BG - Latest Articles

Phosphorus sorption and buffering mechanisms in suspended sediments from the Yangtze Estuary and Hangzhou Bay, China

2013-05-17T00:00:00+02:00

Phosphorus sorption and buffering mechanisms in suspended sediments from the Yangtze Estuary and Hangzhou Bay, China

Biogeosciences, 10, 3341-3348, 2013

Author(s): M. Li, M. J. Whelan, G. Q. Wang, and S. M. White

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 Ca²⁺ and HPO₄²⁻ 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⁻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 (K) in sediments with high fine particle and organic matter contents are relatively high, which implies that they have high buffering capacity. Both EPC₀ and K increase with increasing temperature, indicating a higher P buffering capacity at high temperatures.

A comprehensive benchmarking system for evaluating global vegetation models

2013-05-17T00:00:00+02:00

A comprehensive benchmarking system for evaluating global vegetation models

Biogeosciences, 10, 3313-3340, 2013

Author(s): D. I. Kelley, I. C. Prentice, S. P. Harrison, H. Wang, M. Simard, J. B. Fisher, and K. O. Willis

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.

Sulphur compounds, methane, and phytoplankton: interactions along a north–south transit in the western Pacific Ocean

2013-05-16T00:00:00+02:00

Sulphur compounds, methane, and phytoplankton: interactions along a north–south transit in the western Pacific Ocean

Biogeosciences, 10, 3297-3311, 2013

Author(s): C. Zindler, A. Bracher, C. A. Marandino, B. Taylor, E. Torrecilla, A. Kock, and H. W. Bange

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 a, 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.

Effect of CO₂ enrichment on bacterial metabolism in an Arctic fjord

2013-05-15T00:00:00+02:00

Effect of CO₂ enrichment on bacterial metabolism in an Arctic fjord

Biogeosciences, 10, 3285-3296, 2013

Author(s): C. Motegi, T. Tanaka, J. Piontek, C. P. D. Brussaard, J.-P. Gattuso, and M. G. Weinbauer

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 (pCO₂). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO₂ on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO₂ on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with pCO₂ 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 pCO₂ 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 pCO₂ after nutrient addition (day 14). In addition, increasing pCO₂ enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO₂ may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO₂, 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 pCO₂ was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO₂ potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO₂.

Limitations of microbial hydrocarbon degradation at the Amon mud volcano (Nile deep-sea fan)

2013-05-14T00:00:00+02:00

Limitations of microbial hydrocarbon degradation at the Amon mud volcano (Nile deep-sea fan)

Biogeosciences, 10, 3269-3283, 2013

Author(s): J. Felden, A. Lichtschlag, F. Wenzhöfer, D. de Beer, T. Feseker, P. Pop Ristova, G. de Lange, and A. Boetius

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.

An overview of chemosynthetic symbioses in bivalves from the North Atlantic and Mediterranean Sea

2013-05-14T00:00:00+02:00

An overview of chemosynthetic symbioses in bivalves from the North Atlantic and Mediterranean Sea

Biogeosciences, 10, 3241-3267, 2013

Author(s): S. Duperron, S. M. Gaudron, C. F. Rodrigues, M. R. Cunha, C. Decker, and K. Olu

Deep-sea bivalves found at hydrothermal vents, cold seeps and organic falls are sustained by chemosynthetic bacteria that ensure part or all of their carbon nutrition. These symbioses are of prime importance for the functioning of the ecosystems. Similar symbioses occur in other bivalve species living in shallow and coastal reduced habitats worldwide. In recent years, several deep-sea species have been investigated from continental margins around Europe, West Africa, eastern Americas, the Gulf of Mexico, and from hydrothermal vents on the Mid-Atlantic Ridge. In parallel, numerous, more easily accessible shallow marine species have been studied. Herein we provide a summary of the current knowledge available on chemosymbiotic bivalves in the area ranging west-to-east from the Gulf of Mexico to the Sea of Marmara, and north-to-south from the Arctic to the Gulf of Guinea. Characteristics of symbioses in 53 species from the area are summarized for each of the five bivalve families documented to harbor chemosynthetic symbionts (Mytilidae, Vesicomyidae, Solemyidae, Thyasiridae and Lucinidae). Comparisons are made between the families, with special emphasis on ecology, life cycle, and connectivity. Chemosynthetic symbioses are a major adaptation to ecosystems and habitats exposed to reducing conditions. However, relatively little is known regarding their diversity and functioning, apart from a few "model species" on which effort has focused over the last 30 yr. In the context of increasing concern about biodiversity and ecosystems, and increasing anthropogenic pressure on oceans, we advocate a better assessment of the diversity of bivalve symbioses in order to evaluate the capacities of these remarkable ecological and evolutionary units to withstand environmental change.

Sediment transport along the Cap de Creus Canyon flank during a mild, wet winter

2013-05-14T00:00:00+02:00

Sediment transport along the Cap de Creus Canyon flank during a mild, wet winter

Biogeosciences, 10, 3221-3239, 2013

Author(s): J. Martín, X. Durrieu de Madron, P. Puig, F. Bourrin, A. Palanques, L. Houpert, M. Higueras, A. Sanchez-Vidal, A. M. Calafat, M. Canals, S. Heussner, N. Delsaut, and C. Sotin

Cap de Creus Canyon (CCC) is known as a preferential conduit for particulate matter leaving the Gulf of Lion continental shelf towards the slope and the basin, particularly in winter when storms and dense shelf water cascading coalesce to enhance the seaward export of shelf waters. During the CASCADE (CAscading, Storm, Convection, Advection and Downwelling Events) cruise in March 2011, deployments of recording instruments within the canyon and vertical profiling of the water column properties were conducted to study with high spatial-temporal resolution the impact of such processes on particulate matter fluxes. In the context of the mild and wet 2010–2011 winter, no remarkable dense shelf water formation was observed. On the other hand, the experimental setup allowed for the study of the impact of E-SE storms on the hydrographical structure and the particulate matter fluxes in the CCC. The most remarkable feature in terms of sediment transport was a period of dominant E-SE winds from 12 to 16 March, including two moderate storms (maximum significant wave heights = 4.1–4.6 m). During this period, a plume of freshened, relatively cold and turbid water flowed at high speeds along the southern flank of the CCC in an approximate depth range of 150–350 m. The density of this water mass was lighter than the ambient water in the canyon, indicating that it did not cascade off-shelf and that it merely downwelled into the canyon forced by the strong cyclonic circulation induced over the shelf during the storms and by the subsequent accumulation of seawater along the coast. Suspended sediment load in this turbid intrusion recorded along the southern canyon flank oscillated between 10 and 50 mg L⁻¹, and maximum currents speeds reached values up to 90 cm s⁻¹. A rough estimation of 10⁵ tons of sediment was transported through the canyon along its southern wall during a 3-day-long period of storm-induced downwelling. Following the veering of the wind direction (from SE to NW) on 16 March, downwelling ceased, currents inside the canyon reversed from down- to up-canyon, and the turbid shelf plume was evacuated from the canyon, most probably flowing along the southern canyon flank and being entrained by the general SW circulation after leaving the canyon confinement. This study highlights that remarkable sediment transport occurs in the CCC, and particularly along its southern flank, even during mild and wet winters, in absence of cascading and under limited external forcing. The sediment transport associated with eastern storms like the ones described in this paper tends to enter the canyon by its downstream flank, partially affecting the canyon head region. Sediment transport during these events is not constrained near the seafloor but distributed in a depth range of 200–300 m above the bottom. Our paper broadens the understanding of the complex set of atmosphere-driven sediment transport processes acting in this highly dynamic area of the northwestern Mediterranean Sea.

Effects of soil temperature and moisture on methane uptake and nitrous oxide emissions across three different ecosystem types

2013-05-13T00:00:00+02:00

Effects of soil temperature and moisture on methane uptake and nitrous oxide emissions across three different ecosystem types

Biogeosciences, 10, 3205-3219, 2013

Author(s): G. J. Luo, R. Kiese, B. Wolf, and K. Butterbach-Bahl

In this paper, we investigate similarities of effects of soil environmental drivers on year-round daily soil fluxes of nitrous oxide and methane for three distinct semi-natural or natural ecosystems: temperate spruce forest, Germany; tropical rain forest, Queensland, Australia; and ungrazed semi-arid steppe, Inner Mongolia, China. Annual cumulative fluxes of nitrous oxide and methane varied markedly among ecosystems, with nitrous oxide fluxes being highest for the tropical forest site (tropical forest: 0.96 kg N ha⁻¹ yr⁻¹; temperate forest: 0.67 kg N ha⁻¹ yr⁻¹; steppe: 0.22 kg N ha⁻¹ yr⁻¹), while rates of soil methane uptake were approximately equal for the temperate forest (−3.45 kg C ha⁻¹ yr⁻¹) and the steppe (−3.39 kg C ha⁻¹ yr⁻¹), but lower for the tropical forest site (−2.38 kg C ha⁻¹ yr⁻¹).

In order to allow for cross-site comparison of effects of changes in soil moisture and soil temperature on fluxes of methane and nitrous oxide, we used a normalization approach. Data analysis with normalized data revealed that, across sites, optimum rates of methane uptake are found at environmental conditions representing approximately average site environmental conditions. This might have rather important implications for understanding effects of climate change on soil methane uptake potential, since any shift in environmental conditions is likely to result in a reduction of soil methane uptake ability. For nitrous oxide, our analysis revealed expected patterns: highest nitrous oxide emissions under moist and warm conditions and large nitrous oxide fluxes if soils are exposed to freeze–thawing effects at sufficiently high soil moisture contents. However, the explanatory power of relationships of soil moisture or soil temperature to nitrous oxide fluxes remained rather poor (R² ≤ 0.36). When combined effects of changes in soil moisture and soil temperature were considered, the explanatory power of our empirical relationships with regard to temporal variations in nitrous oxide fluxes were at maximum about 50%. This indicates that other controlling factors such as N and C availability or microbial community dynamics might exert a significant control on the temporal dynamic of nitrous oxide fluxes. Though underlying microbial processes such as nitrification and denitrification are sensitive to changes in the environmental regulating factors, important regulating factors like moisture and temperature seem to have both synergistic and antagonistic effects on the status of other regulating factors. Thus we cannot expect a simple relationship between them and the pattern in the rate of emissions, associated with denitrification or nitrification in the soils.

In conclusion, we hypothesize that our approach of data generalization may prove beneficial for the development of environmental response models, which can be used across sites, and which are needed to help achieve a better understanding of climate change feedbacks on biospheric sinks or sources of nitrous oxide and methane.

