Horizontal migration of algal patches associated with cyanobacterial blooms in an eutrophic shallow lake

Most eutrophic freshwater lakes in China have suffered from harmful cyanobacterial blooms. Phytoplankton horizontal movement plays an important role in the spatial distribution of algal biomass and algal bloom formation in eutrophic freshwater lakes. In this study, an in situ experiment was performed in Lake Zhushan, a semi-enclosed lake bay in the northwestern portion of Lake Taihu, China, to investigate the effects of hydro-meteorological factors on the horizontal movement of algal patches during August 2011, 2012 and 2014. The vertical distribution patterns of chlorophyll a under different wind regimes revealed that the critical wind speed suppressing the formation of algal patches on the water surface was ca. 3.6 m/s in the lake. When wind speed was below the critical value, a significant linear correlation was found between horizontal migration velocity of algal patches and wind speed (R = 0.368, n = 329, p < 0.001), between horizontal migration velocities of algal patches and significant wave height (R = 0.107, n = 154, p < 0.001), and between horizontal migration velocities of algal patches and current flow rate (R = 0.139, n = 262, p < 0.001). The horizontal movement of algal patches at the water surface was influenced by both wind and current directions, and a multiple linear regression equation (R = 0.642, p = 0.002) adequately described the interactive effects of wind speed, flow rate and significant wave height on horizontal migration of algal patches. These results will facilitate a better understanding of the mechanisms of phytoplankton aggregation in the upper layer and algal bloom formation in eutrophic lakes and reservoirs.