Nutrients and hydrography explain the composition of recent Mediterranean planktonic foraminiferal assemblages

The composition of planktonic foraminiferal assemblages in surface sediments of the Mediterranean Sea shows a pronounced zonal gradient. This gradient is different from what is observed in the open ocean, indicating that the assemblage composition may respond to environmental variables other than the otherwise dominant surface water temperature. Here we make use of a dataset of census counts of 18 taxonomic categories in 124 surface-sediment samples extracted from the ForCenS database to understand the main environmental factors affecting the composition of recent Mediterranean planktonic foraminiferal assemblages. The tested explanatory variables include temperature, salinity, chlorophyll, nitrate and phosphate concentrations in the surface waters, temperature, nitrate and phosphate concentrations at depth and thermal, salinity and density vertical gradients. The composition of the assemblages is aligned along two environmental gradients and redundancy analysis reveals that the tested variables explain a large portion (>70%) of the variance in the compositional data. The first environmental gradient reflects the nutrient content in the deep waters, affecting nutrient availability in the productive zone, while the second gradient is driven by higher (northern area) and lower (southern area) upward nutrient advection due to differences in density stratification, conditioned by the regional hydrography. Although sea surface temperature is the most reconstructed environmental variable from fossil assemblages of planktonic foraminifera, it seems to play a secondary role in the Mediterranean Sea. This, has implications for reconstructions of past oceanographic conditions in this region, opening up the possibility to reconstruct productivity instead, or next to, temperature.