Planktonic indicators of trophic states for a shallow lake (Baiyangdian Lake, China)

Eutrophication in lake ecosystems poses a serious threat to the water quality and function of aquatic ecosystems. The species composition and community structure of plankton change directly with variations in lake trophic states. Seasonal sample collections of phytoplankton, zooplankton and environmental variables were conducted in spring, summer, and fall from a large shallow lake, Baiyangdian Lake, China. The species richness, community composition and temporospatial variations of phytoplankton and zooplankton were analyzed. The lake trophic states were assessed using the comprehensive trophic state index (CTSI) and rotifer trophic state index (TSIROT). The results indicated that 69.1% of the lake area showed slight eutrophication, 29.3% showed mesotrophication, and 1.6% showed moderate eutrophication. The Shannon-Wiener diversity index and Pielou evenness index were employed to assess the community diversity of phytoplankton and zooplankton. The dominant taxa and most dominant species of phytoplankton and zooplankton were determined; Cyanophyta and Rotifera were the dominant phytoplankton and zooplankton taxa, respectively. Microcystis sp. and Polyarthra vulgaris were the most dominant species of phytoplankton and zooplankton, respectively. Redundancy analysis (RDA) was applied to identify the key environmental variables that influenced the species, diversity indices and species abundance of phytoplankton and zooplankton. The results showed that ammonia nitrogen (NH4+), total nitrogen (TN), total phosphorus (TP), and dissolved oxygen (DO) were the main environmental factors influencing the species abundance and diversity. The current lake plankton species number and diversity index were lower than those in past decades, as determined by comparing the community characteristics of phytoplankton and zooplankton with historical records. Comparisons of TN:TP ratios with those of other lakes suggested that nitrogen was the limiting nutrient for lake eutrophication. Baiyangdian Lake could have a high potential for eutrophication based on the environmental and ecological characteristics of the lake.