 29 kg m^&minus;2 &pm; 10 kg m⁻² and at 14 kg m⁻² ± 7 kg m⁻², respectively. For the depth of 1 m, the total SOC pool of the Holocene river terrace was estimated at 121 Tg ± 43 Tg, and the SOC pool of the active floodplains was estimated at 120 Tg ± 66 Tg. The mass of SOC stored within the observed seasonally thawed active layer was estimated at about 127 Tg assuming an average maximum active layer depth of 50 cm. The SOC mass which is stored in the perennially frozen ground at the increment 50–100 cm soil depth, which is currently excluded from intense biogeochemical exchange with the atmosphere, was estimated at 113 Tg. The mean nitrogen (N) stocks for the upper 1 m of soils were estimated at 1.2 kg m⁻² ± 0.4 kg m⁻² for the Holocene river terrace and at 0.9 kg m⁻² ± 0.4 kg m^&minus;2 for the active floodplain levels, respectively. For the depth of 1 m, the total N pool of the river terrace was estimated at 4.8 Tg ± 1.5 Tg, and the total N pool of the floodplains was estimated at 7.7 Tg ± 3.6 Tg. Considering the projections for deepening of the seasonally thawed active layer up to 120 cm in the Lena River Delta region within the 21st century, these large carbon and nitrogen stocks could become increasingly available for decomposition and mineralization processes.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3493/2013/<b>External forcings, oceanographic processes and particle flux dynamics in Cap de Creus submarine canyon, NW Mediterranean Sea</b><br /><br />Biogeosciences, 10, 3493-3505, 2013<br /><br />Author(s): A. Rumín-Caparrós, A. Sanchez-Vidal, A. Calafat, M. Canals, J. Martín, P. Puig, and R. Pedrosa-Pàmies<br /><br />Particle fluxes (including major components and grain size), and oceanographic parameters (near-bottom water temperature, current speed and suspended sediment concentration) were measured along the Cap de Creus submarine canyon in the Gulf of Lions (GoL; NW Mediterranean Sea) during two consecutive winter-spring periods (2009–2010 and 2010–2011). The comparison of data obtained with the measurements of meteorological and hydrological parameters (wind speed, turbulent heat flux, river discharge) have shown the important role of atmospheric forcings in transporting particulate matter through the submarine canyon and towards the deep sea. Indeed, atmospheric forcing during 2009–2010 and 2010–2011 winter months showed differences in both intensity and persistence that led to distinct oceanographic responses. Persistent dry northern winds caused strong heat losses (14.2 × 10³ W m^&minus;2) in winter 2009–2010 that triggered a pronounced sea surface cooling compared to winter 2010–2011 (1.6 × 10³ W m⁻² lower). As a consequence, a large volume of dense shelf water formed in winter 2009–2010, which cascaded at high speed (up to &sim;1 m s^&minus;1) down Cap de Creus Canyon as measured by a current-meter in the head of the canyon. The lower heat losses recorded in winter 2010–2011, together with an increased river discharge, resulted in lowered density waters over the shelf, thus preventing the formation and downslope transport of dense shelf water. High total mass fluxes (up to 84.9 g m⁻² d^&minus;1) recorded in winter-spring 2009–2010 indicate that dense shelf water cascading resuspended and transported sediments at least down to the middle canyon. Sediment fluxes were lower (28.9 g m⁻² d^&minus;1) under the quieter conditions of winter 2010–2011. The dominance of the lithogenic fraction in mass fluxes during the two winter-spring periods points to a resuspension origin for most of the particles transported down canyon. The variability in organic matter and opal contents relates to seasonally controlled inputs associated with the plankton spring bloom during March and April of both years.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3479/2013/<b>Interannual variation in the epibenthic megafauna at the shallowest station of the HAUSGARTEN observatory (79° N, 6° E)</b><br /><br />Biogeosciences, 10, 3479-3492, 2013<br /><br />Author(s): K. S. Meyer, M. Bergmann, and T. Soltwedel<br /><br />Epibenthic megafauna play an important role in the deep-sea environment and contribute significantly to benthic biomass, but their population dynamics are still understudied. We used a towed deep-sea camera system to assess the population densities of epibenthic megafauna in 2002, 2007, and 2012 at the shallowest station (HG I, &sim;1300 m) of the deep-sea observatory HAUSGARTEN, in the eastern Fram Strait. Our results indicate that the overall density of megafauna was significantly lower in 2007 than in 2002, but was significantly higher in 2012, resulting in overall greater megafaunal density in 2012. Different species showed different patterns in population density, but the relative proportions of predator/scavengers and suspension-feeding