Low planktic foraminiferal diversity and abundance observed in a 1 2013 West-East Mediterranean Sea transect

13 Planktic foraminifera were collected with 150 μm BONGO nets from the upper 200 m water depth at 20 14 stations across the Mediterranean Sea between 02 May and 02 June, 2013. The main aim was to 15 characterize the species distribution and size-normalized shell weight (SNW). Average foraminifera 16 abundances and diversity are 1.42 ±1.43 ind.∙10 m (ranging from 0.11 to 5.20 ind.∙10 m), with ten 17 overall species found, respectively. Large differences in species assemblages and abundance values are 18 observed between the different Mediterranean sub-basins, with an overall dominance of spinose, 19 symbiont-bearing species indicating oligotrophic conditions. The highest values in absolute abundance 20 are in the Strait of Gibraltar and Alboran Sea. The western basin is dominated by Globorotalia inflata and 21 Globigerina bulloides at slightly lower standing stocks than in the eastern basin. In contrast, the planktic 22 foraminiferal assemblage in the warmer, saltier and more nutrient-limited eastern basin is dominated by 23 Globigerinoides ruber sensu stricto (s.s.). These new collective results in combination with comparison to 24 previous findings, suggest that stratification of the surface water column, nutrient concentration and hence 25 food availability, and temperature seem to be the main factors controlling foraminiferal abundances and 26 distribution. Standing stocks and size-normalized weight (SNW) of G. ruber s.s. and G. bulloides seem 27 more related to food availability than seawater carbonate chemistry. Increasing temperature, salinity, 28 surface ocean stratification and trophic conditions could be the causes of reduced abundance, diversity 29 and species-specific changes in calcification in planktic foraminifera. 30


Introduction
The single-celled foraminifera comprise the most diverse group of calcareous zooplankton of the modern ocean.The majority of foraminifer species are benthic.About 50 morphospecies are planktic, which have Globigerinoides sacculifer of the trilobus-type (on average 0.13 ind.•10 m -3 ; 9.16 %), is especially notable at the Strait of Gibraltar (50.91 %).O. universa is ubiquitous in the Mediterranean with the exception of the three Stations 6, 9, and 14.Its average abundance is 0.12 ind.•10 m -3 (8.70 %).Its dominant size fractions are ≥350 µm.G. ruber sensu lato (s.l.; 0.09 ind.•10 m -3 ; 6.41 %) is found mostly at the same stations as G. ruber s.s., but is usually less abundant.It is most frequent in the ≥350-500-µm test-size fraction, and some individuals >500 µm are found in the Atlantic (Fig. 3;Fig 4).
To show the relative abundance of the various species, some stations were grouped together to achieve a minimum number of foraminifera (>95 tests); the grouping was set by location proximity in which foraminiferal assemblages were similar.The stations at the Strait of Sicily and the western Mediterranean (Stations 20,21,22) are not shown due to a low number of individuals (< 90; Fig. 5).Some similarities can be seen between the Tyrrhenian Sea and the eastern Mediterranean, and also between the Alboran Sea and the southwestern Mediterranean.The Atlantic and the Ionian-Adriatic-Aegean grouping have similar proportions of species.On the other hand, the rest of the locations have a clearly dominant species (G.ruber s.s. at the Tyrrhenian Sea and the eastern Mediterranean; G. inflata at the Alboran Sea), whereas in the southwestern Mediterranean the dominance is less clear, which might be due to a low number of individuals (G.inflata being the main species followed by G. bulloides as at the Alboran Sea station).G.
sacculifer type trilobus has a high relative abundance in the Atlantic and in the Strait of Gibraltar (being the main and the second most abundant species respectively); elsewhere it is less abundant.G. bulloides is most frequent in the entire western Basin and the Atlantic, being the main species at the Strait of Gibraltar.It is less frequent in the Tyrrhenian Sea, and in the eastern Basin and its sub-basins.G. bulloides contrasts with G. ruber s.s., which always represents a small percentage in the western Mediterranean but dominates the Tyrrhenian Sea and the eastern Basin (Fig. 5; Appendix A).
A Pearson test was applied to the main species to see their relative abundance correlation with temperature, salinity, and fluorescence.The correlations found are: G. ruber s.s. is positively correlated Biogeosciences Discuss., doi:10.5194/bg-2016-266, 2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.
with temperature and salinity (p = 0.01), and negatively with fluorescence (p = 0.05).G. inflata is positively correlated with fluorescence (p = 0.05) and G. bulloides has a negative correlation with temperature (p = 0.01).Relative abundance was selected instead of absolute abundance to avoid bias due to the big differences between stations' results in absolute abundance.The remaining species did not pass through a Pearson test as they are not present in all the stations, which makes it difficult to assess a relation between abundance and environmental parameters.