Winter greenhouse gas fluxes (CO₂, CH₄ and N₂O) from a subalpine grassland

2013-05-13T00:00:00+02:00

Winter greenhouse gas fluxes (CO₂, CH₄ and N₂O) from a subalpine grassland

Biogeosciences, 10, 3185-3203, 2013

Author(s): L. Merbold, C. Steinlin, and F. Hagedorn

Although greenhouse gas emissions during winter contribute significantly to annual balances, their quantification is still highly uncertain in snow-covered ecosystems. Here, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes were measured at a subalpine managed grassland in Switzerland using concentration gradients within the snowpack (CO₂, CH₄, N₂O) and the eddy covariance method (CO₂) during the winter 2010/2011. Our objectives were (1) to identify the temporal and spatial variation of greenhouse gases (GHGs) and their drivers, and (2) to estimate the GHG budget of the site during this specific season (1 December–31 March, 121 days). Mean winter fluxes (December–March) based on the gradient method were 0.77 ± 0.54 μmol m⁻² s⁻¹ for CO₂ (1.19 ± 1.05 μmol m⁻² s⁻¹ measured by eddy covariance), −0.14 ± 0.09 nmol m⁻² s⁻¹ for CH₄ and 0.23 ± 0.23 nmol m⁻² s⁻¹ for N₂O, respectively. In comparison with the CO₂ fluxes measured by eddy covariance, the gradient technique underestimated the effluxes by 50%. While CO₂ and CH₄ fluxes decreased with the progressing winter season, N₂O fluxes did not follow a seasonal pattern. The major variables correlating with the fluxes of CO₂ and CH₄ were soil temperature and snow water equivalent, which is based on snow height and snow density. N₂O fluxes were only explained poorly by any of the measured environmental variables. Spatial variability across the valley floor was smallest for CO₂ and largest for N₂O. During the winter season 2010/2011, greenhouse gas fluxes ranged between 550 ± 540 g CO₂ m⁻² estimated by the eddy covariance approach and 543 ± 247 g CO₂ m⁻², −0.4 ± 0.01 g CH₄ m⁻² and 0.11 ± 0.1 g N₂O m⁻² derived by the gradient technique. Total seasonal greenhouse gas emissions from the grassland were between 574 ± 276 and 581 ± 569 g CO₂ eq. m⁻², with N₂O contributing 5% to the overall budget and CH₄ reducing the budget by 0.1%. Cumulative budgets of CO₂ were smaller than emissions reported for other subalpine meadows in the Swiss Alps and the Rocky Mountains. Further investigations on the GHG exchange of grasslands in winter are needed in order to (1) deepen our currently limited knowledge on the environmental drivers of each GHG, (2) to thoroughly constrain annual balances, and (3) to project possible changes in GHG flux magnitude with expected shorter and warmer winter periods.

Carbon isotopic evidence for microbial control of carbon supply to Orca Basin at the seawater–brine interface

2013-05-13T00:00:00+02:00

Carbon isotopic evidence for microbial control of carbon supply to Orca Basin at the seawater–brine interface

Biogeosciences, 10, 3175-3183, 2013

Author(s): S. R. Shah, S. B. Joye, J. A. Brandes, and A. P. McNichol

Orca Basin, an intraslope basin on the Texas-Louisiana continental slope, hosts a hypersaline, anoxic brine in its lowermost 200 m in which limited microbial activity has been reported. This brine contains a large reservoir of reduced and aged carbon, and appears to be stable at decadal time scales: concentrations and isotopic composition of dissolved inorganic (DIC) and organic carbon (DOC) are similar to measurements made in the 1970s. Both DIC and DOC are more "aged" within the brine pool than in overlying water, and the isotopic contrast between brine carbon and seawater carbon is much greater for DIC than DOC. While the stable carbon isotopic composition of brine DIC points towards a combination of methane and organic carbon remineralization as its source, radiocarbon and box model results point to the brine interface as the major source region for DIC, allowing for only limited oxidation of methane diffusing upwards from sediments. This conclusion is consistent with previous studies that identify the seawater–brine interface as the focus of microbial activity associated with Orca Basin brine. Isotopic similarities between DIC and DOC suggest a different relationship between these two carbon reservoirs than is typically observed in deep ocean basins. Radiocarbon values implicate the seawater–brine interface region as the likely source region for DOC to the brine as well as DIC.

The impact of land-use change on floristic diversity at regional scale in southern Sweden 600 BC–AD 2008

2013-05-08T00:00:00+02:00

The impact of land-use change on floristic diversity at regional scale in southern Sweden 600 BC–AD 2008

Biogeosciences, 10, 3159-3173, 2013

Author(s): D. Fredh, A. Broström, M. Rundgren, P. Lagerås, F. Mazier, and L. Zillén

This study explores the relationship between land-use and floristic diversity between 600 BC and AD 2008 in the uplands of southern Sweden. We use fossil pollen assemblages and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model to quantitatively reconstruct land cover at a regional scale. Floristic richness and evenness are estimated using palynological richness and REVEALS-based evenness, respectively. We focus on the period AD 350 to 750 to investigate the impact of an inferred, short-lived (< 200 yr) period of land-use expansion and subsequent land abandonment on vegetation composition and floristic diversity. The observed vegetation response is compared to that recorded during the transition from traditional to modern land-use management at the end of the 19th century. Our results suggest that agricultural land use was most widespread between AD 350 and 1850, which correlates broadly with high values of palynological richness. REVEALS-based evenness was highest between AD 500 and 1600 which indicates a more equal cover among taxa during this time interval. Palynological richness increased during the inferred land-use expansion after AD 350 and decreased during the subsequent regression AD 550–750, while REVEALS-based evenness increased throughout this period. The values of palynological richness during the last few decades are within the range observed during the last 1650 yr. However, REVEALS-based evenness shows much lower values during the last century compared to the previous ca. 2600 yr, which indicates that the composition of present-day vegetation is unusual in a millennial perspective. Our results show that regional scale changes in land use have had clear impacts on floristic diversity in southern Sweden, with a vegetation response time of less than 20 to 50 yr. We show the importance of traditional land use to attain high biodiversity and suggest that ecosystem management should include a regional landscape perspective.

Organic matter composition and stabilization in a polygonal tundra soil of the Lena Delta

2013-05-08T00:00:00+02:00

Organic matter composition and stabilization in a polygonal tundra soil of the Lena Delta

Biogeosciences, 10, 3145-3158, 2013

Author(s): S. Höfle, J. Rethemeyer, C. W. Mueller, and S. John

This study investigated soil organic matter (OM) composition of differently stabilized soil OM fractions in the active layer of a polygonal tundra soil in the Lena Delta, Russia, by applying density and particle size fractionation combined with qualitative OM analysis using solid state ¹³C nuclear magnetic resonance spectroscopy, and lipid analysis combined with ¹⁴C analysis. Bulk soil OM was mainly composed of plant-derived, little-decomposed material with surprisingly high and strongly increasing apparent ¹⁴C ages with active layer depth suggesting slow microbial OM transformation in cold climate. Most soil organic carbon was stored in clay and fine-silt fractions (< 6.3 μm), which were composed of little-decomposed plant material, indicated by the dominance of long n-alkane and n-fatty acid compounds and low alkyl/O-alkyl C ratios. Organo-mineral associations, which are suggested to be a key mechanism of OM stabilization in temperate soils, seem to be less important in the active layer as the mainly plant-derived clay- and fine-silt-sized OM was surprisingly "young", with ¹⁴C contents similar to the bulk soil values. Furthermore, these fractions contained less organic carbon compared to density fractionated OM occluded in soil aggregates – a further important OM stabilization mechanism in temperate soils restricting accessibility of microorganisms. This process seems to be important at greater active layer depth where particulate OM, occluded in soil aggregates, was "older" than free particulate OM.

Unravelling the environmental drivers of deep-sea nematode biodiversity and its relation with carbon mineralisation along a longitudinal primary productivity gradient

2013-05-08T00:00:00+02:00

Unravelling the environmental drivers of deep-sea nematode biodiversity and its relation with carbon mineralisation along a longitudinal primary productivity gradient

Biogeosciences, 10, 3127-3143, 2013

Author(s): E. Pape, T. N. Bezerra, D. O. B. Jones, and A. Vanreusel

Alongside a primary productivity gradient between the Galicia Bank region in the Northeast Atlantic and the more oligotrophic eastern Mediterranean Basin, we investigated the bathymetric (1200–3000 m) and longitudinal variation in several measures for nematode taxon (Shannon–Wiener genus diversity, expected genus richness and generic evenness) and functional diversity (trophic diversity, diversity of life history strategies, biomass diversity and phylogenetic diversity). Our goals were to establish the form of the relation between diversity and productivity (measured as seafloor particulate organic carbon or POC flux), and to verify the positive and negative effect of sediment particle size diversity (SED) and the seasonality in POC flux (SVI), respectively, on diversity, as observed for other oceanographic regions and taxa. In addition, we hypothesised that higher taxon diversity is associated with higher functional diversity, which in turn stimulates nematode carbon mineralisation rates (determined from biomass-dependent respiration estimates). Taxon diversity related positively to seafloor POC flux. Phylogenetic diversity (measured as average taxonomic distinctness) was affected negatively by the magnitude and variability in POC flux, and positively by SED. The latter also showed an inverse relation with trophic diversity. Accounting for differences in total biomass between samples, we observed a positive linear relation between taxon diversity and carbon mineralisation in nematode communities. We could, however, not identify the potential mechanism through which taxon diversity may promote this ecosystem function since none of the functional diversity indices related to both diversity and nematode respiration. The present results suggest potential effects of climate change on deep-sea ecosystem functioning, but further also emphasise the need for a better understanding of nematode functions and their response to evolutionary processes.

Implications of elevated CO₂ on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach

2013-05-08T00:00:00+02:00

Implications of elevated CO₂ on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach

Biogeosciences, 10, 3109-3125, 2013

Author(s): J. Czerny, K. G. Schulz, T. Boxhammer, R. G. J. Bellerby, J. Büdenbender, A. Engel, S. A. Krug, A. Ludwig, K. Nachtigall, G. Nondal, B. Niehoff, A. Silyakova, and U. Riebesell

Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air–sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification applying KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down the mentioned uncertainties. Water-column concentrations of particulate and dissolved organic and inorganic matter were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution as well as estimates of wall growth were developed to close the gaps in element budgets. However, losses elements from the budgets into a sum of insufficiently determined pools were detected, and are principally unavoidable in mesocosm investigation. The comparison of variability patterns of all single measured datasets revealed analytic precision to be the main issue in determination of budgets. Uncertainties in dissolved organic carbon (DOC), nitrogen (DON) and particulate organic phosphorus (POP) were much higher than the summed error in determination of the same elements in all other pools. With estimates provided for all other major elemental pools, mass balance calculations could be used to infer the temporal development of DOC, DON and POP pools.