individuals were both higher in 2012. Variations in megafaunal densities and proportions are likely due to variation in food input to the sea floor, which decreased slightly in the years preceding 2007 and was greatly elevated in the years preceding 2012. Both average evenness and diversity increased over the time period studied, which indicates that HG I may be food-limited and subject to bottom-up control. The community of HG I may be unique in its response to elevated food input, which resulted in higher evenness and diversity in 2012.Mon, 03 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3465/2013/<b>The elemental composition of purple sea urchin (<i>Strongylocentrotus purpuratus</i>) calcite and potential effects of <i>p</i>CO₂ during early life stages</b><br /><br />Biogeosciences, 10, 3465-3477, 2013<br /><br />Author(s): M. LaVigne, T. M. Hill, E. Sanford, B. Gaylord, A. D. Russell, E. A. Lenz, J. D. Hosfelt, and M. K. Young<br /><br />Ocean acidification will likely have negative impacts on invertebrates producing skeletons composed of calcium carbonate. Skeletal solubility is partly controlled by the incorporation of "foreign" ions (e.g. magnesium) into the crystal lattice of these skeletal structures, a process that is sensitive to a variety of biological and environmental factors. Here we explore effects of life stage, oceanographic region of origin, and changes in the partial pressure of carbon dioxide in seawater (<i>p</i>CO₂) on trace elemental composition in the purple sea urchin (<i>Strongylocentrotus purpuratus</i>). We show that, similar to other urchin taxa, adult purple sea urchins have the ability to precipitate skeleton composed of a range of biominerals spanning low- to high-Mg calcites. Mg / Ca and Sr / Ca ratios were substantially lower in adult spines compared to adult tests. On the other hand, trace elemental composition was invariant among adults collected from four oceanographically distinct regions spanning a range of carbonate chemistry conditions (Oregon, Northern California, Central California, and Southern California). Skeletons of newly settled juvenile urchins that originated from adults from the four regions exhibited intermediate Mg / Ca and Sr / Ca between adult spine and test endmembers, indicating that skeleton precipitated during early life stages is more soluble than adult spines and less soluble than adult tests. Mean skeletal Mg / Ca or Sr / Ca of juvenile skeleton did not vary with source region when larvae were reared under present-day, global-average seawater carbonate conditions (400 μatm; pH_T = 8.02 ± 0.03 1 SD; &Omega;_calcite = 3.3 ± 0.2 1 SD). However, when reared under elevated <i>p</i>CO₂ (900 μatm; pH_T = 7.73 ± 0.03; &Omega;_calcite = 1.8 ± 0.1), skeletal Sr / Ca in juveniles exhibited increased variance across the four regions. Although larvae from the northern populations (Oregon, Northern California, Central California) did not exhibit differences in Mg or Sr incorporation under elevated <i>p</i>CO₂ (Sr / Ca = 2.10 ± 0.06 mmol mol⁻¹; Mg / Ca = 67.4 ± 3.9 mmol mol⁻¹), juveniles of Southern California origin partitioned ~8% more Sr into their skeletons when exposed to higher <i>p</i>CO₂ (Sr / Ca = 2.26 ± 0.08 vs. 2.09 ± 0.005 mmol mol⁻¹ 1 SD). Together these results suggest that the diversity of carbonate minerologies present across different skeletal structures and life stages in purple sea urchins does not translate into an equivalent geochemical plasticity of response associated with geographic variation or temporal shifts in seawater properties. Rather, composition of <i>S. purpuratus</i> skeleton precipitated during both early and adult life history stages appears relatively robust to spatial gradients and predicted future changes in carbonate chemistry. An exception to this trend may arise during early life stages, where certain populations of purple sea urchins may alter skeletal mineral precipitation rates and composition beyond a given <i>p</i>CO₂ threshold. This potential for geochemical plasticity during early development in contrast to adult stage geochemical resilience adds to the growing body of evidence that ocean acidification can have differing effects across organismal life stages.Sat, 01 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3455/2013/<b>Low vertical transfer rates of carbon inferred from radiocarbon analysis in an Amazon Podzol</b><br /><br />Biogeosciences, 10, 3455-3464, 2013<br /><br />Author(s): C. A. Sierra, E. M. Jiménez, B. Reu, M. C. Peñuela, A. Thuille, and C. A. Quesada<br /><br />Hydromorphic Podzol soils in the Amazon Basin generally support low-stature forests with some of the lowest amounts of aboveground net primary production (NPP) in the region. However, they can also exhibit large values of belowground NPP that can contribute