Primary production, biomass and abundance of phototrophic picoplankton in the Mediterranean Sea : a review

In the Mediterranean Sea, the contribution of picophytoplankton to primary production varies from 31% (Straits of Messina, with an average total production of 0.18 mg C m -3 h -1 ) to 92% (Ionian Sea, with an average of 1.64 mg C m -3 h -1 ). Although studies have used different methodologies, the contribution of this small phototrophic fraction to whole 14 C assimilation seems to be more important in oligotrophic off-shore waters (71%) compared to the neritic ones (44 %). Diluting effects of fluvial waters (Adriatic Sea) and hydrodynamic phenomena (Straits of Messina) probably influence the physiologic activity of these populations. Picophytoplankton contribution to primary production in brackish and eutrophic environment is clearly lower than in marine waters both in terms of chl a and of 14 C assimilation. Reports of cellular abundance (Cyanobacteria and eucaryotic cells) ranged from 10 5 to 10 8 cells l -1 with a mean value of 10 7 showing no difference between seasons and suggesting a state of particular equilibrium for this population in the planktonic community. Recent data on prochlorophyte cellular density are of the same order of magnitude (10 7 cells l -1 ) of other picophytoplanktonic procaryotic and eucaryotic organisms.