Future elevated pCO₂ was found to enhance net autotrophic community carbon uptake in two of the three experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic carbon under nutrient-recycling summer conditions. This carbon over-consumption effect becomes evident from mass balance calculations, but was too small to be resolved by direct measurements of dissolved organic matter. Faster nutrient uptake by comparatively small algae at high CO₂ after nutrient addition resulted in reduced production rates under future ocean CO₂ conditions at the end of the experiment. This CO₂ mediated shift towards smaller phytoplankton and enhanced cycling of dissolved matter restricted the development of larger phytoplankton, thus pushing the system towards a retention type food chain with overall negative effects on export potential.

Net primary productivity, allocation pattern and carbon use efficiency in an apple orchard assessed by integrating eddy covariance, biometric and continuous soil chamber measurements

2013-05-07T00:00:00+02:00

Net primary productivity, allocation pattern and carbon use efficiency in an apple orchard assessed by integrating eddy covariance, biometric and continuous soil chamber measurements

Biogeosciences, 10, 3089-3108, 2013

Author(s): D. Zanotelli, L. Montagnani, G. Manca, and M. Tagliavini

Carbon use efficiency (CUE), the ratio of net primary production (NPP) over gross primary production (GPP), is a functional parameter that could possibly link the current increasingly accurate global GPP estimates with those of net ecosystem exchange, for which global predictors are still unavailable. Nevertheless, CUE estimates are actually available for only a few ecosystem types, while information regarding agro-ecosystems is scarce, in spite of the simplified spatial structure of these ecosystems that facilitates studies on allocation patterns and temporal growth dynamics.

We combined three largely deployed methods, eddy covariance, soil respiration and biometric measurements, to assess monthly values of CUE, NPP and allocation patterns in different plant organs in an apple orchard during a complete year (2010). We applied a measurement protocol optimized for quantifying monthly values of carbon fluxes in this ecosystem type, which allows for a cross check between estimates obtained from different methods. We also attributed NPP components to standing biomass increments, detritus cycle feeding and lateral exports.

We found that in the apple orchard, both net ecosystem production and gross primary production on a yearly basis, 380 ± 30 g C m⁻² and 1263 ± 189 g C m⁻² respectively, were of a magnitude comparable to those of natural forests growing in similar climate conditions. The largest differences with respect to forests are in the allocation pattern and in the fate of produced biomass. The carbon sequestered from the atmosphere was largely allocated to production of fruit: 49% of annual NPP was taken away from the ecosystem through apple production. Organic material (leaves, fine root litter, pruned wood and early fruit falls) contributing to the detritus cycle was 46% of the NPP. Only 5% was attributable to standing biomass increment, while this NPP component is generally the largest in forests.

The CUE, with an annual average of 0.71 ± 0.12, was higher than the previously suggested constant values of 0.47–0.50. Low nitrogen investment in fruit, the limited root apparatus, and the optimal growth temperature and nutritional condition observed at the site are suggested to be explanatory variables for the high CUE observed.

Nitrogen isotope dynamics and fractionation during sedimentary denitrification in Boknis Eck, Baltic Sea

2013-05-07T00:00:00+02:00

Nitrogen isotope dynamics and fractionation during sedimentary denitrification in Boknis Eck, Baltic Sea

Biogeosciences, 10, 3079-3088, 2013

Author(s): K. Dähnke and B. Thamdrup

The global marine nitrogen cycle is constrained by nitrogen fixation as a source of reactive nitrogen, and denitrification or anammox on the sink side. These processes with their respective isotope effects set the marine nitrate ¹⁵N-isotope value (δ¹⁵N) to a relatively constant average of 5‰. This value can be used to better assess the magnitude of these sources and sink terms, but the underlying assumption is that sedimentary denitrification and anammox, processes responsible for approximately one-third of global nitrogen removal, have little to no isotope effect on nitrate in the water column.

We investigated the isotope fractionation in sediment incubations, measuring net denitrification and nitrogen and oxygen stable isotope fractionation in surface sediments from the coastal Baltic Sea (Boknis Eck, northern Germany), a site with seasonal hypoxia and dynamic nitrogen turnover.

Sediment denitrification was fast, and regardless of current paradigms assuming little fractionation during sediment denitrification, we measured fractionation factors of 18.9‰ for nitrogen and 15.8‰ for oxygen in nitrate. While the input of nitrate to the water column remains speculative, these results challenge the current view of fractionation during sedimentary denitrification and imply that nitrogen budget calculations may need to consider this variability, as both preferential uptake of light nitrate and release of the remaining heavy fraction can significantly alter water column nitrate isotope values at the sediment–water interface.

Surface pathway of radioactive plume of TEPCO Fukushima NPP1 released ¹³⁴Cs and ¹³⁷Cs

2013-05-07T00:00:00+02:00

Surface pathway of radioactive plume of TEPCO Fukushima NPP1 released ¹³⁴Cs and ¹³⁷Cs

Biogeosciences, 10, 3067-3078, 2013

Author(s): M. Aoyama, M. Uematsu, D. Tsumune, and Y. Hamajima

¹³⁴Cs and ¹³⁷Cs were released to the North Pacific Ocean by two major likely pathways, direct discharge from the Fukushima NPP1 accident site and atmospheric deposition off Honshu Islands of Japan, east and northeast of the site. High density observations of ¹³⁴Cs and ¹³⁷Cs in the surface water were carried out by 17 cruises of cargo ships and several research vessel cruises from March 2011 till March 2012. The main body of radioactive surface plume of which activity exceeded 10 Bq m⁻³ travelled along 40° N and reached the International Date Line on March 2012, one year after the accident. A distinct feature of the radioactive plume was that it stayed confined along 40° N when the plume reached the International Date Line. A zonal speed of the radioactive plume was estimated to be about 8 cm s⁻¹ which was consistent with zonal speeds derived by Argo floats at the region.

Seasonal variations of virus- and nanoflagellate-mediated mortality of heterotrophic bacteria in the coastal ecosystem of subtropical western Pacific

2013-05-06T00:00:00+02:00

Seasonal variations of virus- and nanoflagellate-mediated mortality of heterotrophic bacteria in the coastal ecosystem of subtropical western Pacific

Biogeosciences, 10, 3055-3065, 2013

Author(s): A. Y. Tsai, G.-C. Gong, and J. Hung

Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h⁻¹ (April 2011) to 0.42 h⁻¹ (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing + viral lysis) influence the dynamics of the bacteria community.

Seamount physiography and biology in the north-east Atlantic and Mediterranean Sea

2013-05-06T00:00:00+02:00

Seamount physiography and biology in the north-east Atlantic and Mediterranean Sea

Biogeosciences, 10, 3039-3054, 2013

Author(s): T. Morato, K. Ø. Kvile, G. H. Taranto, F. Tempera, B. E. Narayanaswamy, D. Hebbeln, G. M. Menezes, C. Wienberg, R. S. Santos, and T. J. Pitcher

This work aims at characterising the seamount physiography and biology in the OSPAR Convention limits (north-east Atlantic Ocean) and Mediterranean Sea. We first inferred potential abundance, location and morphological characteristics of seamounts, and secondly, summarized the existing biological, geological and oceanographic in situ research, identifying examples of well-studied seamounts. Our study showed that the seamount population in the OSPAR area (north-east Atlantic) and in the Mediterranean Sea is large with around 557 and 101 seamount-like features, respectively. Similarly, seamounts occupy large areas of about 616 000 km² in the OSPAR region and of about 89 500 km² in the Mediterranean Sea. The presence of seamounts in the north-east Atlantic has been known since the late 19th century, but overall knowledge regarding seamount ecology and geology is still relatively poor. Only 37 seamounts in the OSPAR area (3.5% of all seamounts in the region), 22 in the Mediterranean Sea (9.2% of all seamounts in the region) and 25 in the north-east Atlantic south of the OSPAR area have in situ information. Seamounts mapped in both areas are in general very heterogeneous, showing diverse geophysical characteristics. These differences will likely affect the biological diversity and production of resident and associated organisms.

Soil respiration on an aging managed heathland: identifying an appropriate empirical model for predictive purposes

2013-05-06T00:00:00+02:00

Soil respiration on an aging managed heathland: identifying an appropriate empirical model for predictive purposes

Biogeosciences, 10, 3007-3038, 2013

Author(s): G. R. Kopittke, E. E. van Loon, A. Tietema, and D. Asscheman

Heathlands are cultural landscapes which are managed through cyclical cutting, burning or grazing practices. Understanding the carbon (C) fluxes from these ecosystems provides information on the optimal management cycle time to maximise C uptake and minimise C output. The interpretation of field data into annual C loss values requires the use of soil respiration models. These generally include model variables related to the underlying drivers of soil respiration, such as soil temperature, soil moisture and plant activity. Very few studies have used selection procedures in which structurally different models are calibrated, then validated on separate observation datasets and the outcomes critically compared. We present thorough model selection procedures to determine soil heterotrophic (microbial) and autotrophic (root) respiration for a heathland chronosequence and show that soil respiration models are required to correct the effect of experimental design on soil temperature. Measures of photosynthesis, plant biomass, photosynthetically active radiation, root biomass, and microbial biomass did not significantly improve model fit when included with soil temperature. This contradicts many current studies in which these plant variables are used (but not often tested for parameter significance). We critically discuss a number of alternative ecosystem variables associated with soil respiration processes in order to inform future experimental planning and model variable selection at other heathland field sites. The best predictive model used a generalized linear multi-level model with soil temperature as the only variable. Total annual soil C loss from the young, middle and old communities was calculated to be 650, 462 and 435 g C m⁻² yr⁻¹, respectively.

Net sea–air CO₂ flux uncertainties in the Bay of Biscay based on the choice of wind speed products and gas transfer parameterizations

2013-05-03T00:00:00+02:00

Net sea–air CO₂ flux uncertainties in the Bay of Biscay based on the choice of wind speed products and gas transfer parameterizations

Biogeosciences, 10, 2993-3005, 2013

Author(s): P. Otero, X. A. Padin, M. Ruiz-Villarreal, L. M. García-García, A. F. Ríos, and F. F. Pérez

The estimation of sea–air CO₂ fluxes is largely dependent on wind speed through the gas transfer velocity parameterization. In this paper, we quantify uncertainties in the estimation of the CO₂ uptake in the Bay of Biscay resulting from the use of different sources of wind speed such as three different global reanalysis meteorological models (NCEP/NCAR 1, NCEP/DOE 2 and ERA-Interim), one high-resolution regional forecast model (HIRLAM-AEMet), winds derived under the Cross-Calibrated Multi-Platform (CCMP) project, and QuikSCAT winds in combination with some of the most widely used gas transfer velocity parameterizations. Results show that net CO₂ flux estimations during an entire seasonal cycle (September 2002–September 2003) may vary by a factor of ~ 3 depending on the selected wind speed product and the gas exchange parameterization, with the highest impact due to the last one. The comparison of satellite- and model-derived winds with observations at buoys advises against the systematic overestimation of NCEP-2 and the underestimation of NCEP-1. In the coastal region, the presence of land and the time resolution are the main constraints of QuikSCAT, which turns CCMP and ERA-Interim in the preferred options.