2. Size-normalized weight (SNW)
Due to their abundance, G. ruber s.s., G. bulloides, and O. universa where analyzed for their sizenormalized weight (SNW).The high two-dimensional (silhouette) area-to-diameter correlation is best fitted by a power regression (Fig. S2).Comparing the average values from different locations sampled within the Mediterranean, G. ruber s.s.individuals from the Atlantic have the largest size followed by individuals from the Tyrrhenian Sea, and tests from east of the Strait of Sicily.For the other two species G. bulloides and O. universa, the results are statistically not significant, but a similar trend is observed regarding the two basins, with the eastern Mediterranean having the smallest individuals, while the largest individuals occurred in the Atlantic and the northwestern Mediterranean (Fig. S2).The different locations were grouped using the same criteria as in Fig. 5.
Within the Mediterranean, a previous study with comparable results sampled the upper 350 m (Pujol and Grazzini, 1995).For the Alboran Sea, samples were obtained at a similar time of the year (April 1990) with values around 16, 6 and 9 ind.•10m -3 , greater than our Station 3 with 4.14 ind.•10 m -3 .The rest of their samples occurs in a different season of the year and also have notably higher abundances, with larger ones in February than during September-October.Their sampling mean is also higher and approximates to 9.3 ±8.9 ind.•10 m -3 .Regarding Pujol and Grazzini (1995), western Mediterranean abundances are higher than the eastern ones overall, due to more eastern oligotrophic conditions and higher temperature and salinity values that limit foraminiferal production both, during winter and late summer.In concordance with Pujol and Grazzini (1995), no significant differences are observed between samples collected during day and night.
Ten different species are recognized in our study, accounting for a single species (to have comparable results with previous studies) the three varieties of G. ruber: sensu stricto, sensu lato (containing different cryptic species; Aurahs et al., 2009a), and the pink variety.To facilitate comparison, the different G.
Mediterranean, where more species were found: 18 species with Cifelli (1974), and 17 species with Pujol and Grazzini (1995) and with the surface sediments of Thunell (1978).Some of the species not found reach high frequencies in the aforementioned studies: e.g., Turborotalita quinqueloba, Neogloboquadrina pachyderma, and Globorotalia truncatulinoides.The fact that these species were not sampled in the present study may be caused by their absence or presence at extremely low abundances of adult specimens at the sampled stations in May 2013.G. sacculifer type quadrocameratus was not found in previous studies working with plankton tows in the Mediterranean, despite its abundance in sedimentary cores (i.e.Živkovic and Glumac, 2007).
The lower absolute abundance of individuals in our study compared with Pujol and Grazzini (1995), together with low species diversity in the Mediterranean, may indicate a trend of changing conditions in recent years, as it has been reported for temperature and salinity (Yáñez et al., 2010), alkalinity (Cossarini, 2015;Hassoun et al., 2015a), and water mass mixings (Hassoun et al., 2015b).These changing conditions could also imply changes in the ecology and distribution of planktic foraminifera, as discussed below.
The western part of the first leg transect (from the Atlantic to the Strait of Sicily) has a higher percentage of larger size fractions than the eastern part; the main cause of that trend is the species composition.The results are conditioned by the presence of G. inflata (especially in the 350-500 µm fraction) with higher abundances in the west.The same is true for the presence of large O. universa (especially in the >500 µm), plus the contribution of G. siphonifera, which grow largest at stations in which they are most frequent (Fig. 4).

