Interaction between simulated dense Scenedesmus dimorphus (Chlorophyta) bloom and freshwater meta-zooplankton community

Algal bloom has been a subject of much research, especially the occurrence of blue-green algae (cyanobacteria) blooms and their effects on aquatic ecosystems. However, the interaction between green algae blooms and zooplankton community was rarely investigated. In the present study, the effects exerted by Scenedesmus dimorphus (green alga) bloom on the community structure of zooplankton and the top-down control of the bloom process mediated by the zooplankton were evaluated using a series of laboratory cultures. The results showed that a dense S. dimorphus bloom could change the zooplankton community structure by decreasing its diversity indices, leading to the enrichment of a particular zooplankton species, Brachionus calyciflorus. In the presence of mixed species of zooplankton, the density of S. dimorphus in the culture was decreased as determined by a change in total chlorophyll a (Chl a) concentration, which was about 200 μg L^-1 lower than that of the zooplankton-free culture. Furthermore, the number of species belonging to Cladocera, Copepoda and Rotifera all decreased, with all the cladocerans disappeared in the co-culture within 2 weeks of culturing, while the density of rotifers increased from 818 (±243) ind L^-1 at the time of inoculation to 40733 (±2173) ind L^-1 on the 14^th day post-inoculation. Grazing of S. dimorphus by the rotifer B. calyciflorus neutralized its growth, and the gradual increase in B. calyciflorus density eventually led to the collapse of the bloom. Furthermore, grazing by B. calyciflorus also led to a decrease in the maximal photochemical efficiency (F_v/F_m) of photosystem II (PSII). The combined changes occurring in the zooplankton community structure during the process of S. dimorphus bloom and the negative effects of grazing on algal growth, morphology and photosynthetic activities confirmed the key role of zooplankton in the control of algal bloom. The results of the study therefore indicated that dense algal blooms caused by non-toxic algae could still remain a threat to aquatic ecosystems.