Assessing the role of dust deposition on phytoplankton ecophysiology and succession in a low-nutrient low-chlorophyll ecosystem: a mesocosm experiment in the Mediterranean Sea

2013-05-03T00:00:00+02:00

Assessing the role of dust deposition on phytoplankton ecophysiology and succession in a low-nutrient low-chlorophyll ecosystem: a mesocosm experiment in the Mediterranean Sea

Biogeosciences, 10, 2973-2991, 2013

Author(s): V. Giovagnetti, C. Brunet, F. Conversano, F. Tramontano, I. Obernosterer, C. Ridame, and C. Guieu

In this study, we investigate the response of the phytoplankton community, with emphasis on ecophysiology and succession, after two experimental additions of Saharan dust in the surface water layer of a low-nutrient low-chlorophyll ecosystem in the Mediterranean Sea. Three mesocosms were amended with evapocondensed dust to simulate realistic Saharan dust events, while three additional mesocosms were kept unamended and served as controls. The experiment consisted in two consecutive dust additions and samples were daily collected at different depths (−0.1, −5 and −10 m) during one week, starting before each addition occurred. Data concerning HPLC pigment analysis on two size classes (< 3 and > 3 μm), electron transport rate (ETR) vs. irradiance curves, non-photochemical fluorescence quenching (NPQ) and phytoplankton cell abundance (measured by flow cytometry), are presented and discussed in this paper. Results show that picophytoplankton mainly respond to the first dust addition, while the second addition leads to an increase of both pico- and nano-/microphytoplankton. Ecophysiological changes in the phytoplankton community occur, with NPQ and pigment concentration per cell increasing after dust additions. While biomass increases after pulses of new nutrients, ETR does not greatly vary between dust-amended and control conditions, in relation with ecophysiological changes within the phytoplankton community, such as the increase in NPQ and pigment cellular concentration. A quantitative assessment and parameterisation of the onset of a phytoplankton bloom in a nutrient-limited ecosystem is attempted on the basis of the increase in phytoplankton biomass observed during the experiment. The results of this study are discussed focusing on the adaptation of picophytoplankton to nutrient limitation in the surface water layer, as well as on size-dependent competition ability in phytoplankton.

Water supply patterns over Germany under climate change conditions

2013-05-02T00:00:00+02:00

Water supply patterns over Germany under climate change conditions

Biogeosciences, 10, 2959-2972, 2013

Author(s): M. H. Tölle, C. Moseley, O. Panferov, G. Busch, and A. Knohl

A large ensemble of 24 bias-corrected and uncorrected regional climate model (RCM) simulations is used to investigate climate change impacts on water supply patterns over Germany using the seasonal winter and summer Standardized Precipitation Index (SPI) based on 6-month precipitation sums. The climate change signal is studied comparing SPI characteristics for the reference period 1971–2000 with those of the "near" (2036–2065) and the "far" (2071–2100) future. The spread of the climate change signal within the simulation ensemble of bias-corrected versus non-corrected data is discussed. Ensemble scenarios are evaluated against available observation-based data over the reference period 1971–2000. After correcting the model biases, the model ensemble underestimates the variability of the precipitation climatology in the reference period, but replicates the mean characteristics. Projections of water supply patterns based on the SPI for the time periods 2036–2065 and 2071–2100 show wetter winter months during both future time periods. As a result soil drying may be delayed to late spring extending into the summer period, which could have an important effect on sensible heat fluxes. While projections indicate wetting in summer during 2036–2065, drier summers are estimated towards the south-west of Germany for the end of the 21st century. The use of the bias correction intensifies the signal to wetter conditions for both seasons and time periods. The spread in the projection of future water supply patterns between the ensemble members is explored, resulting in high spatial differences that suggest a higher uncertainty of the climate change signal in the southern part of Germany. It is shown that the spread of the climate change signals between SPIs based on single ensemble members is twice as large as the difference between the mean climate change signal of SPIs based on bias-corrected and uncorrected precipitation. This implies that the sensitivity of the SPI to the modelled precipitation bias is small compared to the range of the climate change signals within our ensemble. Therefore, the SPI is a very useful tool for climate change studies allowing us to avoid the additional uncertainties caused by bias corrections.

Trophic state of benthic deep-sea ecosystems from two different continental margins off Iberia

2013-05-02T00:00:00+02:00

Trophic state of benthic deep-sea ecosystems from two different continental margins off Iberia

Biogeosciences, 10, 2945-2957, 2013

Author(s): A. Dell'Anno, A. Pusceddu, C. Corinaldesi, M. Canals, S. Heussner, L. Thomsen, and R. Danovaro

The bioavailability of organic matter in benthic deep-sea ecosystems, commonly used to define their trophic state, can greatly influence key ecological processes such as biomass production and nutrient cycling. Here, we assess the trophic state of deep-sea sediments from open slopes and canyons of the Catalan (NW Mediterranean) and Portuguese (NE Atlantic) continental margins, offshore east and west Iberia, respectively, by using a biomimetic approach based on enzymatic digestion of protein and carbohydrate pools. Patterns of sediment trophic state were analyzed in relation to increasing water depth, including repeated samplings over a 3 yr period in the Catalan margin. Two out of the three sampling periods occurred a few months after dense shelf water cascading events. The benthic deep-sea ecosystems investigated in this study were characterized by high amounts of bioavailable organic matter when compared to other deep-sea sediments. Bioavailable organic matter and its nutritional value were significantly higher in the Portuguese margin than in the Catalan margin, thus reflecting differences in primary productivity of surface waters reported for the two regions. Similarly, sediments of the Catalan margin were characterized by significantly higher food quantity and quality in spring, when the phytoplankton bloom occurs in surface waters, than in summer and autumn. Differences in the benthic trophic state of canyons against open slopes were more evident in the Portuguese than in the Catalan margin. In both continental margins, bioavailable organic C concentrations did not vary or increase with increasing water depth. Overall, our findings suggest that the intensity of primary production processes along with the lateral transfer of organic particles, even amplified by episodic events, can have a role in controlling the quantity and distribution of bioavailable organic detritus and its nutritional value along these continental margin ecosystems.

The large variation in organic carbon consumption in spring in the East China Sea

2013-05-02T00:00:00+02:00

The large variation in organic carbon consumption in spring in the East China Sea

Biogeosciences, 10, 2931-2943, 2013

Author(s): C.-C. Chen, G.-C. Gong, F.-K. Shiah, W.-C. Chou, and C.-C. Hung

A tremendous amount of organic carbon respired by plankton communities has been found in summer in the East China Sea (ECS), and this rate has been significantly correlated with fluvial discharge from the Changjiang River. However, respiration data has rarely been collected in other seasons. To evaluate and reveal the potential controlling mechanism of organic carbon consumption in spring in the ECS, two cruises covering almost the entire ECS shelf were conducted in the spring of 2009 and 2010. These results showed that although the fluvial discharge rates were comparable to the high riverine flow in summer, the plankton community respiration (CR) varied widely between the two springs. In 2009, the level of CR was double that of 2010, with mean (± SD) values of 111.7 (±76.3) and 50.7 (±62.9) mg C m⁻³ d⁻¹, respectively. The CR was positively correlated with concentrations of particulate organic carbon and/or chlorophyll a (Chl a) in 2009 (all p < 0.01). These results suggest that the high CR rate in 2009 can be attributed to high planktonic biomasses. During this period, phytoplankton growth flourished due to allochthonous nutrients discharged from the Changjiang River. Furthermore, higher phytoplankton growth led to the absorption of an enormous amount of fugacity of CO₂ (fCO₂) in the surface waters, even with a significant amount of inorganic carbon regenerated via CR. In 2010, even more riverine runoff nutrients were measured in the ECS than in 2009. Surprisingly, the growth of phytoplankton in 2010 was not stimulated by enriched nutrients, and its growth was likely limited by low water temperature and/or low light intensity. Low temperature might also suppress planktonic metabolism, which could explain why the CR was lower in 2010. During this period, lower surface water fCO₂ may have been driven mainly by physical process(es). To conclude, these results indicate that high organic carbon consumption (i.e. CR) in the spring of 2009 could be attributed to high planktonic biomasses, and the lower CR rate during the cold spring of 2010 might be likely limited by low temperature in the ECS. This further suggests that the high inter-annual variability of organic carbon consumption needs to be kept in mind when budgeting the annual carbon balance.

The Unified North American Soil Map and its implication on the soil organic carbon stock in North America

2013-05-02T00:00:00+02:00

The Unified North American Soil Map and its implication on the soil organic carbon stock in North America

Biogeosciences, 10, 2915-2930, 2013

Author(s): S. Liu, Y. Wei, W. M. Post, R. B. Cook, K. Schaefer, and M. M. Thornton

The Unified North American Soil Map (UNASM) was developed to provide more accurate regional soil information for terrestrial biosphere modeling. The UNASM combines information from state-of-the-art US STATSGO2 and Soil Landscape of Canada (SLCs) databases. The area not covered by these datasets is filled by using the Harmonized World Soil Database version 1.21 (HWSD1.21). The UNASM contains maximum soil depth derived from the data source as well as seven soil attributes (including sand, silt, and clay content, gravel content, organic carbon content, pH, and bulk density) for the topsoil layer (0–30 cm) and the subsoil layer (30–100 cm), respectively, of the spatial resolution of 0.25 degrees in latitude and longitude. There are pronounced differences in the spatial distributions of soil properties and soil organic carbon between UNASM and HWSD, but the UNASM overall provides more detailed and higher-quality information particularly in Alaska and central Canada. To provide more accurate and up-to-date estimate of soil organic carbon stock in North America, we incorporated Northern Circumpolar Soil Carbon Database (NCSCD) into the UNASM. The estimate of total soil organic carbon mass in the upper 100 cm soil profile based on the improved UNASM is 365.96 Pg, of which 23.1% is under trees, 14.1% is in shrubland, and 4.6% is in grassland and cropland. This UNASM data will provide a resource for use in terrestrial ecosystem modeling both for input of soil characteristics and for benchmarking model output.