significantly to the total annual inputs of organic matter into the soil. These hydromorphic Podzol soils also exhibit a horizon rich in organic matter at around 1–2 m depth, presumably as a result of eluviation of dissolved organic matter and sesquioxides of Fe and Al. Therefore, it is likely that these ecosystems store large quantities of carbon by (1) large amounts of C inputs to soils dominated by their high levels of fine-root production, (2) stabilization of organic matter in an illuviation horizon due to significant vertical transfers of C. To assess these ideas we studied soil carbon dynamics using radiocarbon in two adjacent Amazon forests growing on contrasting soils: a hydromorphic Podzol and a well-drained Alisol supporting a high-stature <i>terra firme</i> forest. Our measurements showed similar concentrations of C and radiocarbon in the litter layer and the first 5 cm of the mineral soil for both sites. This result is consistent with the idea that the hydromorphic Podzol soil has similar soil C storage and cycling rates compared to the well-drained Alisol that supports a more opulent vegetation. However, we found important differences in carbon dynamics and transfers along the vertical profile. At both soils, we found similar radiocarbon concentrations in the subsoil, but the carbon released after incubating soil samples presented radiocarbon concentrations of recent origin in the Alisol, but not in the Podzol. There were no indications of incorporation of C fixed after 1950 in the illuvial horizon of the Podzol. With the aid of a simulation model, we predicted that only a minor fraction (1.7%) of the labile carbon decomposed in the topsoil is transferred to the subsoil of the Podzol, while this proportional transfer is about 30% in the Alisol. Furthermore, our estimates were 8 times lower than previous estimations of vertical C transfers in Amazon Podzols, and question the validity of these previous estimations for all Podzols within the Amazon Basin. Our results also challenge our previous ideas about the genesis of these particular soils and suggest that either they are not true Podzols or the podzolization processes had already stopped.Sat, 01 Jun 2013 00:00:00 +0200http://www.biogeosciences.net/10/3445/2013/<b>Photosynthetic parameters in the Beaufort Sea in relation to the phytoplankton community structure</b><br /><br />Biogeosciences, 10, 3445-3454, 2013<br /><br />Author(s): Y. Huot, M. Babin, and F. Bruyant<br /><br />To model phytoplankton primary production from remotely sensed data, a method to estimate photosynthetic parameters describing the photosynthetic rates per unit biomass is required. Variability in these parameters must be related to environmental variables that are measurable remotely. In the Arctic, a limited number of measurements of photosynthetic parameters have been carried out with the concurrent environmental variables needed. Such measurements and their relationship to environmental variables will be required to improve the accuracy of remotely sensed estimates of phytoplankton primary production and our ability to predict future changes. During the MALINA cruise, a large dataset of these parameters was obtained. Together with previously published datasets, we use environmental and trophic variables to provide functional relationships for these parameters. In particular, we describe several specific aspects: the maximum rate of photosynthesis (<i>P</i>_max^chl) normalized to chlorophyll decreases with depth and is higher for communities composed of large cells; the saturation parameter (<i>E</i>_k) decreases with depth but is independent of the community structure; and the initial slope of the photosynthesis versus irradiance curve (<i>&alpha;</i>^chl) normalized to chlorophyll is independent of depth but is higher for communities composed of larger cells. The photosynthetic parameters were not influenced by temperature over the range encountered during the cruise (−2 to 8 &deg;C).Wed, 29 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3421/2013/<b>Coral Patch seamount (NE Atlantic) &ndash; a sedimentological and megafaunal reconnaissance based on video and hydroacoustic surveys</b><br /><br />Biogeosciences, 10, 3421-3443, 2013<br /><br />Author(s): C. Wienberg, P. Wintersteller, L. Beuck, and D. Hebbeln<br /><br />The present study provides new knowledge about the so far largely unexplored Coral Patch seamount which is located in the NE Atlantic Ocean half-way between the Iberian Peninsula and Madeira. For the first time a detailed hydroacoustic mapping (MBES) in conjunction with video surveys (ROV, camera sled) were performed to describe the sedimentological and biological characteristics of this sub-elliptical ENE-WSW elongated seamount. Video observations were restricted to the southwestern summit