Size structure and production of phytoplankton community and carbon pathways channelling in the Southern Tyrrhenian Sea (Western Mediterranean)

In the frame of a multidisciplinary international joint effort, supported by CIESM, phytoplankton composition, distribution and activity in summer and late autumn in the Southern Tyrrhenian Sea were investigated. The main aim of the study was to obtain a preliminary characterization of the autotrophic community structure of a poorly investigated and complex basin during two contrasting seasonal conditions (stratification vs homogeneity). On both occasions the phytoplankton biomass, as chlorophyll a, was low in the euphotic layer of water column (8.9 mg m−2 in July and 10.7 mg m−2 in December), with a clear dominance of the pico-phytoplanktonic component (64% and 76%, respectively), and a pronounced summer Deep Chlorophyll Maximum. Interestingly, primary production was higher in December (429 mg C m−2 d−1) than July (273 mg C m−2 d−1), again with a major contribution of pico-class in both seasons (90% and 52% in late autumn and summer, respectively). The dominant contribution of small size classes to the total photosynthetic efficiency and their fast carbon turnover time within the community hint at a preferential carbon pathway through the small-sized components of the food web that contrasts with the traditional views of the Tyrrhenian Sea as a typical component of the temperate Western Mediterranean Sea.