It remains unclear why the area of a cyanobacterial bloom increases in a shallow lake after a typhoon passes. In this study, the mechanisms of cyanobacterial bloom expansion were investigated by studying meteorological factors, water quality, algal biomass, and bloom area in Lake Taihu before and after typhoons (2007-2016). Our results showed that typhoon-induced sediment resuspension caused a short-term increase in nutrients, but nutrients returned to pre-typhoon levels after the typhoon passages. The short-term nutrient release during a typhoon did not result in an obvious increase in Microcystis cell density in two bays of Lake Taihu (Gonghu and Meiliang). Under strong winds, Microcystis aggregates were uniformly distributed in the water column downwind and then dispersed into different directions by wind-driven currents. In particular, Microcystis in the surface water were transported to the center of Lake Taihu. After a typhoon, dispersed Microcystis refloated and formed blooms. Thus, the bloom area was enlarged compared with before a typhoon. Several days after a typhoon, the bloom area gradually reduced as a result of a steady breeze on the horizontal accumulation of Microcystis.
Keywords: Bloom area; Expansion; Hydrodynamics; Microcystis; Nutrient; Typhoon.