area of Coral Patch seamount (water depth: 560–760 m) and revealed that this part of the summit is dominated by exposed hard substrate, whereas soft sediment is just a minor substrate component. Although exposed hardgrounds are dominant for this summit area and, thus, offer suitable habitat for settlement by benthic organisms, the benthic megafauna shows rather scarce occurrence. In particular, scleractinian framework-building cold-water corals are apparently rare with very few isolated and small-sized live occurrences of the species <i>Lophelia pertusa</i> and <i>Madrepora oculata</i>. In contrast, dead coral framework and coral rubble are more frequent pointing to a higher abundance of cold-water corals on Coral Patch during the recent past. This is even supported by the observation of fishing lines that got entangled with rather fresh-looking coral frameworks. Overall, long lines and various species of commercially important fish were frequently observed emphasising the potential of Coral Patch as an important target for fisheries that may have impacted the entire benthic community. Hydroacoustic seabed classification covered the entire summit of Coral Patch and its northern and southern flanks (water depth: 560–2660 m) and revealed extended areas dominated by mixed and soft sediments at the northern flank and to a minor degree at its easternmost summit and southern flank. Nevertheless, these data also predict most of the summit area to be dominated by exposed bedrock which would offer suitable habitat for benthic organisms. By comparing the locally restricted video observations and the broad-scale monitoring of a much larger and deeper seafloor area as derived by hydroacoustic seabed classification, it becomes obvious that habitat information obtained by in situ sampling may provide a rather scattered pattern about the entire seamount ecosystem. Solely with a combination of both methods, a satisfactory approach to describe the diverse characteristics of a seamount ecosystem can be derived which is in turn indispensable for future scientific monitoring campaigns as well as management and conservation purposes.Mon, 27 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3405/2013/<b>Bioavailability of sinking organic matter in the Blanes canyon and the adjacent open slope (NW Mediterranean Sea)</b><br /><br />Biogeosciences, 10, 3405-3420, 2013<br /><br />Author(s): P. Lopez-Fernandez, S. Bianchelli, A. Pusceddu, A. Calafat, A. Sanchez-Vidal, and R. Danovaro<br /><br />Submarine canyons are sites of intense energy and material exchange between the shelf and the deep adjacent basins. To test the hypothesis that active submarine canyons represent preferential conduits of available food for the deep-sea benthos, two mooring lines were deployed at 1200 m depth from November 2008 to November 2009 inside the Blanes canyon and on the adjacent open slope (Catalan Margin, NW Mediterranean Sea). We investigated the fluxes, biochemical composition and food quality of sinking organic carbon (OC). OC fluxes in the canyon and the open slope varied among sampling periods, though not consistently in the two sites. In particular, while in the open slope the highest OC fluxes were observed in August 2009, in the canyon the highest OC fluxes occurred in April–May 2009. For almost the entire study period, the OC fluxes in the canyon were significantly higher than those in the open slope, whereas OC contents of sinking particles collected in the open slope were consistently higher than those in the canyon. This result confirms that submarine canyons are effective conveyors of OC to the deep sea. Particles transferred to the deep sea floor through the canyons are predominantly of inorganic origin, significantly higher than that reaching the open slope at a similar water depth. Using multivariate statistical tests, two major clusters of sampling periods were identified: one in the canyon that grouped trap samples collected in December 2008, concurrently with the occurrence of a major storm at the sea surface, and associated with increased fluxes of nutritionally available particles from the upper shelf. Another cluster grouped samples from both the canyon and the open slope collected in March 2009, concurrently with the occurrence of the seasonal phytoplankton bloom at the sea surface, and associated with increased fluxes of total phytopigments. Our results confirm the key ecological role of submarine canyons for the functioning of deep-sea ecosystems, and highlight the importance of canyons in linking episodic storms and primary production occurring at the sea surface to the deep sea floor.Thu, 23 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3391/2013/<b>High-resolution measurements of atmospheric molecular hydrogen and its isotopic composition at the West African