2. Factors controlling the abundance of the main species
This discussion is focused on the five main species of our results.The spinose and symbiont-bearing species: G. ruber, O. universa, and G. sacculifer (always referring to the trilobus type), which mainly inhabit tropical and subtropical waters.G. ruber is found as the main species of the Atlantic.O. universa has a quite cosmopolitan standing stock, also being present in warm transitional Atlantic waters (Bé and Tolderlund, 1971).The spinose and nonsymbiotic species G. bulloides, typical of subpolar and transitional regions as well as upwelling areas, but also found in subtropical and tropical waters at a much lower abundance, highlighting its wide temperature range (Thunell, 1978;Bé and Tolderlund, 1971).The non-spinose species G. inflata is considered indigenous from the transitional region in the Atlantic (Bé and Tolderlund, 1971).
( Kuroyanagi and Kawahata, 2004;Wang, 2000); a reason could be that G. ruber s.l. may be less dependent on symbiont activity than G. ruber s.s.(Kuroyanagi and Kawahata, 2004).Cifelli (1974), where it was by far more abundant in the eastern than the western Mediterranean Basin, clearly being the main species found in the Levantine Basin and the south Ionian Sea; for these two locations it seems present during the different seasons, winter included, which is also true for pink variety of G. ruber (see also Thunell, 1978;Pujol and Grazzini, 1995).The dominance of G. ruber s.s. in the eastern Mediterranean Basin relative to the western Basin causes a strong positive correlation with salinity (p = 0.01) in our data set.Its higher relative abundance in the eastern basin may result from symbiont activity in G. ruber, supporting survival in oligotrophic regions, and some independence from chlorophyll-a and macronutrient concentrations (Watkins et al., 1996).The findings of Watkins et al. (1996) are supported by the negative correlations of standing stocks of G. ruber s.s. and fluorescence data of our study (p = 0.05).
The dominance of G. ruber and abundance peaks in May in the eastern Mediterranean coincides with the positive temperature gradient between Station 9 and Station 13 (16.2-17.3ºC; Fig. 1), being more evident for the G. ruber s.s.than for the G. ruber s.l.morphotype.In late summer, G. ruber experiences its largest expansion and presence owing to warmer temperatures, clearly being the main species from the north of Algeria to the Levantine Basin.G. ruber (pink) is the dominant species at the Strait of Sicily and eastwards (Pujol and Grazzini, 1995), whereas in May it only has residual presence in some locations (especially around Crete).In February, presumably due to temperature decrease, G. ruber (pink) almost disappears from the Mediterranean and the other morphotypes are present in low numbers (Pujol and Grazzini, 1995), suggesting that G. ruber s.s. and s.l.have wider cold temperature ranges than the pink variety.Hydrographic conditions and consequently food availability seem to be the factors limiting more its abundance once it has reached its habitable temperature range.