Combining a coupled FTIR-EGA system and in situ DRIFTS for studying soil organic matter in arable soils

2013-05-02T00:00:00+02:00

Combining a coupled FTIR-EGA system and in situ DRIFTS for studying soil organic matter in arable soils

Biogeosciences, 10, 2897-2913, 2013

Author(s): M. S. Demyan, F. Rasche, M. Schütt, N. Smirnova, E. Schulz, and G. Cadisch

An optimized spectroscopic method combining quantitative evolved gas analysis via Fourier transform infrared spectroscopy (FTIR-EGA) in combination with a qualitative in situ thermal reaction monitoring via diffuse reflectance Fourier transform infrared spectroscopy (in situ_T DRIFTS) is being proposed to rapidly characterize soil organic matter (SOM) to study its dynamics and stability. A thermal reaction chamber coupled with an infrared gas cell was used to study the pattern of thermal evolution of carbon dioxide (CO₂) in order to relate evolved gas (i.e., CO₂) to different qualities of SOM. Soil samples were taken from three different arable sites in Germany: (i) the Static Fertilization Experiment, Bad Lauchstädt (Chernozem), from treatments of farmyard manure (FYM), mineral fertilizer (NPK), their combination (FYM + NPK) and control without fertilizer inputs; (ii) Kraichgau; and (iii) Swabian Alb (Cambisols) areas, Southwest Germany. The two latter soils were further fractionated into particulate organic matter (POM), sand and stable aggregates (Sa + A), silt and clay (Si + C), and NaOCl oxidized Si + C (rSOC) to gain OM of different inferred stabilities; respiration was measured from fresh soil samples incubated at 20 °C and 50% water holding capacity for 490 days. A variable long path length gas cell was used to record the mid-infrared absorbance intensity of CO₂ (2400 to 2200 cm⁻¹) being evolved during soil heating from 25 to 700 °C with a heating rate of 68 °C min⁻¹ and holding time of 10 min at 700 °C. Separately, the heating chamber was placed in a diffuse reflectance chamber (DRIFTS) for measuring the mid-infrared absorbance of the soil sample during heating. Thermal stability of the bulk soils and fractions was measured via the temperature of maximum CO₂ evolution (CO₂_max).

Results indicated that the FYM + NPK and FYM treatments of the Chernozem soils had a lower CO₂_max as compared to both NPK and CON treatments. On average, CO₂_max of the Chernozem was much higher (447 °C) as compared to the Cambisol sites (Kraichgau 392 °C; Swabian Alb 384 °C). The POM fraction had the highest CO₂_max (477 °C), while rSOC had a first peak at 265 °C at both sites and a second peak at 392 °C for the Swabian Alb and 482 °C for the Kraichgau. The CO₂_max increased after 490 day incubation, while the C lost during incubation was derived from the whole temperature range but a relatively higher proportion from 200 to 350 °C. In situ_T DRIFTS measurements indicated decreases in vibrational intensities in the order of C-OH = unknown C vibration < C-H < −COO/C =C < C = C with increasing temperature, but interpretation of vibrational changes was complicated by changes in the spectra (i.e., overall vibrational intensity increased with temperature increase) of the sample during heating. The relative quality changes and corresponding temperatures shown by the in situ_T DRIFTS measurements enabled the fitting of four components or peaks to the evolved CO₂ thermogram from the FTIR-EGA measurements. This gave a semi-quantitative measure of the quality of evolved C during the heating experiment, lending more evidence that different qualities of SOM are being evolved at different temperatures from 200 to 700 °C. The CO₂_max was influenced by long-term FYM input and also after 490 days of laboratory incubation, indicating that this measurement is an indicator for the relative overall SOM stability. The combination of FTIR-EGA and in situ_T DRIFTS allows for a quantitative and qualitative monitoring of thermal reactions of SOM, revealing its relative stability, and provides a sound basis for a peak fitting procedure for assigning proportions of evolved CO₂ to different thermal stability components.

Annual carbon balance of a peatland 10 yr following restoration

2013-05-02T00:00:00+02:00

Annual carbon balance of a peatland 10 yr following restoration

Biogeosciences, 10, 2885-2896, 2013

Author(s): M. Strack and Y. C. A. Zuback

Undisturbed peatlands represent long-term net sinks of carbon; however, peat extraction converts these systems into large and persistent sources of greenhouse gases. Although rewetting and restoration following peat extraction have taken place over the last several decades, very few studies have investigated the longer term impact of this restoration on peatland carbon balance. We determined the annual carbon balance of a former horticulturally-extracted peatland restored 10 yr prior to the study and compared these values to the carbon balance measured at neighboring unrestored and natural sites. Carbon dioxide (CO₂) and methane (CH₄) fluxes were measured using the chamber technique biweekly during the growing season from May to October 2010 and three times over the winter period. Dissolved organic carbon (DOC) export was measured from remnant ditches in the unrestored and restored sites. During the growing season the restored site had greater uptake of CO₂ than the natural site when photon flux density was greater than 1000 μmol m⁻² s⁻¹, while the unrestored site remained a source of CO₂. Ecosystem respiration was similar between natural and restored sites, which were both significantly lower than the unrestored site. Methane flux remained low at the restored site except from open water pools, created as part of restoration, and remnant ditches. Export of DOC during the growing season was 5.0 and 28.8 g m⁻² from the restored and unrestored sites, respectively. Due to dry conditions during the study year all sites acted as net carbon sources with annual balance of the natural, restored and unrestored sites of 250.7, 148.0 and 546.6 g C m⁻², respectively. Although hydrological conditions and vegetation community at the restored site remained intermediate between natural and unrestored conditions, peatland restoration resulted in a large reduction in annual carbon loss from the system resulting in a carbon balance more similar to a natural peatland.

A new conceptual model of coral biomineralisation: hypoxia as the physiological driver of skeletal extension

2013-05-02T00:00:00+02:00

A new conceptual model of coral biomineralisation: hypoxia as the physiological driver of skeletal extension

Biogeosciences, 10, 2867-2884, 2013

Author(s): S. Wooldridge

That corals skeletons are built of aragonite crystals with taxonomy-linked ultrastructure has been well understood since the 19th century. Yet, the way by which corals control this crystallization process remains an unsolved question. Here, I outline a new conceptual model of coral biomineralisation that endeavours to relate known skeletal features with homeostatic functions beyond traditional growth (structural) determinants. In particular, I propose that the dominant physiological driver of skeletal extension is night-time hypoxia, which is exacerbated by the respiratory oxygen demands of the coral's algal symbionts (= zooxanthellae). The model thus provides a new narrative to explain the high growth rate of symbiotic corals, by equating skeletal deposition with the "work-rate" of the coral host needed to maintain a stable and beneficial symbiosis. In this way, coral skeletons are interpreted as a continuous (long-run) recording unit of the stability and functioning of the coral–algae endosymbiosis. After providing supportive evidence for the model across multiple scales of observation, I use coral core data from the Great Barrier Reef (Australia) to highlight the disturbed nature of the symbiosis in recent decades, but suggest that its onset is consistent with a trajectory that has been followed since at least the start of the 1900s. In concluding, I outline how the proposed capacity of cnidarians (which includes modern reef corals) to overcome the metabolic limitation of hypoxia via skeletogenesis also provides a new hypothesis to explain the sudden appearance in the fossil record of calcified skeletons at the Precambrian–Cambrian transition – and the ensuing rapid appearance of most major animal phyla.

Ecosystem function and particle flux dynamics across the Mackenzie Shelf (Beaufort Sea, Arctic Ocean): an integrative analysis of spatial variability and biophysical forcings

2013-05-02T00:00:00+02:00

Ecosystem function and particle flux dynamics across the Mackenzie Shelf (Beaufort Sea, Arctic Ocean): an integrative analysis of spatial variability and biophysical forcings

Biogeosciences, 10, 2833-2866, 2013

Author(s): A. Forest, M. Babin, L. Stemmann, M. Picheral, M. Sampei, L. Fortier, Y. Gratton, S. Bélanger, E. Devred, J. Sahlin, D. Doxaran, F. Joux, E. Ortega-Retuerta, J. Martín, W. H. Jeffrey, B. Gasser, and J. Carlos Miquel

A better understanding of how environmental changes affect organic matter fluxes in Arctic marine ecosystems is sorely needed. Here we combine mooring times series, ship-based measurements and remote sensing to assess the variability and forcing factors of vertical fluxes of particulate organic carbon (POC) across the Mackenzie Shelf in 2009. We developed a geospatial model of these fluxes to proceed to an integrative analysis of their determinants in summer. Flux data were obtained with sediment traps moored around 125 m and via a regional empirical algorithm applied to particle size distributions (17 classes from 0.08–4.2 mm) measured by an Underwater Vision Profiler 5. The low fractal dimension (i.e., porous, fluffy particles) derived from the algorithm (1.26 ± 0.34) and the dominance (~ 77%) of rapidly sinking small aggregates (< 0.5 mm) in total fluxes suggested that settling material was the product of recent aggregation processes between marine detritus, gel-like substances, and ballast minerals. Modeled settling velocity of small and large aggregates was, respectively, higher and lower than in previous studies within which a high fractal dimension (i.e., more compact particles) was consequential of deep-trap collection (~400–1300 m). Redundancy analyses and forward selection of abiotic/biotic parameters, linear trends, and spatial structures (i.e., principal coordinates of neighbor matrices, PCNM) were conducted to partition the variation of the 17 POC flux size classes. Flux variability was explained at 69.5% by the addition of a temporal trend, 7 significant PCNM, and 9 biophysical variables. The first PCNM canonical axis (44.5% of spatial variance) reflected the total magnitude of POC fluxes through a shelf-basin gradient controlled by bottom depth and sea ice concentration (p < 0.01). The second most important spatial structure (5.0%) corresponded to areas where shelf break upwelling is known to occur under easterlies and where phytoplankton was dominated by diatoms. Among biophysical parameters, bacterial production and northeasterly wind (upwelling-favorable) were the two strongest corollaries of POC fluxes (r² cum. = 0.37). Bacteria were correlated with small aggregates, while northeasterly wind was associated with large size classes (> 1 mm ESD), but these two factors were weakly related with each other. Copepod biomass was overall negatively correlated (p < 0.05) with vertical POC fluxes, implying that metazoans acted as regulators of export fluxes, even if their role was minor given that our study spanned the onset of diapause. Our results demonstrate that on interior Arctic shelves where productivity is low in mid-summer, localized upwelling zones (nutrient enrichment) may result in the formation of large filamentous phytoaggregates that are not substantially retained by copepod and bacterial communities.

