Dynamics of Species and Size Structures of Phytoplankton at Different Levels of Bottom-Up and Top-Down Effects in Experimental Conditions

An experimental study of the impacts of trophic conditions and the activity of zooplankton and fish on the phytoplankton structure has shown that, at the beginning of the experiment, the species and size structures of algae were dependent on the N : P ratio. This parameter causes the differences in phytoplankton structures between mesotrophic and eutrophic conditions so that, in mesotrophic conditions, the dominant taxa are diatoms, dinoflagellates and chrysophytes while, in eutrophic mesocosms, cyanobacteria are most abundant. Later on, the differences in N : P ratio in the treatments are smoothed out and the dominance shifts to large filamentous green algae. Fish reduce zooplankton control over phytoplankton, thus promoting the development of edible diatoms; this is most clearly manifested in eutrophic conditions.