Evaluating the contamination of microcystins in Lake Taihu, China: The application of equivalent total MC-LR concentration

The frequent occurrence and accumulation of microcystins (MCs) in freshwater systems pose serious threats to the drinking water safety and health of human beings. However, determining the overall toxicity and environmental risks from MC exposure is complex because of the variety of MC analogues and their respective toxicities. To address this issue, we conducted a survey of particulate (intracellular) and dissolved (extracellular) MC in Lake Taihu from 14 sampling sites in the northern part of the lake, and 32 stations throughout the entire lake, over a 16-month period. We propose a novel indicator, total MC-LR concentration (TLR), defined as the total concentration of MC-LR after transforming other MC variants (i.e., MC-RR and MC-YR) to an equivalent toxicity of MC-LR. Intracellular concentrations of TLR (iTLR) were usually observed, with the maximum values in July and October 2013 corresponding to periods of peaks in phytoplankton biomass. In contrast, extracellular concentrations of TLR (eTLR) were highest in May and June 2014. These differences in temporal patterns exhibited by LR, TLR, and TMC between intracellular and extracellular MC may be attributed to the influence of environmental variables. In addition, the distribution of iTLR and eTLR in the entire lake showed clear spatial heterogeneity. MC concentrations were greatest in the northern part of the lake during warm months, especially in Meiliang Bay. Based on the strong linear relationships between TLR and the concentration of chlorophyll-a (Chl-a), as well as TLR and the cell density of cyanobacteria, we propose not-to-exceed safety thresholds for Chl-a of 21.28 μg/L in the northern lake and 23.26 μg/L in the whole lake, which are paired with safety thresholds for cyanobacterial cell densities of 2.21 × 108 cells/L and 1.15 × 108 cells/L, respectively. The application of this newly proposed indicator, TLR, may contribute to better evaluation of overall MC toxicity and provide guidance on recommended limits for Chl-a concentration and cyanobacterial cell density in other freshwater ecosystems.