Interconnectivity vs. isolation of prokaryotic communities in European deep-sea mud volcanoes

2013-05-02T00:00:00+02:00

Interconnectivity vs. isolation of prokaryotic communities in European deep-sea mud volcanoes

Biogeosciences, 10, 2821-2831, 2013

Author(s): M. G. Pachiadaki and K. A. Kormas

During the past two decades, European cold seep ecosystems have attracted the scientific interest and to date there are several studies which have investigated the community structure and biodiversity of individual sites. In order to gain a better insight into the biology, biodiversity, and biogeography of seep-associated microbial communities along Europe's continental margins, a comparative approach was applied in the present work. By exploiting the publicly available data on 16S rRNA gene sequences retrieved from sediments of the Håkon Mosby mud volcano, Gulf of Cádiz and the eastern Mediterranean mud volcanoes/pockmarks (Anaximander area and Nile Fan), we investigated the prokaryotic biological components connecting these geographically isolated systems. The construction of interaction networks for both archaeal and bacterial shared operational taxonomic units (OTUs) among the different sites, revealed the presence of persistent OTUs, which can be considered as "key-players". One archaeal OTU (HQ588641) belonging to the ANME-3 group and one δ-Proteobacteria (HQ588562) were found in all five investigated areas. Other Archaea OTUs shared between four sites or less, belonged to the ANME-2c, -2a, MBG-D, -B and Thaumarchaeota. All other shared Bacteria belonged to the δ- and γ-Proteobacteria, with the exception of one JS1 affiliate OTU. The distribution of the majority of the shared OTUs seems to be restricted in cold seeps, mud volcanoes and other marine methane-rich environments. Although the investigated sites were connected through a small number of OTUs, these microorganisms hold central ecophysiological roles in these sediments, namely methane- and sulfur-mediated mineralization.

The real limits to marine life: a further critique of the Respiration Index

2013-05-02T00:00:00+02:00

The real limits to marine life: a further critique of the Respiration Index

Biogeosciences, 10, 2815-2819, 2013

Author(s): B. A. Seibel and J. J. Childress

The recently proposed "Respiration Index" (RI = log PO₂/PCO₂) suggests that aerobic metabolism is limited by the ratio of reactants (oxygen) to products (carbon dioxide) according to the thermodynamics of cellular respiration. Here, we demonstrate further that, because of the large standard free energy change for organic carbon oxidation (ΔG° = −686 kcal mol⁻¹), carbon dioxide can never reach concentrations that would limit the thermodynamics of this reaction. A PCO₂ to PO₂ ratio of 10⁵⁰³ would be required to reach equilibrium (equilibrium constant, K_eq = 10⁵⁰³), where ΔG = 0. Thus, a Respiration Index of −503 would be the real thermodynamic limit to aerobic life. Such a Respiration Index is never reached, either in the cell or in the environment. Moreover, cellular respiration and oxygen provision are kinetically controlled such that, within limits, environmental oxygen and CO₂ concentrations have little to do with intracellular concentrations. The RI is fundamentally different from the aragonite saturation state, a thermodynamic index used to quantify the potential effect of CO₂ on calcification rates, because of its failure to incorporate the equilibrium constant of the reaction. Not only is the RI invalid, but its use leads to incorrect and misleading predictions of the threat of changing oxygen and carbon dioxide to marine life. We provide a physiological framework that identifies oxygen thresholds and allows for synergistic effects of ocean acidification and global warming.

How do tree competition and stand dynamics lead to spatial patterns in monospecific mangroves?

2013-04-30T00:00:00+02:00

How do tree competition and stand dynamics lead to spatial patterns in monospecific mangroves?

Biogeosciences, 10, 2803-2814, 2013

Author(s): M. N. I. Khan, S. Sharma, U. Berger, N. Koedam, F. Dahdouh-Guebas, and A. Hagihara

Information on mangrove stand development is rare because long-term monitoring data is often lacking. Such information is important in order to plan management measures effectively. Novel approaches based on existing datasets are required to bridge this gap of knowledge. This study uses a unique combination of field data analyses with simulation experiments in order to demonstrate how information on mangrove dynamics can be extracted if data are sparse. The paper provides a baseline characterization of stand development in a monospecific pioneer mangrove stand of Kandelia obovata. Point pattern analyses revealed that in the young stage, self-thinning has started but has not yet lead to a regularity of spatial tree distribution in the entire stand, and trees located in smaller clumps hinder each other in growth but do not lead to a significant size class differentiation. However, after ca. 2 decades the self-thinning and the size class differentiation start to become more visible. A mutual inhibition of growth was observed within 2 m circular distance (r) in the young stage and within 3 m distance after two decades of stand development as confirmed by the negative values of mark correlation function. As a stand grows older the spatial pattern of individuals become more regular from a clustered pattern. In order to understand and predict the future stand development, simulation experiments were carried out by means of the individual-based model KiWi.

Water column distribution and carbon isotopic signal of cholesterol, brassicasterol and particulate organic carbon in the Atlantic sector of the Southern Ocean

2013-04-29T00:00:00+02:00

Water column distribution and carbon isotopic signal of cholesterol, brassicasterol and particulate organic carbon in the Atlantic sector of the Southern Ocean

Biogeosciences, 10, 2787-2801, 2013

Author(s): A.-J. Cavagna, F. Dehairs, S. Bouillon, V. Woule-Ebongué, F. Planchon, B. Delille, and I. Bouloubassi

The combination of concentrations and δ¹³C signatures of Particulate Organic Carbon (POC) and sterols provides a powerful approach to study ecological and environmental changes in both the modern and ancient ocean. We applied this tool to study the biogeochemical changes in the modern ocean water column during the BONUS-GoodHope survey (February–March 2008) from Cape Basin to the northern part of the Weddell Gyre. Cholesterol and brassicasterol were chosen as ideal biomarkers of the heterotrophic and autotrophic carbon pools, respectively, because of their ubiquitous and relatively refractory nature.

We document depth distributions of concentrations (relative to bulk POC) and δ¹³C signatures of cholesterol and brassicasterol combined with CO₂ aq. surface concentration variation. While the relationship between CO₂ aq. and δ¹³C of bulk POC and biomarkers have been reported by others for the surface water, our data show that this persists in mesopelagic and deep waters, suggesting that δ¹³C signatures of certain biomarkers in the water column could be applied as proxies for surface water CO₂ aq. We observed a general increase in sterol δ¹³C signatures with depth, which is likely related to a combination of particle size effects, selective feeding on larger cells by zooplankton, and growth rate related effects. Our data suggest a key role of zooplankton fecal aggregates in carbon export for this part of the Southern Ocean (SO). Additionally, in the southern part of the transect south of the Polar Front (PF), the release of sea-ice algae during the ice demise in the Seasonal Ice Zone (SIZ) is hypothesized to influence the isotopic signature of sterols in the open ocean. Overall, the combined use of δ¹³C values and concentrations measurements of both bulk organic C and specific sterols throughout the water column offers the promising potential to explore the recent history of plankton and the fate of organic matter in the SO.

Seasonal and spatial comparisons of phytoplankton growth and mortality rates due to microzooplankton grazing in the northern South China Sea

2013-04-29T00:00:00+02:00

Seasonal and spatial comparisons of phytoplankton growth and mortality rates due to microzooplankton grazing in the northern South China Sea

Biogeosciences, 10, 2775-2785, 2013

Author(s): B. Chen, L. Zheng, B. Huang, S. Song, and H. Liu

We conducted a comprehensive investigation on the microzooplankton herbivory effect on phytoplankton in the northern South China Sea (SCS) using the seawater dilution technique at surface and deep chlorophyll maximum (DCM) layers on two cruises (July–August of 2009 and January of 2010). We compared vertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal (summer vs. winter) differences of phytoplankton growth (μ₀) and microzooplankton grazing rates (m). During summer, both μ₀ and m were significantly higher at the surface than at the DCM layer, which was below the mixed layer. During winter, surface μ₀ was significantly higher than at the DCM, while m was not significantly different between the two layers, both of which were within the mixed layer. Surface μ₀ was, on average, significantly higher in summer than in winter, while average surface m was not different between the two seasons. There were no cross-shelf gradients of μ₀ in summer or winter surface waters. In surface waters, μ₀ was not correlated with ambient nitrate concentrations, and the effect of nutrient enrichment on phytoplankton growth was not pronounced. There was a decreasing trend of m from shelf to basin surface waters in summer, but not in winter. Microzooplankton grazing effect on phytoplankton (m/μ₀) was relatively small in the summer basin waters, indicating a decoupling of microzooplankton grazing and phytoplankton growth at this time. On average, microzooplankton grazed 73% and 65% of the daily primary production in summer and winter, respectively.

UV/PAR radiation and DOM properties in surface coastal waters of the Canadian shelf of the Beaufort Sea during summer 2009

2013-04-26T00:00:00+02:00

UV/PAR radiation and DOM properties in surface coastal waters of the Canadian shelf of the Beaufort Sea during summer 2009

Biogeosciences, 10, 2761-2774, 2013

Author(s): J. Para, B. Charrière, A. Matsuoka, W. L. Miller, J. F. Rontani, and R. Sempéré

Surface waters from the Beaufort Sea in the Arctic Ocean were evaluated for dissolved organic carbon (DOC), and optical characteristics including UV (ultraviolet) radiation and PAR (photosynthetically active radiation) diffuse attenuation (K_d), and chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) as part of the MALINA field campaign (30 July to 27 August). Spectral absorption coefficients (a_CDOM (350 nm) (m⁻¹)) were significantly correlated to both diffuse attenuation coefficients (K_d) in the UV-A and UV-B and to DOC concentrations. This indicates CDOM as the dominant attenuator of both UV and PAR solar radiation and suggests its use as an optical proxy for DOC concentrations in this region. While the Mackenzie input is the main driver of CDOM dynamics in low salinity waters, locally, primary production can create significant increases in CDOM. Extrapolating CDOM to DOC relationships, we estimate that ∼16% of the DOC in the Mackenzie River does not absorb radiation at 350 nm. The discharges of DOC and its chromophoric subset (CDOM) by the Mackenzie River during the MALINA cruise are estimated as ∼0.22 TgC and 0.18 TgC, respectively. Three dissolved fluorescent components (C1–C3) were identified by fluorescence excitation/emission matrix spectroscopy (EEMS) and parallel factor (PARAFAC) analysis. Our results showed an aquatic dissolved organic matter (DOM) component (C1), probably produced in the numerous lakes of the watershed, that co-dominated with a terrestrial humic-like component (C2) in the Mackenzie Delta Sector. This aquatic DOM could partially explain the high CDOM spectral slopes observed in the Beaufort Sea.