2. 2. Globorotalia inflata
The presence of G. inflata is related with cool waters and high food availability (Pujol and Grazzini, 1995), following high phosphate concentrations (Ottens, 1992).This explains its higher abundance at the cooler nutrient-rich western basin, and its progressive scarcity in the warmer oligotrophic eastern Mediterranean (Fig. 1;Cifelli, 1974;Thunell, 1978).The same pattern is observed in late summer.From spring to late summer shows a displacement from the eastern Alboran Sea to the northwestern Mediterranean, decreasing frequency at the Algero-Provençal Basin and the southwestern Mediterranean Basin, maintaining the residual presence at the eastern basin.In winter, with cooler temperatures, the opposite process happens, and G. inflata becomes the dominant species at the southwestern basin, with high frequencies in the Strait of Sicily and just east of it.Eastwards its presence is maintained at only residual levels (Pujol and Grazzini, 1995).Its distribution along the seasons shows that G. inflata is scarce or absent in warmer, stratified and nutrient-depleted regions in the Mediterranean.
Despite having similar temperature ranges than the southwestern Mediterranean, G. inflata is absent in the Tyrrhenian Sea and the northwestern Mediterranean, and it had also found to be scarce in June (Cifelli, 1974).In addition, G. inflata shows a positive correlation with fluorescence (p = 0.05), suggesting that food depletion plays a more important role in limiting its distribution than warm temperatures.
Alboran Sea spring distribution of G. inflata, with G. bulloides as a clear secondary species, matches with other studies (Pujol and Grazzini, 1995;Raden et al., 2011).G. inflata peak abundances appear more to the west than those reported by Cifelli (1974) to the east of the Balearic Islands.Those peaks can be associated with nutrient-rich upwelling areas rich in foraminifer prey inside its temperature range (Fig. 1; Fig. 2).

2. 3. Globigerina bulloides
Following Cifelli (1974), G. bulloides is the dominant specie in the Atlantic stations close to the Strait of Gibraltar, whereas in our study it shares presence with other species (Station 1; Fig. 3a).The G. bulloides dominance at the Strait of Gibraltar during late spring-early summer confirms the finding of Cifelli (1974).The abundance peak of G. bulloides at the Strait of Gibraltar coincides with high nutrient concentration and upwelling (Figs. 1, 2, and 3), making Station 2 the most rich in planktic foraminifera of all the transect.This confirms its association with upwelling areas, where phyto-and zooplanktonic blooms control its abundances, as it is an opportunistic species (Pujol and Grazzini, 1995;Sousa et al., 2014).It correlates with fluorescence peaks since it feeds on phytoplankton (Mortyn and Charles, 2003; Fig. 1).
In April (Pujol and Grazzini, 1995;van Raden et al., 2011) and May, it is found to be the second most abundant species, surpassed by G. inflata, in the westernmost Alboran Sea.One month later it is found to be the dominant species displacing G. inflata, which is still dominant in the eastern Alboran Sea (Cifelli, 1974).Its ubiquity and its higher abundance in the western basin with respect to the east is supported by previous studies (i.e., Cifelli, 1974;Thunell, 1978), with a higher difference in abundance in February than in September-October (Pujol and Grazzini, 1995).In late summer, its presence is more secondary, with abundance peaks around the Strait of Sicily and south of Sardinia.Abundance peaks at the same locations plus the Gulf of Lion occur during winter, but with larger absolute abundances (Pujol and Grazzini, 1995).
G. bulloides decreases in abundance when food is depleted, observable in the eastern Mediterranean, where it always has lower absolute abundances than in the west.During spring to late summer in the eastern basin, G. bulloides has a minor presence, being more present at the near east of the Strait of Sicily (Cifelli, 1974;Pujol and Grazzini, 1995).During winter its abundance increases and it becomes the second main species in the Levantine Basin preceded by G. ruber, and also it is one of the main species in the Ionian Sea.Levantine waters have permanent eddies that can help phytoplankton blooms, explaining the presence of G. bulloides in winter (Pujol and Grazzini, 1995).It is noticeable that northwards of the Levantine Basin and the Aegean Sea its abundances are comparable to those in the western basin regarding surface sediment data from Thunell (1978).G. bulloides has more affinity for cooler upwelled waters than warmer more stratified waters (Sousa et al., 2014;Thunell, 1978), being present in subtropical waters only in cooler months (Ottens, 1992).The coldest station of the first leg (Strait of Gibraltar, 14.2 ºC) tracked by BONGO nets coincides with its abundance peak, and it is absent in the warmest station (off the Nile Delta, 17.6 ºC; Fig. 1a), which also is one of the scarcest in foraminiferal prey (Fig. 1c; Fig. 2).Its negative correlation with temperature (p = 0.01) matches with its low abundance in the eastern basin and its higher abundances in the western basin (northwestern basin included, despite its low absolute abundances but being the main species there), and with its seasonal distribution.Its presence and distribution seems to be limited by a combination of low nutrient concentration and limited food availability, caused by stratification of the surface water column, and increased sea surface temperatures (SSTs).

