Aims Our objectives were to examine the community structure of phytoplankton and the associated environmental factors in Lake Changhu in the summer of 2012, and to investigate the degree of eutrophication in the lake.
Methods Biological characteristics of the alga and integrative nutritional state index were used for evaluation of eutrophication in Lake Changhu. Phytoplankton and water samples were collected at 20 sites. The water samples were fixed, precipitated and concentrated for qualitative and quantitative analysis. Variables related to water conditions such as chlorophyll a, total phosphorus, total nitrogen, total suspended solids, secchi disk depth and chemical oxygen demand were monitored.
Important findings Fifty-three species (genera) of phytoplankton were identified, belonging to Chlorophyta, Cyanophyta, Bacillariophyta, Xanthophyta, Euglenophyta, Pyrrophyta and Cryptophyta, respectively. Chlorophyta (24 species), which accounted for 38.9% of the total, was the most abundant, followed by Cyanophyta (15 species, accounting for 36.0% of the total) and Bacillariophyta (7 species, accounting for 14.1% of the total). There were 10 dominant species and Oscillatoria amphibia was a common species in four areas, with maximum dominancy of 0.72. The phytoplankton density varied from 12.03 × 106 to 62.13 × 106 cell·L–1 with an average of 27.71 × 106 cells·L–1. The highest cell density occurred in the Yuanxinhu area, followed by the Haizihu area and the Mahongtai area; the Miaohu area was observed to have the lowest cell density. Biodiversity index of phytoplankton varied from 0.89 to 3.24, and evenness index varied in a range of 0.23–0.83. Based on the two methods of eutrophication evaluation, the water was in moderately eutrophic and eutrophic state in Lake Changhu in the summer of 2012. Canonical correlation analysis suggested that the total nitrogen, total suspended solids, total phosphorus, dissolved oxygen and nitrite nitrogen were the main environmental factors affecting the spatial distribution of phytoplankton community in Lake Changhu in summer. Most of the Cyanophyta (Dactylococcopsis acicularis, Oscillatoria amphibia, Phormidium and Anabeana) had a great demand for total nitrogen. Affected by geographical feature, human activities and the hydrodynamic features, the sampling sites showed apparent regional differentiations as revealed by canonical correspondence analysis.