Analysis on the reasons for the large bloom area of Lake Taihu in 2017

Abstract

After a series of comprehensive restoring, the area of Microcystis blooms in Lake Taihu Basin has gradually decreased by 2015, and preliminary results were achieved. However, in 2017, an unprecedented expansion of the Microcystis blooms area (1403 km2)occurred in Lake Taihu, and the reason behind which has become a focus of researchers' attention. In this study, water quality and Microcystis cell density data, nutrient and Microcystis cell density data, meteorological data and blooms area data were collected twice a month from 6 sampling sites in Gonghu Bay and Meiliang Bay of Lake Taihu from 2011 to 2017. The results showed that the 7-year average concentrations of total nitrogen (TN)and total phosphorus (TP)in Lake Taihu was 1.89 and 0.076 mg/L in 2011-2017, while in 2017 TN concentration was 1.60 mg/L and TP concentration was 0.083 mg/L, which was 9.0% higher than the 7-year average TP concentration. In terms of temperature, the average temperature in 2011-2017 was 17.3℃, while in 2017 it was 18.0℃, which was 0.7℃ higher than that in 7 years. The average cell density of Microcystis in Lake Taihu in 7 years was 0.53 × 108 cells/L, while in 2017 it was 1.18 × 108 cells/L, which was 2.21 times of the average in 7 years. In such a data background, RDA annual mean analysis and wind speed interval proportion analysis results indicated that the rise of TP concentration and the high temperature also showed a significant positive correlation with the area of Microcystis blooms. And the number of weak wind days from May to September has the greatest impact on the area of algal blooms, while the wind speed of more than 4 m/s would significantly reduce the probability of large-scale outbreaks. Based on the above data and analysis, the large area of algal blooms in 2017 is attributed to the increasing of Microcystis cell density due to the rise of TP concentration and high temperature in the whole lake, which provides a rich material basis for the formation of Microcystis blooms. Futhermore, the weak wind in the season of large-scale propagation of Microcystis. The wind speed of about 1-2 m/s is easy to facilitate Microcystis aggregation and Microcystis blooms formation.