Composition and dynamics of phytoplankton of the Ionian Sea (eastern Mediterranean)

The relationship between phytoplankton distribution and dynamics, and the resident water masses in the Ionian Sea was investigated in the spring of 1999 by flow cytometry, HPLC, microscopy, 14C incorporation, and serial dilutions. More than 50% of total biomass was represented by ultraphytoplankton, with proportions increasing with depth (up to 80%) and eastward (up to 60%). Synechococcus sp. cyanobacteria dominated ultraphytoplankton numbers and biomass. Hydrological stability explained the relative vertical distribution of Synechococcus sp. and Prochlorococcus sp. cyanobacteria and their degree of photoacclimation. The northwestern area (‘‘Italian Side’’) was in a transition from bloom to oligotrophic conditions, reflected in high biological instability in terms of phytoplankton composition, photoacclimative properties, and photosynthetic efficiency (P/B of 11.40 mg C mg chl-1 h-1). The influence of the Adriatic SurfaceWater, carrying a trace of the spring bloom was visible from satellite imagery (SeaWiFS). The eastern part of the sampled area (‘‘Greek Side,’’ GS) was hydrologically more stable (low estimated vertical velocities), resulting in higher photosynthetic efficiency (P/B of 20.47 mg C mg chl-1 h-1) and phytoplankton photoacclimative properties. The Atlantic Water (AW) was the most oligotrophic in terms of nutrient concentration, chla, and productivity (208.0 mg C m-2 d-1), but also the most variable in terms of mesoscale features. Growth of Synechococcus sp. and Prochlorococcus sp. was slow (0.36 d-1 at max), while picoeukaryotes grew well in the deep chlorophyll maximum (DCM) of the GS (0.67 d-1) Picoeukaryote growth was tightly coupled with grazing (0.80 d-1), indicating efficient biomass recycling. Biological characterization and DCM dynamics in the three areas are discussed in relation to physical features present at the time of sampling.