Spatial variability of particle-attached and free-living bacterial diversity in surface waters from the Mackenzie River to the Beaufort Sea (Canadian Arctic)

2013-04-25T00:00:00+02:00

Spatial variability of particle-attached and free-living bacterial diversity in surface waters from the Mackenzie River to the Beaufort Sea (Canadian Arctic)

Biogeosciences, 10, 2747-2759, 2013

Author(s): E. Ortega-Retuerta, F. Joux, W. H. Jeffrey, and J. F. Ghiglione

We explored the patterns of total and active bacterial community structure in a gradient covering surface waters from the Mackenzie River to the coastal Beaufort Sea in the Canadian Arctic Ocean, with a particular focus on free-living (FL) vs. particle-attached (PA) communities. Capillary electrophoresis–single-strand conformation polymorphism (CE-SSCP) showed significant differences when comparing river, coast and open sea bacterial community structures. In contrast to the river and coastal waters, total (16S rDNA-based) and active (16S rRNA-based) communities in the open sea samples were not significantly different, suggesting that most present bacterial groups were equally active in this area. Additionally, we observed significant differences between PA and FL bacterial community structure in the open sea, but similar structure in the two fractions for coastal and river samples. Direct multivariate statistical analyses showed that total community structure was mainly driven by salinity (a proxy of dissolved organic carbon and chromophoric dissolved organic matter), suspended particles, amino acids and chlorophyll a. Pyrosequencing of 16S rRNA genes from selected samples confirmed significant differences between river, coastal and sea samples. The PA fraction was only different (15.7% similarity) from the FL one in the open sea sample. Furthermore, PA samples generally showed higher diversity (Shannon, Simpson and Chao indices) than FL samples. At the class level, Opitutae was most abundant in the PA fraction of the sea sample, followed by Flavobacteria and Gammaproteobacteria, while the FL sea sample was dominated by Alphaproteobacteria. Finally, for the coast and river samples and both PA and FL fractions, Betaproteobacteria, Alphaproteobacteria and Actinobacteria were dominant. These results highlight the coexistence of particle specialists and generalists and the role of particle quality in structuring bacterial communities in the area. These results may also serve as a basis to predict further changes in bacterial communities should climate change lead to further increases in river discharge and related particle loads.

Multi-scale interactions between local hydrography, seabed topography, and community assembly on cold-water coral reefs

2013-04-24T00:00:00+02:00

Multi-scale interactions between local hydrography, seabed topography, and community assembly on cold-water coral reefs

Biogeosciences, 10, 2737-2746, 2013

Author(s): L.-A. Henry, J. Moreno Navas, and J. M. Roberts

We investigated how interactions between hydrography, topography and species ecology influence the assembly of species and functional traits across multiple spatial scales of a cold-water coral reef seascape. In a novel approach for these ecosystems, we used a spatially resolved complex three-dimensional flow model of hydrography to help explain assembly patterns. Forward-selection of distance-based Moran's eigenvector mapping (dbMEM) variables identified two submodels of spatial scales at which communities change: broad-scale (across reef) and fine-scale (within reef). Variance partitioning identified bathymetric and hydrographic gradients important in creating broad-scale assembly of species and traits. In contrast, fine-scale assembly was related more to processes that created spatially autocorrelated patches of fauna, such as philopatric recruitment in sessile fauna, and social interactions and food supply in scavenging detritivores and mobile predators. Our study shows how habitat modification of reef connectivity and hydrography by bottom fishing and renewable energy installations could alter the structure and function of an entire cold-water coral reef seascape.

Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the central Baltic Sea

2013-04-23T00:00:00+02:00

Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the central Baltic Sea

Biogeosciences, 10, 2725-2735, 2013

Author(s): M. Blumenberg, C. Berndmeyer, M. Moros, M. Muschalla, O. Schmale, and V. Thiel

The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification has developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and it controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Some individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and from soil bacteria (adenosylhopane) through allochthonous input after the Littorina transgression, whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a positive correlation of BHP abundances with C_org and δ¹⁵N.

Factors challenging our ability to detect long-term trends in ocean chlorophyll

2013-04-23T00:00:00+02:00

Factors challenging our ability to detect long-term trends in ocean chlorophyll

Biogeosciences, 10, 2711-2724, 2013

Author(s): C. Beaulieu, S. A. Henson, Jorge L. Sarmiento, J. P. Dunne, S. C. Doney, R. R. Rykaczewski, and L. Bopp

Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll a concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean and Land Colour Instrument (OLCI) expected in 2014 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll a, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll a. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies.

Coccolithophore surface distributions in the North Atlantic and their modulation of the air-sea flux of CO₂ from 10 years of satellite Earth observation data

2013-04-23T00:00:00+02:00

Coccolithophore surface distributions in the North Atlantic and their modulation of the air-sea flux of CO₂ from 10 years of satellite Earth observation data

Biogeosciences, 10, 2699-2709, 2013

Author(s): J. D. Shutler, P. E. Land, C. W. Brown, H. S. Findlay, C. J. Donlon, M. Medland, R. Snooke, and J. C. Blackford

Coccolithophores are the primary oceanic phytoplankton responsible for the production of calcium carbonate (CaCO₃). These climatically important plankton play a key role in the oceanic carbon cycle as a major contributor of carbon to the open ocean carbonate pump (~50%) and their calcification can affect the atmosphere-to-ocean (air-sea) uptake of carbon dioxide (CO₂) through increasing the seawater partial pressure of CO₂ (pCO₂). Here we document variations in the areal extent of surface blooms of the globally important coccolithophore, Emiliania huxleyi, in the North Atlantic over a 10-year period (1998–2007), using Earth observation data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). We calculate the annual mean sea surface areal coverage of E. huxleyi in the North Atlantic to be 474 000 ± 104 000 km², which results in a net CaCO₃ carbon (CaCO₃-C) production of 0.14–1.71 Tg CaCO₃-C per year. However, this surface coverage (and, thus, net production) can fluctuate inter-annually by −54/+8% about the mean value and is strongly correlated with the El Niño/Southern Oscillation (ENSO) climate oscillation index (r=0.75, p<0.02). Our analysis evaluates the spatial extent over which the E. huxleyi blooms in the North Atlantic can increase the pCO₂ and, thus, decrease the localised air-sea flux of atmospheric CO₂. In regions where the blooms are prevalent, the average reduction in the monthly air-sea CO₂ flux can reach 55%. The maximum reduction of the monthly air-sea CO₂ flux in the time series is 155%. This work suggests that the high variability, frequency and distribution of these calcifying plankton and their impact on pCO₂ should be considered if we are to fully understand the variability of the North Atlantic air-to-sea flux of CO₂. We estimate that these blooms can reduce the annual N. Atlantic net sink atmospheric CO₂ by between 3–28%.

Analysis of a 39-year continuous atmospheric CO₂ record from Baring Head, New Zealand

2013-04-23T00:00:00+02:00

Analysis of a 39-year continuous atmospheric CO₂ record from Baring Head, New Zealand

Biogeosciences, 10, 2683-2697, 2013

Author(s): B. B. Stephens, G. W. Brailsford, A. J. Gomez, K. Riedel, S. E. Mikaloff Fletcher, S. Nichol, and M. Manning

We present an analysis of a 39-year record of continuous atmospheric CO₂ observations made at Baring Head, New Zealand, filtered for steady background CO₂ mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO₂ time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended episodes of strong southerly winds with low CO₂ variability. The filtered Baring Head CO₂ record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO₂ fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO₂ growth rate in the Baring Head record closely match the El Niño-Southern Oscillation, reflecting the global reach of CO₂ mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO₂ record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern Hemisphere fossil-fuel burning and Southern Ocean CO₂ uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO₂ uptake, as a result of increased zonal winds, is too small to be detectable as significant differences in atmospheric CO₂ between mid to high latitude Southern Hemisphere observing stations.

Nitrous oxide emissions from European agriculture – an analysis of variability and drivers of emissions from field experiments

2013-04-22T00:00:00+02:00

Nitrous oxide emissions from European agriculture – an analysis of variability and drivers of emissions from field experiments

Biogeosciences, 10, 2671-2682, 2013

Author(s): R. M. Rees, J. Augustin, G. Alberti, B. C. Ball, P. Boeckx, A. Cantarel, S. Castaldi, N. Chirinda, B. Chojnicki, M. Giebels, H. Gordon, B. Grosz, L. Horvath, R. Juszczak, Å. Kasimir Klemedtsson, L. Klemedtsson, S. Medinets, A. Machon, F. Mapanda, J. Nyamangara, J. E. Olesen, D. S. Reay, L. Sanchez, A. Sanz Cobena, K. A. Smith, A. Sowerby, M. Sommer, J. F. Soussana, M. Stenberg, C. F. E. Topp, O. van Cleemput, A. Vallejo, C. A. Watson, and M. Wuta

Nitrous oxide emissions from a network of agricultural experiments in Europe were used to explore the relative importance of site and management controls of emissions. At each site, a selection of management interventions were compared within replicated experimental designs in plot-based experiments. Arable experiments were conducted at Beano in Italy, El Encin in Spain, Foulum in Denmark, Logården in Sweden, Maulde in Belgium, Paulinenaue in Germany, and Tulloch in the UK. Grassland experiments were conducted at Crichton, Nafferton and Peaknaze in the UK, Gödöllö in Hungary, Rzecin in Poland, Zarnekow in Germany and Theix in France. Nitrous oxide emissions were measured at each site over a period of at least two years using static chambers. Emissions varied widely between sites and as a result of manipulation treatments. Average site emissions (throughout the study period) varied between 0.04 and 21.21 kg N₂O-N ha⁻¹ yr⁻¹, with the largest fluxes and variability associated with the grassland sites. Total nitrogen addition was found to be the single most important determinant of emissions, accounting for 15% of the variance (using linear regression) in the data from the arable sites (p < 0.0001), and 77% in the grassland sites. The annual emissions from arable sites were significantly greater than those that would be predicted by IPCC default emission factors. Variability of N₂O emissions within sites that occurred as a result of manipulation treatments was greater than that resulting from site-to-site and year-to-year variation, highlighting the importance of management interventions in contributing to greenhouse gas mitigation.

Major consequences of an intense dense shelf water cascading event on deep-sea benthic trophic conditions and meiofaunal biodiversity

2013-04-22T00:00:00+02:00

Major consequences of an intense dense shelf water cascading event on deep-sea benthic trophic conditions and meiofaunal biodiversity

Biogeosciences, 10, 2659-2670, 2013

Author(s): A. Pusceddu, M. Mea, M. Canals, S. Heussner, X. Durrieu de Madron, A. Sanchez-Vidal, S. Bianchelli, C. Corinaldesi, A. Dell'Anno, L. Thomsen, and R. Danovaro

Numerous submarine canyons around the world are preferential conduits for episodic dense shelf water cascading (DSWC), which quickly modifies physical and chemical ambient conditions while transporting large amounts of material towards the base of slope and basin. Observations conducted during the last 20 yr in the Lacaze-Duthiers and Cap de Creus canyons (Gulf of Lion, NW Mediterranean Sea) report several intense DSWC events. The effects of DSWC on deep-sea ecosystems are almost unknown. To investigate the effects of these episodic events, we analysed changes in the meiofaunal biodiversity inside and outside the canyon. Sediment samples were collected at depths varying from ca. 1000 to > 2100 m in May 2004 (before a major event), April 2005 (during a major cascading event) and in October 2005, August 2006, April 2008 and April 2009 (after a major event). We report here that the late winter–early spring 2005 cascading led to a reduction of the organic matter contents in canyon floor sediments down to 1800 m depth, whereas surface sediments at about 2200 m depth showed an increase. Our findings suggest that the nutritional material removed from the shallower continental shelf, canyon floor and flanks, and also the adjacent open slope was rapidly transported to the deep margin. During the cascading event the meiofaunal abundance and biodiversity in the studied deep-sea sediments were significantly lower than after the event. Benthic assemblages during the cascading were significantly different from those in all other sampling periods in both the canyon and deep margin. After only six months from the cessation of the cascading, benthic assemblages in the impacted sediments were again similar to those observed in other sampling periods, thus illustrating a quick recovery. Since the present climate change is expected to increase the intensity and frequency of these episodic events, we anticipate that they will increasingly affect benthic bathyal ecosystems, which may eventually challenge their resilience.

