The lakes in this study were divided into four types: river connected lakes (RL), seasonal river connected lakes (SL), city lakes (NC) and non-urban lakes (NE). Among the four lake types, city lakes were the most serious, with TN concentration 4-6 times and TP concentration 7-16 times that of the other three lake types. The seasonal river connected lakes and non-urban lakes were less eutrophic than city lakes, and the river connected lakes were faced with seriously eutrophication because of contamination input from the littoral zone.
The changes of NH4＋-N, NO3－-N, NO2－-N and TN:TP ratio were studied during growth and non-growth seasons in the 33 lakes. There were significant positive correlations among all nutrient concentrations, and the correlations were stronger in growth season than in non-growth season.
In addition, TN:TP decreased with TP concentration decline, and TN and inorganic nitrogen concentrations were obviously lower in growth season than in non-growth season, suggesting that the decrease of nitrogen (especially NH4＋-N and NO3－-N) was an important reason for the decrease of TN:TP in growth season.
The biomass of cyanobacteria was very higher in growth season than in non-growth season, and the highest value in growth season was 5.719 mg L-1 in Sanjiao lake, the lowest value was 0.001 mg L-1 in Tian’e Zhou. Cyanobacteria in the seasonal lakes were much higher than those in other type lakes. In growth season, cyanobacteria in non-urban lakes and seasonal lakes were more than 40% in total phytoplankton biomass among the four type lakes, cyanobacteria in river connected lakes and city lakes were less than 20% in total phytoplankton biomass.
A survey of 30 lakes was conducted to reveal how environmental and biological variables were associated with the concentration of the cyanotoxin, microcystin (MC). The results showed that mean MC concentration in seasonally river-connected lakes (SL) was nearly 33 times that in permanently river-connected lakes (RL), and more than six times that in city lakes (NC) and non-urban lakes (NE) which were not connected to the Yangtze River. The highest MC (8.574 μg L) was detected in Dianshan Lake. MC-RR and MC-LR were the primary toxin variants in our data. MC-RR, MC-YR and MC-LR were significantly correlated with Chl a, biomass of cyanobacteria, Microcystis and Anabaena, indicating that microcystins were mainly produced by Microcystis and Anabaena sp. in these lakes. Nonlinear interval maxima regression indicated that the relationships of Secchi depth, total nitrogen (TN), total phosphorus (TP) and NH4＋ with MC were characterized by negative exponential curves. The relationships between MC and TN, TP, NO3－+ NO2－ were fitted well with a unimodal curve. Multivariate analyses by principal component and classifying analysis indicated that MC was mainly affected by Microcystis among the biological factors and closely related with temperature among physicochemical factors.