Spatiotemporal patterns and threshold of chlorophyll-a in Lake Taihu based on microcystins.

Chlorophyll-a (Chl-a) is considered as an indicator of phytoplankton biomass dynamically reflecting the growth of algae. Therefore, determination of Chl-a threshold is of vital importance to the health of aquatic ecosystems and drinking water security. This research is aimed to investigate the spatial and temporal distributions of Chl-a and microcystin (MC) concentrations using Geographic Information System (GIS) and identify the Chl-a threshold in Lake Taihu based on available guideline values of MCs. Nearly, the same characteristics of spatiotemporal variation of Chl-a and MCs were observed in Lake Taihu. Overall, the lakewide distributions of Chl-a and MCs were highly variable over time and space. The Chl-a concentration in the winter and spring was relatively low, and gradually increasing in summer and autumn, with the maximum concentration observed in August. But the maximum MCs concentration appeared in October, 2 months lagging behind the Chl-a. The highest annual average Chl-a and MCs concentrations were observed in Zhushan Bay, Meiliang Bay, and Gonghu Bay in northwest of Lake Taihu, following by West Zone and Center Zone. Dongtaihu Bay, East Zone, and South Zone always present good water quality. Referring to the guideline value of MCs, the Chl-a threshold was determined as 10-15 mg·m-3 based on the linear regression correlation between Chl-a and MCs. The establishment of Chl-a threshold is useful for eutrophication control, water quality management, and drinking water utilities in developing water safety plans.

[Research on the threshold of Chl-a in Lake Taihu based on microcystins].

Water samples were collected in Lake Taihu from June to October in 2013 in order to investigate the threshold of chlorophyll a (Chl-a). The concentrations of three microcystins isomers (MC-LR, MC-RR, MC-YR) were detected by means of solid phase extraction and high performance liquid chromatography-tandem mass spectrometry. The correlations between various MCs and eutrophication factors, for instance of total nitrogen (TN), total phosphorus (TP), chlorophyll a, permanganate index etc were analyzed. The threshold of Chl-a was studied based on the relationships between MC-LR, MCs and Chl-a. The results showed that Lake Taihu was severely polluted by MCs and its spatial distribution could be described as follows: the concentration in Meiliang Bay was the highest, followed by Gonghu Bay and Western Lake, and Lake Center; the least polluted areas were in Lake Xuhu and Southern Lake. The concentration of MC-LR was the highest among the 3 MCs. The correlation analysis indicated that MC-LR, MC-RR, MC-YR and MCs had very positive correlation with permanganate index, TN, TP and Chl-a (P < 0.01). The threshold value of Chl-a was 12.26 mg x m(-3) according to the standard thresholds of MC-LR and MCs in drinking water. The threshold value of Chl-a in Lake Taihu was very close to the standard in the State of North Carolina, which demonstrated that the threshold value provided in this study was reasonable.