The prevalence and persistence of microcystin in seven subtropical reservoirs in China and associated potential risks for human health

Microcystins (MCs) are the most frequent cyanotoxins occurring in inland freshwaters, posing a serious threat to drinking water safety and human health. However, the distribution and risk of MCs in subtropical reservoirs in China are still unclear. In this study, we investigated the MCs pollution in seven subtropical reservoirs in Fujian Province, China. The results showed that most samples (87.74%) from all reservoirs were in a mesotrophic state during the survey period, and a variety of potential toxigenic cyanobacteria were detected, which resulted in the detection rate of MCs reaching 100%. The MCs concentration of surface samples ranged from 0.19 to 1.06 mu g/L, and the proportion of MC-YR (43.72%) was the highest, followed by MC-RR (37.88%). The maximum was usually obtained in warmer seasons. The MCs concentration in stratified water samples ranged from 0.20 to 0.54 mu g/L, while the MCs concentration in bottom water samples was sometimes higher than that in surface samples. MCs were positively correlated with most of the physicochemical parameters and the densities of phytoplankton phyla, but only negatively correlated with water depth, total nitrogen and nitrate nitrogen. MCs concentration was significantly positively correlated with the density of Microcystis and Cylindrospermopsis. Finally, risk assessments of MCs indicate that the risk from MCs pollution in all reservoirs studied remains low, particularly to humans. However, since the reservoir has different community compositions of toxin-producing cyanobacteria from that of shallow lakes, the long-term and regular monitoring and study of cyanotoxins should be strengthened in future under a changing climate.