Biogeographical and seasonal distribution of pteropod populations in the Western and Central Mediterranean Sea inferred from sediment traps

Pteropods are a group of cosmopolitan holoplanktic gastropods that produce an aragonite shell and play an important role in both marine ecosystems and geochemical cycles. In addition to being affected by anthropogenic impacts that include warming and changes in carbonate system parameters, the Mediterranean Sea is considered to be understudied concerning pteropods dynamics and abundances. This work aims to document the modern spatial and temporal distributions of pteropods populations in the Northwestern and Central Mediterranean Sea (Gulf of Lions and Strait of Sicily), respectively. We present data from two sediment-trap records that cover the timeframe between early 1996 and early 2004 for the Gulf of Lions and late 2013 to late 2014 for the Sicily Strait. A total of 843 pteropod shells and 18 different species were identified. Limacina inflata, Creseis virgula and Creseis clava were the most abundant species in the Gulf of Lions, while in the Sicily Strait, C. conica replaced C. clava as the most abundant species. These taxons represented around 70% of the total individuals identified in both sites. Overall, our results suggest a greater pteropod abundance in the Gulf of Lions than in the Sicily Strait, most likely due to enhanced food conditions. In the Gulf of Lions, maximum fluxes occurred in autumn (32.5% of the annual pteropod fluxes registered in October), while in the Sicily Strait peak fluxes occurred in winter (30.5% of the annual pteropod fluxes registered in January). Comparison of temporal changes pteropod fluxes with satellite sea surface temperature (SST), and chlorophyll-a concentration suggest a possible positive effect of high algal accumulation and cool water conditions in the Strait of Sicily on the main pteropod groups. In turn, no clear relationships between pteropod groups, SST and chlorophyll-a were identified in the Gulf of Lions, highlighting the effect of salinity and carbonate system parameters. Overall, and despite the limitations associated with the use of sediment traps for pteropod population monitoring, the consistency of our results with the literature supports the use of sediment traps as useful tools for documenting the diversity and temporal distribution of pteropods

Reduction in size of the calcifying phytoplankton Calcidiscus leptoporus to environmental changes between the Holocene and modern Subantarctic Southern Ocean

Unidad de excelencia María de Maeztu CEX2019-000940-MThe Subantarctic Zone of the Southern Ocean plays a disproportionally large role on the Earth system. Model projections predict rapid environmental change in the coming decades, including ocean acidification, warming, and changes in nutrient supply which pose a serious risk for marine ecosystems. Yet despite the importance of the Subantarctic Zone, annual and inter-annual time series are extremely rare, leading to important uncertainties about the current state of its ecosystems and hindering predictions of future response to climate change. Moreover, as the longest observational time series available are only a few decades long, it remains unknown whether marine pelagic ecosystems have already responded to ongoing environmental change during the industrial era. Here, we take advantage of multiple sampling efforts - monitoring of surface layer water properties together with sediment trap, seafloor sediment and sediment core sampling - to reconstruct the modern and pre-industrial state of the keystone calcifying phytoplankton Calcidiscus leptoporus, central to the global marine carbonate cycle. Morphometric measurements reveal that modern C. leptoporus coccoliths are 15% lighter and 25% smaller than those preserved in the underlying Holocene-aged sediments. The cumulative effect of multiple environmental factors appears responsible for the coccolith size variations since the Last Deglaciation, with warming and ocean acidification most likely playing a predominant role during the industrial era. Notably, extrapolation of our results suggests a future reduction in cell and coccolith size which will have a negative impact on the efficiency of the biological pump in the Southern Ocean through a reduction of carbonate ballasting. Lastly, our results tentatively suggest that C. leptoporus coccolith size could be used as a palaeo-proxy for growth rate. Future culture experiments will be needed to test this hypothesis

Calcification response of planktic foraminifera to environmental change in the Western Mediterranean Sea during the industrial era

The aim of this work is to investigate the variability of planktic foraminifera calcification in the northwestern Mediterranean Sea on seasonal, interannual and pre-industrial Holocene time scales. This study is based on data from a 12-year-long sediment trap record retrieved in the in the Gulf of Lions and seabed sediment samples from the Gulf of Lions and the promontory of Menorca. Three different planktic foraminifera species were selected based on their different ecology and abundance: Globigerina bulloides, Neogloboquadrina incompta, and Globorotalia truncatulinoides. A total of 273 samples were weighted in both sediment trap and seabed samples. As the traditionally used sieve fractions method is considered unreliable because of the effect of morphometric parameters on the foraminifera weight, we measured area and diameter to constrain the effect of these parameters. The results of our study show substantial different seasonal calcification patterns across species: G. bulloides showed a slight calcification increase during the high productivity period, while both N. incompta and G. truncatulinoides display a higher calcification during the low productivity period. The comparison of these patterns with environmental parameters revealed that Optimum Growth Conditions temperature and carbonate system parameters are the most likely to influence seasonal calcification in the Gulf of Lions. Interannual analysis suggest that both G. bulloides and N. incompta slightly reduced their calcification between 1994 and 2005, while G. truncatulinoides exhibited a constant and pronounced increase in its calcification that translated in an increase of 20 % of its shell weight for the 400–500 µm narrow size class. While our data suggest that carbonate system parameters are the most likely environmental parameter driving foraminifera calcification changes over the years. Finally, comparison between sediment trap data and seabed sediments allowed us to assess the changes of planktic foraminifera calcification during the late Holocene, including the preindustrial era. Several lines of evidence strongly indicate that selective dissolution did not bias the results in any of our data sets. Our results showed a clear calcification reduction between pre-industrial Holocene and recent data with G. truncatulinoides experiencing the largest calcification decrease (32–40 %) followed by N. incompta (20–27 %) and G. bulloides (18–24 %). Overall, our results provide evidence of clear reduction in planktic foraminifera calcification in the Mediterranean most likely associated with ongoing ocean acidification and consistent with previous observations in other settings of the world’s oceans