Corrigendum to "Copepod community growth rates in relation to body size, temperature, and food availability in the East China Sea: a test of metabolic theory of ecology" published in Biogeoscience, 10, 1877–1892, 2013

2013-04-19T00:00:00+02:00

Corrigendum to "Copepod community growth rates in relation to body size, temperature, and food availability in the East China Sea: a test of metabolic theory of ecology" published in Biogeoscience, 10, 1877–1892, 2013

Biogeosciences, 10, 2657-2657, 2013

Author(s): K. Y. Lin, A. R. Sastri, G. C. Gong, and C. H. Hsieh

No abstract available.

Corrigendum to "Topo-edaphic controls over woody plant biomass in South African savannas" published in Biogeosciences, 9, 1809–1821, 2012

2013-04-19T00:00:00+02:00

Corrigendum to "Topo-edaphic controls over woody plant biomass in South African savannas" published in Biogeosciences, 9, 1809–1821, 2012

Biogeosciences, 10, 2655-2655, 2013

Author(s): M. S. Colgan, G. P. Asner, S. R. Levick, R. E. Martin, and O. A. Chadwick

No abstract available.

Impact of global change on coastal oxygen dynamics and risk of hypoxia

2013-04-19T00:00:00+02:00

Impact of global change on coastal oxygen dynamics and risk of hypoxia

Biogeosciences, 10, 2633-2653, 2013

Author(s): L. Meire, K. E. R. Soetaert, and F. J. R. Meysman

Climate change and changing nutrient loadings are the two main aspects of global change that are linked to the increase in the prevalence of coastal hypoxia – the depletion of oxygen in the bottom waters of coastal areas. However, it remains uncertain how strongly these two drivers will each increase the risk of hypoxia over the next decades. Through model simulations we have investigated the relative influence of climate change and nutrient run-off on the bottom water oxygen dynamics in the Oyster Grounds, an area in the central North Sea experiencing summer stratification. Simulations were performed with a one-dimensional ecosystem model that couples hydrodynamics, pelagic biogeochemistry and sediment diagenesis. Climatological conditions for the North Sea over the next 100 yr were derived from a global-scale climate model. Our results indicate that changing climatological conditions will increase the risk of hypoxia. The bottom water oxygen concentration in late summer is predicted to decrease by 24 μM or 11.5% in the year 2100. More intense stratification is the dominant factor responsible for this decrease (58%), followed by the reduced solubility of oxygen at higher water temperature (27%), while the remaining part could be attributed to enhanced metabolic rates in warmer bottom waters (15%). Relative to these climate change effects, changes in nutrient runoff are also important and may even have a stronger impact on the bottom water oxygenation. Decreased nutrient loadings strongly decrease the probability of hypoxic events. This stresses the importance of continued eutrophication management in coastal areas, which could function as a mitigation tool to counteract the effects of rising temperatures.

Temporal variation of nitrate and phosphate transport in headwater catchments: the hydrological controls and land use alteration

2013-04-18T00:00:00+02:00

Temporal variation of nitrate and phosphate transport in headwater catchments: the hydrological controls and land use alteration

Biogeosciences, 10, 2617-2632, 2013

Author(s): T.-Y. Lee, J.-C. Huang, S.-J. Kao, and C.-P. Tung

Oceania rivers are hotspots of DIN (dissolved inorganic nitrogen) and DIP (dissolved inorganic phosphorus) transport due to humid/warm climate, typhoon-induced episodic rainfall and high tectonic activity that create an environment favorable for high/rapid runoff and soil erosion. In spite of its uniqueness, effects of hydrologic controls and land use on the transport behaviors of DIN and DIP are rarely documented. A 2 yr monitoring study for DIN and DIP from three headwater catchments with different cultivation gradient (0 To 8.9%) was implemented during a ~ 3 day interval with an additional monitoring campaign at a 3 h interval during typhoon periods. Results showed the DIN yields in the pristine, moderately cultivated (2.7%), and intensively cultivated (8.9%) watersheds were 8.3, 26, and 37 kg N ha⁻¹ yr⁻¹, respectively. For the DIP yields, they were 0.36, 0.35, and 0.56 kg P ha⁻¹ yr⁻¹, respectively. Higher year-round DIN concentrations and five times larger in DIN yields in intensively cultivated watersheds indicate DIN is more sensitive to land use changes. The high background DIN yield from the relatively pristine watershed was likely due to high atmospheric nitrogen deposition and large subterranean N pool. The correlations between runoff and concentration reveals that typhoon floods purge out more DIN from the subterranean reservoir, i.e., soil, by contrast, runoff washes off surface soil resulting in higher suspended sediment with higher DIP. Collectively, typhoon runoff contributes 20–70% and 47–80%, respectively, to the annual DIN and DIP exports. The DIN yield to DIP yield ratio varied from 97 to 410, which is higher than the global mean of ~ 18. Such a high ratio indicates a P-limiting condition in stream and the downstream aquatic environment. Based on our field observation, we constructed a conceptual model illustrating different remobilization mechanisms for DIN and DIP from headwaters in a mountainous river, which is analogous to typical Oceania rivers and the headwater of large rivers in similar climate zones. Our study advanced our understanding about the role of cyclones, which exert hydrological control, and land use on nutrient export in the Oceania region, benefiting watershed management under the context of climate change.

Seasonal, daily and diel N₂ effluxes in permeable carbonate sediments

2013-04-18T00:00:00+02:00

Seasonal, daily and diel N₂ effluxes in permeable carbonate sediments

Biogeosciences, 10, 2601-2615, 2013

Author(s): B. D. Eyre, I. R. Santos, and D. T. Maher

Benthic metabolism and inorganic nitrogen and N₂ flux rates (denitrification) were measured in permeable carbonate sands from Heron Island (Great Barrier Reef). Some of the N₂ flux rates were among the highest measured in sediments. All benthic fluxes showed a significant difference between seasons with higher rates in summer and late summer. There was no distinct response of the benthic system to mass coral spawning. Instead, changes in benthic fluxes over 12 days in summer appear to be driven by tidal changes in water depth and associated changes in phytosynthetically active radiation reaching the sediments. Dark N₂ fluxes were strongly correlated to benthic oxygen consumption across all sites and seasons (r² = 0.63; p < 0.005; slope = 0.035). However, there were seasonal differences with a steeper slope in summer than winter, reflecting either more efficient coupling between respiration and nitrification–denitrification at higher temperatures or different sources of organic matter. Adding data from published studies on carbonate sands revealed two slopes in the dark N₂ flux versus benthic oxygen consumption relationship. The lower slope (0.035) was most likely due to high carbon : nitrogen (C : N) organic matter from coral reefs, and associated assimilation of nitrogen by heterotrophic bacteria including enhanced heterotrophic N-fixation, but competition by benthic microalgae or inefficient coupling between respiration and nitrification–denitrification cannot be excluded. The steeper slope (0.089) was most likely due to respiration being driven by low C : N phytodetritus. If the different slopes were driven by the sources of organic matter, then global estimates of continental shelf denitrification are probably about right. In contrast, global estimates of continental shelf denitrification may be over-estimated if the low slope was due to inefficient coupling between respiration and nitrification–denitrification and also due to reduced N₂ effluxes in the light associated with competition by benthic microalgae for nitrogen and N-fixation.

Impacts of dust deposition on dissolved trace metal concentrations (Mn, Al and Fe) during a mesocosm experiment

2013-04-18T00:00:00+02:00

Impacts of dust deposition on dissolved trace metal concentrations (Mn, Al and Fe) during a mesocosm experiment

Biogeosciences, 10, 2583-2600, 2013

Author(s): K. Wuttig, T. Wagener, M. Bressac, A. Dammshäuser, P. Streu, C. Guieu, and P. L. Croot

The deposition of atmospheric dust is the primary process supplying trace elements abundant in crustal rocks (e.g. Al, Mn and Fe) to the surface ocean. Upon deposition, the residence time in surface waters for each of these elements differs according to their chemical speciation and biological utilization. Presently, however, the chemical and physical processes occurring after atmospheric deposition are poorly constrained, principally because of the difficulty in following natural dust events in situ. In the present work we examined the temporal changes in the biogeochemistry of crustal metals (in particular Al, Mn and Fe) after an artificial dust deposition event. The experiment was contained inside trace metal clean mesocosms (0–12.5 m depths) deployed in the surface waters of the northwestern Mediterranean, close to the coast of Corsica within the frame of the DUNE project (a DUst experiment in a low Nutrient, low chlorophyll Ecosystem). Two consecutive artificial dust deposition events, each mimicking a wet deposition of 10 g m⁻² of dust, were performed during the course of this DUNE-2 experiment. The changes in dissolved manganese (Mn), iron (Fe) and aluminum (Al) concentrations were followed immediately after the seeding with dust and over the following week. The Mn, Fe and Al inventories and loss or dissolution rates were determined. The evolution of the inventories after the two consecutive additions of dust showed distinct behaviors for dissolved Mn, Al and Fe. Even though the mixing conditions differed from one seeding to the other, Mn and Al showed clear increases directly after both seedings due to dissolution processes. Three days after the dust additions, Al concentrations decreased as a consequence of scavenging on sinking particles. Al appeared to be highly affected by the concentrations of biogenic particles, with an order of magnitude difference in its loss rates related to the increase of biomass after the addition of dust. In the case of dissolved Fe, it appears that the first dust addition resulted in a decrease as it was scavenged by sinking dust particles, whereas the second seeding induced dissolution of Fe from the dust particles due to the excess Fe binding ligand concentrations present at that time. This difference, which might be related to a change in Fe binding ligand concentration in the mesocosms, highlights the complex processes that control the solubility of Fe. Based on the inventories at the mesocosm scale, the estimations of the fractional solubility of metals from dust particles in seawater were 1.44 ± 0.19% and 0.91 ± 0.83% for Al and 41 ± 9% and 27 ± 19% for Mn for the first and the second dust addition. These values are in good agreement with laboratory-based estimates. For Fe no fractional solubility was obtained after the first seeding, but 0.12 ± 0.03% was estimated after the second seeding. Overall, the trace metal dataset presented here makes a significant contribution to enhancing our knowledge on the processes influencing trace metal release from Saharan dust and the subsequent processes of bio-uptake and scavenging in a low nutrient, low chlorophyll area.

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