coast of Mauritania</b><br /><br />Biogeosciences, 10, 3391-3403, 2013<br /><br />Author(s): S. Walter, A. Kock, and T. Röckmann<br /><br />Oceans are a net source of molecular hydrogen (H₂) to the atmosphere, where nitrogen (N₂) fixation is assumed to be the main biological production pathway followed by photochemical production from organic material. The sources can be distinguished using isotope measurements because of clearly differing isotopic signatures of the produced hydrogen. <br><br> Here we present the first ship-borne measurements of atmospheric molecular H₂ mixing ratio and isotopic composition at the West African coast of Mauritania (16–25° W, 17–24° N). This area is one of the biologically most active regions of the world's oceans with seasonal upwelling events and characterized by strongly differing hydrographical/biological properties and phytoplankton community structures. The aim of this study was to identify areas of H₂ production and distinguish H₂ sources by isotopic signatures of atmospheric H₂. For this more than 100 air samples were taken during two cruises in February 2007 and 2008. During both cruises a transect from the Cape Verde Islands towards the Mauritanian Coast was sampled to cover differing oceanic regions such as upwelling and oligotrophic regimes. In 2007, additionally, four days were sampled at high resolution of one sample per hour to investigate a possible diurnal cycle of atmospheric H₂. <br><br> Our results indicate the influence of local sources and suggest the Banc d'Arguin as a pool for precursors for photochemical H₂ production, whereas oceanic N₂ fixation could not be identified as a source for atmospheric H₂ during these two cruises. The variability in diurnal cycles is probably influenced by released precursors for photochemical H₂ production and also affected by a varying origin of air masses. This means for future investigations that only measuring the mixing ratio of H₂ is insufficient to explain the variability of an atmospheric diurnal cycle and support is needed, e.g. by isotopic measurements. Nevertheless, measurements of atmospheric H₂ mixing ratios, which are easy to conduct online during ship cruises, could be a useful indicator of production areas of biological precursors such as volatile organic compounds (VOCs) for further investigations.Thu, 23 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3375/2013/<b>Pathway of CH₄ production, fraction of CH₄ oxidized, and ¹³C isotope fractionation in a straw-incorporated rice field</b><br /><br />Biogeosciences, 10, 3375-3389, 2013<br /><br />Author(s): G. B. Zhang, Y. Ji, J. Ma, G. Liu, H. Xu, and K. Yagi<br /><br />Straw incorporation generally increases CH₄ emission from rice fields, but its effects on the mechanism of CH₄ emission, especially on the pathway of CH₄ production and the fraction of CH₄ oxidized, are not well known. To investigate the methanogenic pathway, the fraction of CH₄ oxidized as well as the stable carbon isotope fractionation during the oxidation and transport of CH₄ as affected by straw incorporation, observations were conducted of production and oxidation of CH₄ in paddy soil and rice roots and &delta;¹³C-values of produced CH₄ and CO₂, and emitted CH₄ in incubation and field experiments. Straw incorporation significantly enhanced CH₄ production potentials of the paddy soil and rice roots. However, it increased the relative contribution of acetate to total CH₄ production (<i>F</i>_ac) in the paddy soil by &sim;10–30%, but decreased <i>F</i>_ac-value of the rice roots by &sim;5–20%. Compared with rice roots, paddy soil was more important in acetoclastic methanogenesis, with <i>F</i>_ac-value being 6–30% higher. Straw incorporation highly decreased the fraction of CH₄ oxidized (<i>F</i>_ox) by 41–71%, probably attributed to the fact that it increased CH₄ oxidation potential whereas CH₄ production potential was increased to a larger extent. There was little CH₄ formed during aerobic incubation, and the produced CH₄ was more ¹³C-enriched relative to that of anaerobic incubation. Assuming &delta;¹³C-values of CH₄ aerobically produced in paddy soil to be the &delta;¹³C-values of residual CH₄ after being oxidized, (<i>F</i>_ox-value still appeared to be 45–68% lower when straw was incorporated. Oxidation fractionation factor (&alpha;_ox) was higher with straw incorporation (1.033) than without straw incorporation (1.025). The &delta;¹³C-values of CH₄ emitted after cutting of the plants (&minus;50 to &minus;43&permil;) were more positive than those of before (&minus;58 to &minus;55&permil;), suggesting a transport fractionation factor (&varepsilon;_transport) was &minus;8.0&permil; with straw incorporation and &minus;12.0&permil; without straw incorporation. Causes of this difference may be related to the diffusion process in