Cyanobacterial surface blooms pose significant environmental and public health challenges by degrading water quality and producing harmful toxins. Understanding the physical and biochemical processes governing the formation and persistence of these blooms is crucial for improving predictive models and developing effective mitigation strategies. This project aimed to explore how hydrodynamic forces, such as wind-driven turbulence, biochemical surface interactions, and environmental conditions influence bloom dynamics in freshwater ecosystems. 

The project achieved substantial progress through laboratory experiments, modelling studies, and field observations. Key findings include: 

1. Wind and Hydrodynamics: Laboratory experiments using annular flumes demonstrated that weak turbulence promotes bloom formation by stabilizing colonies at the air-water interface, while strong turbulence disperses scum and limits colony aggregation. The presence of Microcystis was shown to reduce surface tension, facilitating scum reformation and lateral expansion during weak wind conditions. 

2. Trait-Based Modelling: A coupled hydrodynamic and trait-based phytoplankton model revealed that turbulence acts as a selective force, shaping the photosynthetic traits of cyanobacterial populations. Diverse photosynthetic capacities within a population accelerated bloom formation, emphasizing the ecological importance of intraspecific variation. 

3. Temperature Adaptation: Simulations demonstrated that the thermal history of cyanobacteria alters their thermal optima, enabling the dominance of psychrophilic strains during cold-water periods while suppressing summer blooms. This challenges the long-standing view that cyanobacteria universally thrive in warmer temperatures. 

4. Colony Morphology and Light Intensity: Laboratory experiments highlighted that light intensity influences colony morphology and buoyancy. More compact colonies remained buoyant under low light conditions, promoting surface retention, while larger colonies formed under high light exhibited decreased buoyancy. 

5. Field Protocols for Monitoring: A novel protocol using laser in-situ scattering technology was developed and validated for simultaneously measuring multiple bloom indicators, including biovolume concentration, cell density, and vertical distribution. This approach significantly improves the accuracy of monitoring cyanobacterial blooms and their migration patterns. 

Collaborative efforts with a Chinese research team facilitated the complementary field measurements in Lake Dianchi and laboratory analyses of light effects on Microcystis. The jointly achieved project outcomes, which include four peer-reviewed publications and one manuscript under review, provide new insights into bloom formation dynamics and inform strategies for mitigating harmful cyanobacterial blooms in freshwater ecosystems.

Project PIs: A. Lorke (RPTU) and X. Wu (Institute of Hydrobiology, Chinese Academy of Sciences)

Key publications:

• Wu, H., Wu, X., Rovelli, L., & Lorke, A. (2024). Dynamics of Microcystis surface scum formation under different wind conditions: the role of hydrodynamic processes at the air-water interface Frontiers in Plant Science, 15, 1370874. https://doi.org/10.3389/fpls.2024.1370874
• Wu, H., Wu, X., Rovelli, L., & Lorke, A. (2024). Selection of photosynthetic traits by turbulent mixing governs formation of cyanobacterial blooms in shallow eutrophic lakes. The ISME Journal, 18(1), wrae021. https://doi.org/10.1093/ismejo/wrae021
• Xu, G., Zhang, Y., Yang, T., Wu, H., Lorke, A., Pan, M., Xiao, B., & Wu, X. (2023). Effect of light-mediated variations of colony morphology on the buoyancy regulation of Microcystis colonies. Water Research, 235, 119839. https://doi.org/10.1016/j.watres.2023.119839
• Zhang, Y., Yang, T., Zhang, Y., Xu, G., Lorke, A., Pan, M., He, F., Li, Q., Xiao, B., & Wu, X. (2024). Assessment of in-situ monitoring and tracking the vertical migration of cyanobacterial blooms using LISST-HAB. Water Research, 257, 121693. https://doi.org/10.1016/j.watres.2024.121693

PhD thesis: Wu, Huaming: Interactions of cyanobacterial blooms with physical processes in lakes. Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau. 2024. https://doi.org/10.26204/KLUEDO/8422