Unexpected predominance of photosynthetic picoeukaryotes in shallow eutrophic lakes

Abstract

The spatial and temporal dynamics of different picophytoplankton groups were investigated by flow cytometry over a 1-year period in two large shallow eutrophic lakes in China (Lake Taihu and Chaohu). The picophytoplankton were composed of photosynthetic picoeukaryotes (PPEs), phycocyanin-rich picocyanobacteria (PC-cells) and single-cell Microcystis. They accounted for a maximum of 70% of the total plankton chlorophyll a concentrations during a period of non-cyanobacterial blooms in winter and spring. The PPEs were the most abundant group ranging from 2.2 to 199.5 × 103 cells mL−1 in the two lakes. The PC-cells were less abundant (average abundance: 1.6 × 104 cells mL−1). Pronounced temporal and spatial patterns of picophytoplankton composition and abundance were observed and PPEs were dominant in winter and spring while PC-cells prevailed in summer and autumn. The statistical analysis demonstrated that this population succession was explained by different environmental factors. PPEs were more correlated with physical parameters (temperature, oxidation-reduction potential, dissolved oxygen and pH) and less linked to nutrient changes, whereas PC-cells were closely related to nutrient conditions and disappeared with high nutrient concentrations. Ultimately, Microcystis single cells were detected mainly in the late spring and autumn, and thus, the detection of these Microcystis forms by flow cytometry suggests a potential method to monitor cyanobacterial blooms in these lakes for early warning signals.