transport as affected by growth of rice plants and pressure in the rhizosphere. The experiment shows that straw incorporation increases the contribution of acetate to total methanogenesis in paddy soil but decreases it on rice roots, and it significantly decreases the fraction of CH₄ oxidized in the field and expands oxidation fractionation while reducing transport fractionation.Wed, 22 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3359/2013/<b>Density and distribution of megafauna at the Håkon Mosby mud volcano (the Barents Sea) based on image analysis</b><br /><br />Biogeosciences, 10, 3359-3374, 2013<br /><br />Author(s): E. Rybakova (Goroslavskaya), S. Galkin, M. Bergmann, T. Soltwedel, and A. Gebruk<br /><br />During a survey of the Håkon Mosby mud volcano (HMMV), located on the Bear Island fan in the southwest Barents Sea at &sim;1250 m water depth, different habitats inside the volcano caldera and outside it were photographed using a towed camera platform, an Ocean Floor Observation System (OFOS). Three transects were performed across the caldera and one outside, in the background area, each transect was &sim;2 km in length. We compared the density, taxa richness and diversity of nonsymbiotrophic megafauna in areas inside the volcano caldera with different bacterial mat and pogonophoran tubeworm cover. Significant variations in megafaunal composition, density and distribution were found between considered areas. Total megafaunal density was highest in areas of dense pogonophoran populations (mean 52.9 ind. m^&minus;2) followed by areas of plain light-coloured sediment that were devoid of bacterial mats and tube worms (mean 37.7 ind. m^&minus;2). The lowest densities were recorded in areas of dense bacterial mats (mean &le;1.4 ind. m^&minus;2). Five taxa contributed to most of the observed variation: the ophiuroid <i>Ophiocten gracilis</i>, lysianassid amphipods, the pycnogonid <i>Nymphon macronix</i>, the caprellid <i>Metacaprella horrida</i> and the fish <i>Lycodes squamiventer</i>. In agreement with previous studies, three zones within the HMMV caldera were distinguished, based on different habitats and megafaunal composition: "bacterial mats", "pogonophoran fields" and "plain light-coloured sediments". The zones were arranged almost concentrically around the central part of the caldera that was devoid of visible megafauna. The total number of taxa showed little variation inside (24 spp.) and outside the caldera (26 spp.). The density, diversity and composition of megafauna varied substantially between plain light-coloured sediment areas inside the caldera and the HMMV background. Megafaunal density was lower in the background (mean 25.3 ind. m^&minus;2) compared to areas of plain light-coloured sediments inside the caldera. So the effect of the mud-volcano environment on benthic communities is expressed in increasing of biomass, changing of taxa composition and proportions of most taxonomic groups.Tue, 21 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3349/2013/<b>Intrusion of coastal waters into the pelagic eastern Mediterranean: in situ and satellite-based characterization</b><br /><br />Biogeosciences, 10, 3349-3357, 2013<br /><br />Author(s): S. Efrati, Y. Lehahn, E. Rahav, N. Kress, B. Herut, I. Gertman, R. Goldman, T. Ozer, M. Lazar, and E. Heifetz<br /><br />A combined dataset of near-real-time multi-satellite observations and in situ measurements from a high-resolution survey is used for characterizing physical-biogeochemical properties of a patch stretching from the coast to the open sea in the Levantine Basin (LB) of the eastern Mediterranean (EM). Spatial analysis of the combined dataset indicates that the patch is a semi-enclosed system, bounded within the mixed layer and separated from ambient waters by transport barriers induced by horizontal stirring. As such, the patch is characterized by physical-biogeochemical properties that significantly differ from those of the waters surrounding it, with lower salinity and higher temperatures, concentrations of silicic acid and chlorophyll <i>a</i>, and abundance of <i>Synechococcus</i> and picoeukaryote cells. Based on estimates of patch dimensions (&sim;40 km width and &sim;25 m depth) and propagation speed (&sim;0.09 m s⁻¹), the volume flux associated with the patch is found to be on the order of 0.1 Sv. Our observations suggest that horizontal stirring by surface currents is likely to have an important impact on the ultra-oligotrophic Levantine Basin ecosystem, through (1) transport of nutrients and coastally derived material, and (2) formation of local, dynamically isolated niches. In addition, this work provides a satellite-based framework for planning and executing high-resolution sampling strategies in the interface between the coast and the open sea.Tue, 21 