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Lake Taihu is the third-largest freshwater lake in China and has been suffering from cyanobacterial blooms for over two decades. The northern part of the lake, Meiliang Bay, is known to be at high risk of dense and sustained Microcystis blooms and toxins. This study aimed to investigate and record the annual and seasonal dynamics of toxic genotype, Microcystis morphospecies succession and microcystin variation. It also aimed to find out the underlying driving factors influencing the dynamic changes. Microcystin (MC) and the Microcystis genotype were quantified using HPLC and quantitative real-time PCR, respectively. Our study, over three consecutive years, showed that the pattern of morphospecies succession was seasonally distinct and annually consistent. During the same period in 2012, 2013 and 2014, the average MC were, on dry weight basis, 733 μg·g−1, 844 μg·g−1, 870 μg·g−1, respectively. The proportion of toxic Microcystis accounted for 41%, 44% and 52%, respectively. Cell bound microcystin was found to correlate with the percentage of toxic Microcystis. Based on historical and current data, we conclude that annual bloom toxicity was relatively stable or possibly increased over the last decade.

Long-term interannual and monthly (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) data were analyzed to elucidate the effects of meteorological factors and nutrient levels on phytoplankton biomass in the cyanobacteria-dominated Waihai basin of Lake Dianchi. The interannual ln(chl. a) exhibited positive correlations with the mean air temperature, mean minimum air temperature, and mean maximum air temperature; in addition, a positive relationship between Δln(chl. a) and ΔTP was observed throughout the period. Additionally, ln(chl. a) exhibited a positive correlation with the TP concentration, negative correlations with the sunshine hours and wind speed during the dry season, and positive correlations with the TN and TP concentrations during the rainy season. Furthermore, TP was the most influential factor affecting cyanobacterial bloom dynamics throughout the entire period and during the dry season, and TN and TP were the most important factors during the rainy season, as determined by relative importance analysis. The results of this study based on interannual analysis demonstrated that both meteorological factors and nutrient levels have important roles in controlling cyanobacterial bloom dynamics. The relative importance of these factors may change according to precipitation patterns. Thus, climate change regulation and eutrophication management should be considered in strategies for bloom control. Decreasing the TP load should be prioritized throughout the entire period and during the dry season, and decreasing the TN and TP loads should be considered initially during the rainy season. In addition, further studies of more frequent and complete data acquired over a longer period of time should be conducted in the future.

The succession amongst different cyanobacteria genera remains understudied and poorly understood as much of the focus has been on monospecific blooms. The spring-summer succession between Aphanizomenon flos-aquae and Microcystis and its underlying mechanism were investigated from 2009 to 2012 in hypereutrophic Dianchi Lake in China. Regularly, A. flos-aquae grows rapidly and forms bloom in March at water temperatures around 15°C, followed by Microcystis blooming around 18°C in April. Since 2010, the pattern of succession has changed, featuring an increase of Microcystis but decrease of A. flos-aquae biovolume, leading to near disappearance of A. flos-aquae in 2012. Coincidently, there was a sharp increase of nitrogen concentration in 2010, going up to 5.67 mg/l in 2011, and a big increase in the mass ratio of TN to TP from 13.6 in 2009 to 21.1 in 2011. We hypothesized that temperature is the most influential factor governing the initiation of rapid growth and succession between A. flos-aquae and Microcystis, while increase of TN and/or ratio of TN to TP may trigger the decline and disappearance of A. flos-aquae, time of recruitment, and the population dynamics of Microcystis. Our findings are not only meaningful to the understanding of the cyanobacterial bloom mechanism but also to the management of shallow eutrophic lakes.

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