Effect of the Rainfall on Extinction of Cyanobacteria Bloom and Its Mechanism Analysis

Abstract

There were three rainfall events with different intensity in the Xiangxi Bay (XXB) from May 24 to June 2 in 2016. The factors such as hydrodynamics, water temperature, optical properties, and chlorophyll a concentrations during the rainfall events were analyzed. During the May 27 moderate rain period, the upstream flow of the reservoir bay increased by 1.9 times and the average mixing layer depth in the whole reservoir increased 8.2 m, compared to those before the rainfall event. During the June 1 light rain period, the average mixing layer depth in the whole reservoir increased 1.6 m and the average chlorophyll concentration reduced 2.02 μg·L-1, compared with those before the rainfall event. During the June 2 heavy rain period, the upstream flow of the reservoir bay increased by 4 times, the average mixing layer depth in the whole reservoir increased 7.9 m and the average chlorophyll concentration reduced 14.64 μg·L-1, compared with those before the rainfall event. The algae moved from the upstream to the downstream with water that reduced the concentration of algae in the XXB. The water temperature stratification weakened during the rain event and the average mixing layer depth in the whole reservoir increased, destroying the algal growth environment. After the rainfall, under suitable light and temperature conditions for 2-3 d, the water temperature stratification of the reservoir was recovered and rapid growth and reproduction of algae occurred. As a result, the chlorophyll concentrations in the reservoir increased. Rainfall has a periodic inhibitory effect on the outbreak of algal blooms; however, it cannot fundamentally solve the problem of tribal bay blooms.