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3341/2013/<b>Phosphorus sorption and buffering mechanisms in suspended sediments from the Yangtze Estuary and Hangzhou Bay, China</b><br /><br />Biogeosciences, 10, 3341-3348, 2013<br /><br />Author(s): M. Li, M. J. Whelan, G. Q. Wang, and S. M. White<br /><br />The adsorption isotherm and the mechanism of the buffering effect are important controls on phosphorus (P) behaviors in estuaries and are important for estimating phosphate concentrations in aquatic environments. In this paper, we derive phosphate adsorption isotherms in order to investigate sediment adsorption and buffering capacity for phosphorus discharged from sewage outfalls in the Yangtze Estuary and Hangzhou Bay near Shanghai, China. Experiments were also carried out at different temperatures in order to explore the buffering effects for phosphate. The results show that P sorption in sediments with low fine particle fractions was best described using exponential equations. Some P interactions between water and sediment may be caused by the precipitation of CaHPO₄ from Ca^2&plus; and HPO₄^2&minus; when the phosphate concentration in the liquid phase is high. Results from the buffering experiments suggest that the Zero Equilibrium Phosphate Concentrations (EPC₀) vary from 0.014 mg L⁻¹ to 0.061 mg L^&minus;1, which are consistent with measured phosphate concentrations in water samples collected at the same time as sediment sampling. Values of EPC₀ and linear sorption coefficients (<i>K</i>) in sediments with high fine particle and organic matter contents are relatively high, which implies that they have high buffering capacity. Both EPC₀ and <i>K</i> increase with increasing temperature, indicating a higher P buffering capacity at high temperatures.Fri, 17 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3313/2013/<b>A comprehensive benchmarking system for evaluating global vegetation models</b><br /><br />Biogeosciences, 10, 3313-3340, 2013<br /><br />Author(s): D. I. Kelley, I. C. Prentice, S. P. Harrison, H. Wang, M. Simard, J. B. Fisher, and K. O. Willis<br /><br />We present a benchmark system for global vegetation models. This system provides a quantitative evaluation of multiple simulated vegetation properties, including primary production; seasonal net ecosystem production; vegetation cover; composition and height; fire regime; and runoff. The benchmarks are derived from remotely sensed gridded datasets and site-based observations. The datasets allow comparisons of annual average conditions and seasonal and inter-annual variability, and they allow the impact of spatial and temporal biases in means and variability to be assessed separately. Specifically designed metrics quantify model performance for each process, and are compared to scores based on the temporal or spatial mean value of the observations and a "random" model produced by bootstrap resampling of the observations. The benchmark system is applied to three models: a simple light-use efficiency and water-balance model (the Simple Diagnostic Biosphere Model: SDBM), the Lund-Potsdam-Jena (LPJ) and Land Processes and eXchanges (LPX) dynamic global vegetation models (DGVMs). In general, the SDBM performs better than either of the DGVMs. It reproduces independent measurements of net primary production (NPP) but underestimates the amplitude of the observed CO₂ seasonal cycle. The two DGVMs show little difference for most benchmarks (including the inter-annual variability in the growth rate and seasonal cycle of atmospheric CO₂), but LPX represents burnt fraction demonstrably more accurately. Benchmarking also identified several weaknesses common to both DGVMs. The benchmarking system provides a quantitative approach for evaluating how adequately processes are represented in a model, identifying errors and biases, tracking improvements in performance through model development, and discriminating among models. Adoption of such a system would do much to improve confidence in terrestrial model predictions of climate change impacts and feedbacks.Fri, 17 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3297/2013/<b>Sulphur compounds, methane, and phytoplankton: interactions along a north–south transit in the western Pacific Ocean</b><br /><br />Biogeosciences, 10, 3297-3311, 2013<br /><br />Author(s): C. Zindler, A. Bracher, C. A. Marandino, B. Taylor, E. Torrecilla, A. Kock, and H. W. Bange<br /><br />Here we present results of the first comprehensive study of sulphur compounds and methane in the oligotrophic tropical western Pacific Ocean. The concentrations of dimethylsuphide (DMS), dimethylsulphoniopropionate (DMSP), dimethylsulphoxide (DMSO), and methane (CH₄), as well as various phytoplankton marker pigments in the surface ocean were measured along a north–south transit from