Sea surface temperature control of taxon specific phytoplankton production along an oligotrophic gradient in the Mediterranean Sea

• Inverse correlations were found between phytoplankton biomass, productivity, and composition in water masses with sea surface temperature below 19 °C.
• No correlations were found between phytoplankton biomass, productivity, and composition in water masses with sea surface temperature above 19 °C.

The current study aimed to assess changes in phytoplankton composition and productivity along an oligotrophic gradient in relation to changes in sea surface temperature (SST). Phytoplankton pigments, nutrients, and physical water column properties were studied along a longitudinal transect in the Mediterranean Sea (MED) in May–June 2013, covering its western (wMED) and eastern (eMED) basins. Pigments were used to determine phytoplankton taxonomic composition and taxon specific carbon fixation, whereas beam attenuation was used to calculate particulate organic carbon (POC). Nitrate, phosphate, and silicate concentrations integrated over 0–125 m declined from wMED to eMED (on average 13, 9, and 2 fold lower in eMED compared with wMED, respectively) and correlated inversely with SST (16.2–23.0 °C) for the whole transect in the Mediterranean Sea.N:P ratios in the euphotic zone averaged 5.6 and 3.1 for wMED and eMED respectively, suggesting that phytoplankton was nitrate limited. Average phytoplankton productivity and biomass were 4 and 2.5 fold higher in the wMED than in the eMED, respectively. Relative abundances of diatoms, prasinophytes, dinoflagellates, and cryptophytes showed inverse correlations with SST and positive correlations with nutrient concentrations. In contrast, pelagophytes, chlorophytes, euglenophytes, Synechoccocus and Prochlorococcus showed positive correlations with SST and negative correlations with nutrient concentrations. Particulate organic carbon (POC) of the upper 200 m showed up to 3 fold higher concentrations in the wMED than in the eMED, and correlated positively with chlorophyll concentrations, productivity, and nutrient concentrations, and inversely with SST. Inverse correlations were observed for phytoplankton biomass and SST below 19 °C. SST above 19 °C as observed in the eMED did not correlate with phytoplankton biomass and productivity, showing that in this temperature range phytoplankton productivity is uncoupled from nutrient supply from deep water. This suggests different responses of Mediterranean phytoplankton to changes in SST, depending on the temperature range.