As one of the few remaining lakes that are freely connected with the Yangtze River, Poyang Lake exhibits a unique aquatic ecology which is different from the disconnected lakes. In order to study the mechanism of limnological responses to water level fluctuations (WLFs), samples were collected weekly over 10 months from September 2011 to December 2012 in Xingzi and Duchang, and the effect of fluctuations in water level on nutrients and phytoplankton chlorophyll a (Chla) concentrations was investigated in Poyang Lake. Chla concentrations were strongly related to WLFs, with higher Chla concentrations in the higher water phase. The regression analysis indicated that Chla concentrations were significantly and positively correlated with water temperature (P<0.0001) and secchi depth (SD) (P<0.0001) but negatively correlated with total and inorganic nitrogen concentrations (TN,P<0.01; NOx-N,P<0.01; NH4+-N:P<0.05). SD and total and inorganic nitrogen concentrations were all significantly correlated with WLFs of Poyang Lake (SD,P<0.01; TN,P<0.0001; NOx-N:P<0.01; NH4+-N,P<0.0001). Two different water levels were identified in the lake by principal components analysis:the low water level with high nutrient values and the high water level with high water temperature, SD, and Chla values. These results supported the hypothesis that WLFs clearly exerted an overall impact on the growth of phytoplankton in Poyang Lake of the Yangtze River floodplain. WLFs affected the water transparency (expressed by SD), nutrients, and Chla concentrations of Poyang Lake, likely due to flush flood pulse, re-suspension of suspended matter, dilution effects, and human activities.
Keywords: Chla; Poyang Lake; The Yangtze River; secchi depth; total and inorganic nutrients; water temperature.