2. 4. Orbulina universa
O. universa was found ubiquitous by Pujol and Grazzini (1995), being present at all the stations and seasons, reaching peak abundances in the southwestern Mediterranean both in late-summer and winter.
Regarding our data, it follows the same pattern during spring, only absent from three stations.No peak area is clear in spring in our data and that of Cifelli (1974), but slightly higher abundances in the western basin compared to the east are modest.That small difference can be caused by more nutrient-rich upwelling areas (Sousa et al., 2014;Morard et al., 2013) in the western basin or by high salinities in the eastern basin.

2. Globigerinoides sacculifer type trilobus
In June, the distribution of G. sacculifer is quite ubiquitous and has 5 % presence at the Strait of Gibraltar (Cifelli, 1974); meanwhile our results show a 25 % presence one month before and absence at seven stations.Also, lower percentages are found in April at the Alboran Sea (Pujol and Grazzini, 1995).In September-October it shows high abundances and is one of the main species from north of Minorca to the southwestern Mediterranean until the Strait of Sicily, where it is rarely found, presumably due to warmer waters than in May, even if this is not supported by our Pearson correlation.In late summer it decreases considerably and progressively eastwards, where the highly dominant G. ruber is maintained as the most important species (Pujol and Grazzini, 1995), probably due to slightly higher temperature and salinity tolerance (see also Bijma et al., 1990).On the other hand, in February G. sacculifer disappears from the north Levantine Basin and its abundances lowers considerably, being a residual species in terms of relative abundance in all the Mediterranean (Pujol and Grazzini, 1995), suggesting temperatures too cold for it.

3. Factors controlling planktic foraminiferal test weight
The size-normalized weight (SNW) of tests of both G. ruber s.s. and G. bulloides are statistically significant, and follow a systematic change from the Atlantic towards the eastern Mediterranean (Fig. 6).
Therefore, the SNW of these two species is interpreted and discussed for environmental effects and Despite the finding that only one out of three genotypes (i.e.Type III, after Darling and Wade, 2008) occurs in the Mediterranean Sea (Mediterranean species, after de Vargas et al., 1999), Weight-area relation data do not show any statistically significant systematic distribution (Fig. S4c).The Mediterranean Type III has been found to include two sub-types, Type IIIa and Type IIIb (André et al., 2014).The different genotypes and morphotypes of O. universa tolerate wide ranges of salinity and temperature in surface waters (e.g., de Vargas et al., 1999).Whereas the various types of O. universa differ in the size of pores (de Vargas et al., 1999;Morard et al., 2009), their pore-size is also affected by environmental conditions including water temperature (e.g., Bé et al., 1973).Likewise, thickness of the test wall has been described to vary between types (de Vargas et al., 1999;Morard et al., 2009), and is as well affected by environmental conditions and ontogenetic stage of specimens.Adult O. universa have been show to continuously add calcite layers to the proximal surface of the same sphere (Spero, 1988;Spero et al., 2015).Since environmental and biological factors may affect individuals of the different genotypes of O. universa to varying degrees, we could not detect any systematic change in SNW in the data presented here.
The various interfering effects, which control the SNW of O. universa in the Mediterranean Sea, may also explain differences in the weight-diameter relation data reported from other regions of the world ocean: Bijma et al. (2002) weighed O. universa of the 500-600 µm size fraction in the Caribbean Sea and report a weight ranging at 28-60 µg.Lombard et al. (2010) give a weight of 20-70 µg for specimens sampled off Catalina Island, California, in the same size fraction of the 500-600 µm.Our weight-diameter relation data range at 24-45 µg (Fig. S3c) for the same size fraction of the 500-600 µm, ranging at the lower limit of the weight-diameter relations measured in the Caribbean (Bijma et al., 2002) and off California (Lombard et al., 2010), which may be caused either by differences in genotypes or environmental conditions, or both.In our samples from the Mediterranean, individuals exceeding 60 µg have diameters larger than 650 µm.The reason why the SNW of O. universa is particularly low and highly variable in the Mediterranean despite of high carbonate ion concentration ([CO 3 2-]) and pH (Fig. 1) might be sought in factors other than, and in addition to, chemical and physical conditions, namely the changing availability of food along the transect from the Atlantic Ocean to the Levantine Basin.

