Huaming Wu

• Cyanobacterial harmful algal blooms (CyanHABs) are becoming increasingly frequent and intense across large geographic areas. These blooms disrupt aquatic ecosystems and pose risks to water usage due to the production of cyanotoxins by toxic strains like Microcystis spp. However, the interactions of cyanobacteria with physical environments (including their potential feedback effects on these environments), particularly at a deeper level (e.g., trait level), remain largely unknown. In this thesis, laboratory experiments, field investigations, and a one-dimensional trait-based coupled hydrodynamic-phytoplankton model were used to explore the interactions of CyanHABs with physical drivers, including wind-driven hydrodynamics, turbidity, and temperature. The experiments showed that weak winds create stable conditions that increase the size of Microcystis colonies at the air-water interface, likely through capillary forces. Microcystis also reduce surface tension, decreasing surface flow velocity and potentially driving lateral surface convection, which promotes the reformation and expansion of surface scum. The coupled hydrodynamic-phytoplankton model further examined the effects of turbulence and turbidity on cyanobacterial populations at the trait level. It revealed that both turbulence and turbidity impact the photosynthetic capacity (Pmax) of cyanobacterial populations through light competition. High diversity in Pmax also accelerated the formation of surface blooms. This model, applied to a large shallow lake (Lake Dianchi, China), was also used to study the effect of temperature on CyanHABs, particularly how they occur under unfavorable cold-water conditions. The results showed that diversification in optimum growth temperature (Topt) enhances psychrophilic strains, promoting cold-water blooms while mitigating summer blooms. Topt-diverse populations exhibit an inverse relationship with summer temperatures, challenging the current cyanobacteria-temperature paradigm and indicating that environments can shape population responses in opposite directions based on trait diversity. This thesis highlights previously unrecognized mechanisms (i.e., altering physical environments and trait distributions) by which cyanobacteria cope with varying conditions, emphasizing the need to integrate these processes into models to better understand how physical drivers will affect dynamics of CyanHABs under future climatic conditions.