The Effects of TN:TP Ratios on the Phytoplankton and Colonial Cyanobacteria in Eutrophic Shallow Lakes

The aim of this study was to assess the effects of nutrient and other environmental factors on phytoplankton community composition and the density of colonial cyanobacteria along the gradients of total nitrogen (TN) to total phosphorus (TP) ratios. Fifteen urban eutrophic lakes with different nutrients concentrations were sampled on a bimonthly basis for two years. In total, the absolutely dominance of colonial cyanobacteria in summer occurred with the relative low TN:TP ratios, and relative higher TN:TP ratio was observed when the unicellular algae dominated the community in winter. The relationships between phytoplankton density and nutrients were analyzed using the analysis of variance (ANOVA) and regression analysis (RA), which attributed the dominant of colonial species to the high concentration of phosphorus (P) and the lower TN:TP ratios. The results of ANOVA demonstrated that the chlorophyll a (Chl-a) had a significant positive correlation with TP when the concentration of TP less than 500mg/m³, and increased smoothly when TP greater than 500mg/m³. The regression analysis between colonial cyanobacteria and TN:TP showed a negative correlation when the TN:TP less than 22 and reversed to positive when it exceed 22. Since the verity of above, we concluded that the TN:TP ratios and TP play similarly efficient role in regulating the colonial cyanobacteria densities.