Unique Spatiotemporal Distribution Of Total And Toxic Microcystis Populations In Water And Sediment In Lake Taihu

Cyanobacteria blooms caused by eutrophication not noly destroy the structure and function of ecosystem, but also release highly stable toxic metabolites, and thus have become a cosmopolitan public health and environmental problem. Microcystis spp. are among the most widespread toxic cyanobacterial species. Lake Taihu, the third largest freshwater lake in China, has occurred lake-wide Microcystis blooms annually in the past few decades, which has raised concerns on the pattern of Microcystis occurrence and environmental factors favoring algal blooms in this large freshwater system. However, previous studies of the Microcystis abundance in Lake Taihu had mostly focused on bloom-affected areas (northern part of the lake) or season (the summer bloom season). Besides, Microcystis can survive for a long time and accumulate in the surface sediment in eutrophic lakes. Therefore, it is also required to interpret the spatiotemporal distribution of total and toxic Microcystis populations in the surface sediment.To fully understand the total and toxic Microcystis spatiotemporal distribution and the environmental factors favoring algal blooms in Lake Taihu, surface water and sediment samples were collected from five representative areas with different eutrophication levels (E: eastern lake, S:southern lake, W:western lake, Z:Zhushan Bay, M:Meiliang Bay) during four seasons in 2013. The abundance of total and toxic Microcystis populations was examined by quantitative polymerase chain reaction (qPCR) method targeting Microcystis-specific 16S rRNA and mcyA genes, respectively. Then the Pearson correlation test was used to analyze the relationship between the Microcystis abundance and environment factors.In surface water, Microcystis spp. were detected at all sampling areas in four seasons, but the spatiotemporal distribution were different. It suggested that eutrophication and seasons both affected the distribution of Microcystis population. Besides, our results showed that the total Microcystis population accumulated mainly in southern lake in winter, while most of total Microcystis cells concentrated in northwestern areas (W, Z, and M) during summer and autumn. Thus, the prevention and control strategies of Microcystis blooms should be adjusted seasonally. In winter, we should take southern lake-focused strategyies, while in summer take northwestern areas-focused strategies.In the sediment, Microcystis spp. were also ubiquitous, and the abundance of total Microcystis (1.82×109 copies/g) and toxic Microcystis (4.19×106 copies/g) were high in summer. As the dead cyanobacteria tend to combine with polluted sediment and cause black blooms by anaerobic fermenting, the high abundance of Microcystis in the sediment of Lake Taihu would lead to black blooms, which will deteriorate water quality and threaten the safety of water supply. Besides, in sediment samples, the proportion of toxic and total Microcystis spp. was higher in winer and spring than summer and autumn, whereas the situation was on the contrary in water. It suggested that, cyanobacteria in sediment appear to play a major role in maintaining the occurrence of blooms of toxic Microcystis spp. Therefore, ecological dredging shoukd be one of the best methods of controlling Microcystis blooms.In the water, CODMn showed a significant positive correlation with toxic Microcystis and total Microcystis levels in this study. Besides, toxic Microcystis population had a strong positive relationship with TP in water samples, and the total Microcystis biomass was positively correlated with TN in the sediment. It indicated that CODMn not only can be the index of the relative contents of organic substances, but also can be the indicator for Microcystis biomass. Futhermore, reducing the input of nutrient (especially of N and P loading) should be the effective way in controlling Microcystis blooms.The findings of this study will facilitate the understanding of the spatiotemporal distribution of Microcystis and environmental factors favoring Microcystis blooms in water and sediment, and thus provide the basis for effective strategies of control and treatment of Microcystis blooms in Lake Taihu.