ANNUAL VARIATION OF NITROGEN, PHOSPHORUS AND SILICA LEVELS, AND THEIR EFFECTS ON PHYTOPLANKTON DISTRIBUTION (DIATOMS AND CYANOBACTERIA) AND EUTROPHICATION OF SMALL SHALLOW LAKES

Like nitrogen (N) and phosphorus (P), silica (Si) is an important nutrient in primary productivity. Reduction of dissolved silica (DSi) induced by anthropomorphic activities is a universal phenomenon. Such activities include dam construction. Obstruction of rivers and lakes can prevent or reduce the quantity of Si transported into the lakes. At the same time, the pollutants from human activities add more N and P into the lakes, so that transmission of Si is reduced. Since Si in the lake water can limit the growth of phytoplankton, this induces a succession of dominant phytoplankton species towards a non-diatom class. From the past to the present, it is widely considered that the outbreak of certain harmful phytoplankton (such as cyanobacteria) in lake water is strongly related to the high contents of N and P in the water. The parameters of the aquatic index of physicochemical [Cond, DO, pH, temperature, transparency, DIN (Dissolved Inorganic Nitrogen), DIP (Dissolved Inorganic Phosphorus), DSi (Dissolved Silica)] and their effect on phytoplankton (diatoms, cyanobacteria and Chl-a) growth are analyzed for the Moon Lake in Wuhan, China. The results show that there is abundant of DIN and DIP in the Moon Lake, but the DSi content is low and unevenly distributed. The primary productivity at a high place in the lake, and the density of cyanobacteria is much higher than that of diatoms. Nutrient structure of the lake water body is irrational. After the death of diatoms, the nutritional combination of the lake water (rich in N and P, but some low and uneven DSi). The growth of cyanobacteria can be stimulated by these conditions, leading to further reduction in the abundance of diatoms. As such, cyanobacteria will become the dominant species in Moon Lake. While other physicochemical factors in water may contribute, the degree of maximum output of diatoms is primarily controlled by the level of DSi. The level may play an important role in the shift from diatoms to cyanobacteria among the phytoplankton community in the lake.