Effects of nutrient temporal variations on toxic genotype and microcystin concentration in two eutrophic lakes

Harmful cyanobacterial blooms are a growing threat to freshwater ecosystems worldwide due to the production of microcystin (MC), which can have detrimental effects on water quality and human health. The relations between MC-producing Microcystis, MC production, and environmental variables especially nutrient conditions in eutrophic lakes, Lake Taihu and Lake Yanghe, were investigated during the bloom season of 2015. Results showed that toxigenic cells contributed to 8.94-75.68% and 7.87-58.69% of the total Microcystis in Lake Taihu and Lake Yanghe, respectively. The dynamics of toxigenic cells and MC production were positively associated with NH3-N concentration in Lake Taihu, while positively associated with the concentrations of TP, TDP and PO4-P in Lake Yanghe, indicating that the dominant nutrient factor affecting the toxic blooms was nitrogen in Lake Taihu, whereas it was phosphorus in Lake Yanghe. The significant relationship between TLR eq (total MC after transformation of MC-RR and MC-YR into MC-LR) and Chlorophyll-a (Chl-a) concentration implied that Chl-a could be an alternative measure to predict MC risk in the two lakes, and the safe threshold value of Chl-a was proposed as 25.38 and 31.06 μg/L in Lake Taihu and Lake Yanghe, respectively.