Japan to Australia in October 2009. DMS (0.9 nmol L⁻¹), dissolved DMSP (DMSP_d, 1.6 nmol L⁻¹) and particulate DMSP (DMSP_p, 2 nmol L⁻¹) concentrations were generally low, while dissolved DMSO (DMSO_d, 4.4 nmol L⁻¹) and particulate DMSO (DMSO_p, 11.5 nmol L⁻¹) concentrations were comparably enhanced. Positive correlations were found between DMSO and DMSP as well as DMSP and DMSO with chlorophyll <i>a</i>, which suggests a similar source for both compounds. Similar phytoplankton groups were identified as being important for the DMSO and DMSP pool, thus, the same algae taxa might produce both DMSP and DMSO. In contrast, phytoplankton seemed to play only a minor role for the DMS distribution in the western Pacific Ocean. The observed DMSP_p : DMSO_p ratios were very low and seem to be characteristic of oligotrophic tropical waters representing the extreme endpoint of the global DMSP_p : DMSO_p ratio vs SST relationship. It is most likely that nutrient limitation and oxidative stress in the tropical western Pacific Ocean triggered enhanced DMSO production leading to an accumulation of DMSO in the sea surface. Positive correlations between DMSP_d and CH₄, as well as between DMSO (particulate and total) and CH₄, were found along the transit. We conclude that DMSP and DMSO and/or their degradation products might serve as potential substrates for CH₄ production in the oxic surface layer of the western Pacific Ocean.Thu, 16 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3285/2013/<b>Effect of CO₂ enrichment on bacterial metabolism in an Arctic fjord</b><br /><br />Biogeosciences, 10, 3285-3296, 2013<br /><br />Author(s): C. Motegi, T. Tanaka, J. Piontek, C. P. D. Brussaard, J.-P. Gattuso, and M. G. Weinbauer<br /><br />The anthropogenic increase of carbon dioxide (CO₂) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO₂ (<i>p</i>CO₂). Although bacteria play a major role in carbon cycling, little is known about the impact of rising <i>p</i>CO₂ on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising <i>p</i>CO₂ on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with <i>p</i>CO₂ levels ranging from ca. 180 to 1400 μatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient addition. Over the wide range of <i>p</i>CO₂ investigated, the patterns in BP and growth rate of bulk and free-living communities were generally similar over time. However, BP of the bulk community significantly decreased with increasing <i>p</i>CO₂ after nutrient addition (day 14). In addition, increasing <i>p</i>CO₂ enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that <i>p</i>CO₂ may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including <i>p</i>CO₂, explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast to BP, no clear trend and effect of changes of <i>p</i>CO₂ was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in <i>p</i>CO₂ potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO₂.Wed, 15 May 2013 00:00:00 +0200http://www.biogeosciences.net/10/3269/2013/<b>Limitations of microbial hydrocarbon degradation at the Amon mud volcano (Nile deep-sea fan)</b><br /><br />Biogeosciences, 10, 3269-3283, 2013<br /><br />Author(s): J. Felden, A. Lichtschlag, F. Wenzhöfer, D. de Beer, T. Feseker, P. Pop Ristova, G. de Lange, and A. Boetius<br /><br />The Amon mud volcano (MV), located at 1250 m water depth on the Nile deep-sea fan, is known for its active emission of methane and non-methane hydrocarbons into the hydrosphere. Previous investigations showed a low efficiency of hydrocarbon-degrading anaerobic microbial communities inhabiting the Amon MV center in the presence of sulfate and hydrocarbons in the seeping subsurface fluids. By comparing spatial and temporal patterns of in situ biogeochemical fluxes, temperature gradients, pore water composition, and microbial activities over 3 yr, we investigated why the activity of anaerobic hydrocarbon degraders can be low despite high energy supplies. We found that the central dome of the Amon MV, as well as a lateral mud flow at its base, showed signs of recent exposure of hot subsurface muds lacking active hydrocarbon degrading communities. In these highly disturbed areas, anaerobic degradation of methane was less than 2% of the methane flux. Rather high oxygen consumption rates compared to low sulfide production suggest a faster development of more rapidly growing aerobic hydrocarbon degraders in highly disturbed areas. In contrast, the more stabilized muds surrounding the central gas and fluid conduits hosted active anaerobic hydrocarbon-degrading microbial communities. The low microbial activity in the hydrocarbon-vented areas of Amon MV is thus a consequence of kinetic limitations by heat and mud expulsion, whereas most of the outer MV area is limited by hydrocarbon transport.Tue, 14 May 2013 00:00:00 +0200