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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.

Exceptional vertical migration ability provides the cyanobacterium Microcystis with competitive advantages in bloom formation and dominance establishment. Studies have been conducted on the vertical migration patterns of Microcystis since the 1970s; however, bloom simulations remain limited. Here, we used a simple model based on the viscous drag force of turbulence and analysed the motion characteristics of Microcystis colonies. The vertical distribution of turbulent kinetic energy (K Z ), cell concentration and colony size profiles in Lake Taihu (Meiliang Bay and Gonghu Bay) and the critical vertical turbulent kinetic energy of colonies (TK Z , i.e. the anti-turbulence ability of colonies) were tested. The results showed that, under steady K Z profiles, colonies had relative rest positions (RRPs) where K Z  ≈ TK Z and colonies of the same size gathered together. The vertical migration patterns were affected more by turbulence than by density (colony mass density) if the average K Z of the water column (MK Z ) was not equal to TK Z . If MK Z  ≈ TK Z , the colonies could exhibit a diurnal pattern of sinking at noon and floating upwards before dawn without steady RRPs. Our results suggest that studies on RRPs may offer optimizations for bloom forecast and control in the future due to easier simulation of K Z profiles than that of flow fields.

The widespread use of detergents increases the concentration of surfactant in lakes and reservoirs. High surfactant loads produces toxicity to algae; however, the influence of the increasing surfactant on the competition between algae is not clear. In this paper, different amounts of linear alkylbenzene sulfonate (LAS) were added to test the effects of LAS on the competition between Microcystis aeruginosa and Scenedesmus obliquus under eutrophic condition. In single culture, the growth of S. obliquus was promoted under lower LAS concentrations (1 and 20 mg L(-1)), but cell density of S. obliquus reduced when treated with higher LAS concentration (100 mg L(-1)). The growth of M. aeruginosa was inhibited markedly with 20 and 100 mg L(-1) LAS. Compared with single culture, the result was opposite in co-cultures and the cell density of S. obliquus increased significantly when treated with LAS of 1, 20, and 100 mg L(-1). The specific growth rates of S. obliquus and M. aeruginosa in both cultures were 0.4-0.5 day(-1) and 0.6-0.7 day(-1), respectively, except that the specific growth rate of M. aeruginosa in both cultures treated with 100 mg L(-1) LAS was about 0.2 day(-1). M. aeruginosa dominated over S. obliquus in the co-culture without LAS, while the competition was completely opposite with the addition of 20 mg L(-1) LAS. The growth of S. obliquus treated with 20 mg L(-1) LAS was not affected significantly in single culture but was promoted by 75 % in co-culture. Moreover, the growth of S. obliquus in co-culture treated with 100 mg L(-1) LAS was promoted by more than 97 %. These results suggested that the increasing LAS would overturn the competition of algae in freshwater ecosystems.