## Results  
### Seasonal Variations of Cyanobacteria, MCs and T&O Compounds in the EDWS  
In the EDWS of Lake Chaohu, cyanobacteria and their associated metabolites including MCs and T&O compounds were studied monthly from August 2011 to July 2012 (Table 1). The chlorophyll-_a_ concentrations ranged from 5.4 to 68.0 \(\mathrm{\SIUnitSymbolMicro}\)L with the lowest and highest average concentrations detected in the spring (9.0 \(\pm\) 3.0 \(\mathrm{\SIUnitSymbolMicro}\)L) and in the summer (38.5 \(\pm\) 14.0 \(\mathrm{\SIUnitSymbolMicro}\)L), respectively. The dominant phytoplankton was cyanobacteria with _Microcystis_ and _Dolichospermum_ making up more than 98% of the cyanobacterial abundance except in the spring (85%). Cyanobacterial density was lowest in the spring with only _Dolichospermum_ detected in most samples. From summer to the middle of autumn, the _Microcystis_ abundance increased considerably as the dominant species and the cyanobacteria reached a peak density of 195,000 cells/mL simultaneously. In late autumn and winter, the amount of filamentous species belonging to the genera _Dolichospermum_ increased gradually and again became the dominant species with the maximum number of cyanobacteria (200,000 cells/mL) measured. Co-occurrence of three MC congeners and four T&O compounds with different seasonal trends have been observed in this water source (Table 1). The total concentrations of the three studied MC congeners (MC-LR, MC-RR and MC-YR) ranged from 0.28 to 8.86 \(\mathrm{\SIUnitSymbolMicro}\)L. In the spring, the average concentration of each MC congener was at its lowest value corresponding to the lowest cyanobacterial density of the whole year. In summer and autumn, MCs occurred at higher concentrations with _Microcystis_ being dominant. The MC-LR concentration reached its maximum amount in the summer with average values of 2.80 and 3.22 \(\mathrm{\SIUnitSymbolMicro}\)L for EMC-LR and IMC-LR, respectively. By comparison, the highest concentrations of MC-RR (3.73 \(\mathrm{\SIUnitSymbolMicro}\)L) and MC-YR (3.58 \(\mathrm{\SIUnitSymbolMicro}\)L) were both detected in autumn leading to the peak concentration of the total MCs during the study period, while the concentrations of the three other MC variants remained at high levels with a sum of 4.11 \(\mathrm{\SIUnitSymbolMicro}\)L in winter. The concentrations of MCs were detected above the WHO [4] recommended guideline value in summer and autumn, and even maintained a high level of nearly 1.0 \(\mathrm{\SIUnitSymbolMicro}\)L in winter. Compared to MCs, the four studied T&O compounds showed more complex seasonal dynamics in the source water (Table 1). Furthermore, the seasonal variations of dissolved T&O compounds were inconsistent with those of particle compounds. In the spring, the maximum concentrations of _d_-GEO, _d_-CYC and _d_-ION were obtained at the lowest cyanobacterial density. In the summer, the maximum concentration of MIB was detected at the highest concentrations of both _d_-MIB (193.4 \(\mathrm{\SIUnitSymbolMicro}\)L) and _p_-MIB (34.0 \(\mathrm{\SIUnitSymbolMicro}\)L). Additionally, higher levels of _d_-CYC and _p_-CYC were observed at concentrations around 50 \(\mathrm{\SIUnitSymbolMicro}\)L, while the peak concentration of _p_-ION (152.7 \(\mathrm{\SIUnitSymbolMicro}\)L) was also recorded. In autumn, relatively low concentrations of total T&O compounds were observed but also exceeded their OTCs in some samples. In winter, the maximum concentration of CYC (125.0 \(\mathrm{\SIUnitSymbolMicro}\)L) was observed with the highest _p_-CYC concentration (106.3 \(\mathrm{\SIUnitSymbolMicro}\)L) detected, while the concentration of _p_-ION was up to 115.8 \(\mathrm{\SIUnitSymbolMicro}\)L. The four studied T&O compounds could exceed their OTCs all the year round. During the study period, 69%, 39%, 72% and 86% of the samples for GEO, MIB, CYC and ION exceeded their corresponding OTCs, respectively.