Research On Influence Of Nutrient Level And Meteorological Condition On Cyanobacteria Bloom In Taihu Lake

Taihu Lake is China’s third largest freshwater lake. It plays a significant role in supplying drinking water, restraining flood, and maintaining ecological balance. Because of the basin’s rapid economic development, Taihu Lake suffers serious pollution. The water quality is worse than Class V in most waters of Taihu Lake. There is too much nitrogen and phosphorus in Taihu Lake, and these nutrient factors are the limiting factors that affect the water quality rating. At present, eutrophication and Cyanobacteria Bloom are the most important ecological problems that Taihu Lake faces.This research collects water data and meteorological data of Taihu Lake from October 1991 to December 2006, and uses some statistical analysis methods, like correlation analysis and one-way analysis of variance (one-way ANOVA), to reveal the effects of nutrient and meteorological factors and their different levels on the Cyanobacteria Bloom in Taihu Lake.The trend analysis reveals that the mean Taihu Lake’s chlorophyll a and cyanobacteria biomass in 1991-2006 experienced two (1996-1997,2004-2005) and three peaks (1994-1995,1998-1999,2004), the mean Taihu Lake’s total nitrogen and total phosphorus’s annual variation is similar to chlorophyll a, the annual variation of temperature, wind speed and rainfall is small, but the most warm years and the mean cyanobacteria biomass peak years are the same is some degree, and the temperature has a trend that high temperature appeared earlier and disappeared later in a year.The correlation analysis results show that the current factors that closely related to Cyanobacteria Bloom includes total phosphorus, temperature, and the previous factors includes total nitrogen (the quarter before the previous quarter). Wind speed (3,4 and 5 months ago) and rainfall in the previous quarter in some cases also show a significant correlation with chlorophyll a and cyanobacteria biomass, but it is difficult to determine whether it is the independent effect of these factors. Total phosphorus and biological factors have an independent correlation out of other factors’control. Temperature and early total nitrogen (the quarter before the previous quarter) interfere with each other, perhaps because the two have the similar seasonal changes.The classification analysis results are:total phosphorus is divided into three levels (<0.088mg/L,0.088mg/L-0.132mg/L,>0.132mg/L), temperature is divided into two levels (<17.8 ℃,17.8℃), early total nitrogen (the quarter before the previous quarter) is divided into four levels (<2.32mg/L,2.32mg/L-2.93mg/L,2.93mg/L-3.51mg/L, 3.51mg/L-4.35mg/L), wind speed and early wind speed (3,4 and 5 months ago) will not be graded, rainfall is divided into two levels, the previous quarter’s rainfall is divided into seven levels, temperatures in the previous two months is divided into five levels. The increase in total phosphorus can cause the increase of the frequency of Cyanobacteria Bloom (characterized by chlorophyll a), and the first and third levels of total phosphorus have good negative and positive indications for Cyanobacteria Bloom (characterized by chlorophyll a). Temperature increases, frequency of Cyanobacteria Bloom (characterized by chlorophyll a, or cyanobacteria biomass) increases, and 17.8° C can be used as a reference of Cyanobacteria Bloom alert. Early total nitrogen (the quarter before the previous quarter) increases, frequency of Cyanobacteria Bloom (characterized by chlorophyll a, or cyanobacteria biomass) increases, and the first and the fourth levels of early total nitrogen (the quarter before the previous quarter) have good negative and positive indications for Cyanobacteria Bloom. Although there are some levels of other factors having good indications for Cyanobacteria Bloom, but these results are difficult to explain.Therefore, the nutrient level is still the most fundamental factor in the Cyanobacteria Bloom in Taihu Lake, and the integrated control of the nitrogen, phosphorus is the most fundamental means to decrease Cyanobacteria Bloom in Taihu Lake. Temperature plays an important role in promoting Cyanobacteria Bloom in Taihu Lake. Taihu Lake’s trend that high temperature appearing earlier and disappearing later is evident, this may mean that the Taihu Lake’s Cyanobacteria Bloom problem will become more severe in the future.