Factors affecting the SNW of G. ruber and G. bulloides
In the same way as in O. universa, the SNW of G. ruber s.s.seems not to be controlled by carbonate chemistry, and to be affected by other factors like nutrient concentration and food availability.which are statistically significant.High SNW in the Atlantic and Tyrrhenian Sea correlates with enhanced primary production: enhanced fluorescence (Fig. 1d) and presumably enhanced food availability (Fig. 6; Fig. 2, also noticeable in Fig. S2d-e and Fig. S4d-e).At the same sites, larger IQR indicates more variability in test calcite production of G. ruber s.s.specimens, although a limited number of samples together with the low and uneven sampling size impede any further interpretation of the data (Fig. 6).
Under more oligotrophic conditions, low SNW of G. ruber s.s.might be caused by limited food availability.
The relationship between food availability and SNW in G. bulloides is opposite to that in G. ruber s.s.
(Fig. 6).The SNW of G. bulloides tests increases from the Atlantic toward the eastern Mediterranean.At the same time, variability in SNW data increases with increasing absolute SNW, which resembles the distribution of data in G. ruber s.s.(Fig. 6): In both species G. ruber s.s. and G. bulloides larger IQRs are found toward higher absolute SNW.
An opposite trend in SNW of the two species G. ruber s.s. and G. bulloides had earlier been described from the Arabian Sea, and could neither be assigned to changes in [CO 3 -2 ] of ambient seawater nor growth conditions.Due to its symbionts, G. ruber would rather have an advantage over symbiont-barren G. bulloides in oligotrophic waters, and support formation of test calcite through CO 2 consumption and increasing [CO 3 -2 ] and pH (see also Köhler-Rink and Kühl, 2005).Those finding may still point toward differences in growth conditions: Reproduction of both G. ruber and G. bulloides might be retarded under less optimal conditions, and additional calcite layers might be added to the proximal text surface before reproduction, similar to the process described for O. universa (see above).Therefore, tests may grow heavier under less optimal than optimal alimentation, given that carbonate chemistry of ambient seawater does not limit the formation of test calcite.
Comparing weight-diameter relations, G. ruber (255-350 µm size fraction) from plankton tows of the western Arabian Sea have an average weight of 11.5 ±0.69 µg (de Moel et al., 2009), which is heavier than the individuals from our study (5.9 ±0.31 µg; Fig. S3a; Appendix A).The difference in weightdiameter relation may indicate that G. ruber was produced under more ideal conditions for shell calcite formation in the Arabian Sea especially during non-upwelling periods and still higher overall primary productivity and food availability.However, the comparison might be biased by the fact that G. ruber s.s.In general, higher SNW occurs at lower latitudes and lower SNW at higher latitudes (see also Schmidt et al., 2004).All of these findings support our idea of an effect of limited alimentation on reproduction.Increased longevity and Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.
ongoing production of additional calcite layers at the proximal side of shells may result in an increased SNW, given that carbonate chemistry does not limit calcite formation in planktic foraminifera.G. ruber was the most abundant species, including more G. ruber s.s.than s.l.morphotypes.Its dominance in the east compared to the west, is assumed to be caused by stratification of the surface water column, enhanced SST, and trophic conditions.G. ruber is a symbiont-bearing species, which might be an advantage over symbiont-barren species like G. bulloides under oligotrophic and food-limited conditions as in the Levantine Basin.G. bulloides was most abundant in upwelled waters in the Strait of Gibraltar, in the Alboran Sea, and in the western Mediterranean.O. universa was present at rather balanced standing stocks along the entire transect from the west to the east.In general, distribution patterns of the main planktic foraminiferal species in the Mediterranean seem to be mainly related to a combination of food availability and temperature.Eastern Mediterranean (Stations 9,10,11,12,13) Ionian-Adriatic-Aegean (Stations 14,15,17,16,(16)(17)(18) Tyrrhenian Sea (Station 19)

