Plankton community dynamics during decay of a cyanobacteria bloom: a mesocosm experiment

The aim of the work was to study the effects of a decaying cyanobacteria bloom on nutrient dynamics, plankton community development and production rates of bacteria and primary producers. It was hypothesised that the system would turn more heterotrophic following the decay of the bloom. A 10-day outdoors mesocosm experiment was performed in early June in a brackish-water environment. Non-toxic filamentous cyanobacteria Aphanizomenon flos-aquae were added to the treatment, whereas the control lacked cyanobacteria. A. flos-aquae decayed rapidly, and was absent from the units by day 2. Significantly higher bacteria abundances, lower nanoflagellate densities and higher ciliate abundances were found, suggesting a bottom-up regulated process in the treatments bags. N:P ratios were low (6-12), suggesting N-limitation. Bacteria correlated negatively with numbers of heterotrophic nanoflagellates (HNF), suggesting grazing on bacteria by HNF. Primary production correlated positively with irradiance, chlorophyll a and inorganic nutrients in all units. The rapidly decaying A. flos-aquae biomass imposed a significant bottom-up regulation in the treatment mesocosms, and the system turned from autotrophic into more heterotrophic with time. The rapid decay also caused some similarities and parallel changes between the treatment and the control.