Plankton Dynamics and Distributionin the Eastern Mediterranean Sea
The similarities and differences in phytoplankton response to anthropogenic nutrient enrichment were studied in an open system (North Aegean Sea) and a coastal site of Saronikos gulf and in the Eastern Mediterranean. Although by physico-chemical parameters two different environments have been compared, the two regions exhibited similar features. The analysis of the data provides information on the contribution of each size class to the total autotrophic biomass and how nutrients and hydrographical variability of the water column affects the picophytoplankton assemblage composition. Phytoplankton size structure depends on a variety of factors, which ultimately are controlled by hydrodynamics of the system (Riegman et al., 1993). In general, the food web structure is also dependent on the nutrient availability. At low nutrient concentration, the picophytoplankton is supposed to be the relatively most important group, whereas under extensive nutrient supply larger size fractions dominate the phytoplankton. These larger organisms can be eaten directly by the mesozooplankton, which is the main component of the diet of the planktivorous fish. Picophytoplankton is not directly consumed by the mesozooplankton but is mainly consumed by protists. Some authors have pointed out that stability-instability conditions in the water column play a major role in controlling phytoplankton size structure. On one hand, hydrodynamical forcing controls nutrient supply to the euphotic layer. High nutrient concentrations cause an increase in the biomass and primary production of larger phytoplankton (Chisholm, 1992; Agawin et al., 2000). On the other hand, water column stability determines the size-differential residence time of phytoplankton cells within the euphotic layer, either favouring large-sized phytoplankton losses by passive sinking or accumulating them through the effect of vertical motion (Margalef, 1985; Malone, 1980; Rodrigues et al., 2001).