Conclusions
Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License. the northwestern Mediterranean.It is found in low numbers in the southwestern Mediterranean, Atlantic, and Strait of Gibraltar stations.Individuals >350 µm in test diameter are rare (Fig. 3; Fig 4).G. inflata is the second most abundant species (0.29 ind.•10 m -3 ; 20.19%), mainly due to its high abundance in the Alboran Sea (3.5 ind.•10 m -3 ; 61.08% of the sample).It is present in the western Mediterranean until the Strait of Sicily.East of the Strait of Sicily, it is only found with low abundances at the westernmost stations.The dominant size fraction is 350-500 µm (Fig. 3; Fig 4).G. bulloides has an average abundance of 0.24 ind.•10 m -3 (17.20 %), mainly due to its abundance in the Strait of Gibraltar (2.31 ind.•10 m -3 ; 47.34 %).It is slightly most abundant in the southwestern Mediterranean and the Tyrrhenian Sea.It is a quite ubiquitous species being absent at four stations.It is rarely occurs in the >350-µm test-size fraction (Fig. 3; Fig 4).
Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.G. ruber s.s. and s.l.varieties are found in the Atlantic with slightly larger absolute abundances and higher relative abundances than in the western Mediterranean Basin, where it is found in low abundances.Temperature may be the main cause, with warmer Atlantic waters (16.1 ºC) with respect to the western Mediterranean (14.3 ºC in the SW, 14.0 ºC in the NW; Fig. 1), as demonstrated by positive significant correlations with temperature in the G. ruber s.s.variety (p = 0.01).The G. ruber results confirm the findings of the June 1969 cruise of BiogeosciencesDiscuss., doi:10.5194/bg-2016Discuss., doi:10.5194/bg--266, 2016     Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.
Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.biological prerequisites in the following.In contrast, changes of the SNW of O. universa are statistically insignificant (Figs.S2c, S3c, and S4c), and cannot be used to identify and quantify particular environmental effects.5.3.1 Unknown control of the SNW of O. universa The lack of statistical significance in the SNW data of O. universa in our data set possibly caused by an insufficient understanding of the ecology of the different morphotypes and genotypes of O. universa.
and s.l.morphotypes were analyzed together in the study ofde Moel et al. (2009).Data for supra-regional comparison of weight-diameter relation of G. bulloides from the water column are found for the 200-250 µm size fraction: in the north Atlantic (56-63 °N) in June 2009(Aldridge et al., 2012) with a range of 1.75-2.92µg (r 2 = 0.52).For that size fraction our results (36 °N) show heavier tests in the Alboran Sea (3.46 ±0.15 µg), and similar weights at the Strait of Gibraltar (2.57±0.00 µg; Fig.S3b).For the same water depth as in our samples,Schiebel et al. (2007) found a heavier average weightdiameter relation in fall (5.19 ±0.25 µg) than during spring (4.21 ±0.2 µg) in the eastern north Atlantic (47 °N), and 5.51 ±0.31 µg during the SW monsoon in the Arabian Sea (16 °N).
Absolute and relative abundances of planktic foraminifera were studied from plankton tow samples across the Mediterranean in May 2013.The samples reflect high differences in species abundance and assemblages between the different basins and sub-basins of the Mediterranean Sea.Absolute abundance and diversity of planktic foraminifer assemblages are low in comparison to other regions of the world ocean.Average standing stocks in the upper 200 m of the water column range from 1.42 ±1.43 ind.•10 m - 3 , including ten morphospecies in total.Planktic foraminifer assemblages are indicative of changing temperatures and salinities, as well as trophic conditions, between the eastern and the western Mediterranean Sea.Highest standing stocks of total planktic foraminifera occurred in the Strait of Gibraltar and the Alboran Sea.Overall, the largest foraminifera occurred in the western part of the transect, caused by the assemblages composition, and the presence of large G. inflata.

FiguresFig. 1 .
Figures Fig. 1.(a) Temperature (ºC), (b) salinity, (c) fluorescence (μg•l -1 ), (d) pH, and (e) [CO 3 ] -2 (µmol•kg -1 ) values of the water column of the transect.Values follow a color scale (under every graph), also values shown in the isometric lines.X axis: water depth.Y axis: longitude (degrees).Measurement locations indicated with white dots, with the coinciding stations numbered at top.The station number and the map section correlates with the map at right of this description.Note reversed color scale at (d) and (e).

Fig. 2 .Fig. 3 .
Fig. 2. Sampled stations with BONGO nets (dots).The numbers in the picture represent the station codes:First leg: 1 to 13, second leg: 14 to 22. Colour scale at right represents the values of surface chlorophyll concentration (in μg/l), retrieved from MODIS Aqua (L2), from the closest day as possible of the first leg transect.Fig.3. Absolute abundance of planktic foraminifera from BONGO nets during (a) leg 1 and (b) leg 2. Category 'Others' is comprised of G. siphonifera, G. sacculifer quadrocameratus-type, H. pelagica, G. ruber (pink), G. menardii and G. sacculifer sacculifer-type.Note different Y axis scale in the graphs.

Fig. 4 .
Fig. 4. Percentage of each planktic foraminifera size fraction in each station from (a) leg 1 and (b) leg 2.

Fig. 5 .Fig. 6 .
Fig. 5. Relative abundance of planktic foraminifera.Category 'Others' is comprised of G. siphonifera, G. sacculifer quadrocameratus-type, H. pelagica, G. ruber (pink), G. menardii and G. sacculifer sacculifertype.Less than 1% values are not shown.Number in parenthesis indicates the total individuals of each location.Fig. 6.Size-normalized weight of G. ruber s.s. and G. bulloides in box-and-whisker plots representation for the different location groupings in the Mediterranean.Box extends from the lower (Q 1 ) to upper (Q 3 )quartiles values of the data, with a line at the median (Q 2 ).Whiskers extend from the quartiles to values comprised within a 1.5 interquartile range (IQR = Q 3 -Q 1 ) distance: Q 1 -1.5•IQR;Q 3 + 1.5•IQR.

Figure 6
Figure 6 Production of the shell and the size-normalized weight (SNW) of tests of the most frequent species G. ruber s.s. and G. bulloides are most affected by trophic conditions and food availability, given that carbonate chemistry in the Mediterranean does not limit calcite test formation.G. ruber is more affine to oligotrophic conditions, and grows heaviest tests in less food-limited waters in the west near Gibraltar and the Tyrrhenian Sea.In contrast, G. bulloides grows heaviest tests under more food-limited conditions in the eastern Mediterranean Sea.We speculate that reproduction is hindered when the species-specific food sources are limited, while individuals continue adding calcite to the outer shell, and grow heavier tests than individuals that reproduced earlier in ontogeny.
These observations highlight the need for more interdisciplinary studies on the causes of changing foraminiferal assemblages and decreasing shell production, especially in the Mediterranean as a marginal basin, which is assumed particularly sensitive to changes of the environment and global climate.Biogeosciences Discuss., doi:10.5194/bg-2016-266,2016 Manuscript under review for journal Biogeosciences Published: 14 July 2016 c Author(s) 2016.CC-BY 3.0 License.