SEMINAR: Spatial and Temporal Dynamics of Cyanobacterial Toxins in Freshwater Systems: Implications for Water Quality Management

Excessive growth of cyanobacterial biomass, commonly known as cyanobacterial blooms, appear to be increasing in magnitude and frequency worldwide, thus posing an additional threat to the safety and security of water resources With the increasing global water stress, there is a need for effective management ofcyanobacterial blooms in water systems. So far, it has been a great challenge to manage the occurrence and assess public health risks associated with cyanobacterial blooms due to difficulty in predicting the level of cyanobacterial biomass and microcystin concentration. The effectiveness of strategies currently used in the management and public health risk assessment depend upon the understanding of the dynamics of cyanobacteria and microcystin under natural conditions. This research aims to: i) assess the relationship between cyanobacterial biomass and microcystin concentration, which is the indirect microcystin risk assessment traditionally used by water authorities; ii) explore the environmental drivers of the dynamics of cyanobacterial dominance and microcystin production and assess site specificity of the environmental drivers; and iii) investigate how changes in the structure of phytoplankton community and cyanobacterial composition affect the dynamics of microcystin production.

The results revealed that cyanobacterial and microcystin spatiotemporal dynamics affect the biomass-toxin relationship and pose a significant challenge in the indirect microcystin risk assessment. The correlation between the biomass and toxin is weak and site-specific, and large changes in total microcystin concentrations occur even at stable cyanobacterial biomass concentrations. The divergence in the biomass-toxin correlation suggests a direct effect of the environmental factors on the dynamics of cyanobacteria and microcystin production. Environmental factors, particularly nutrient concentrations are significantly correlated to the spatiotemporal dynamics of cyanobacteria and microcystin production. Low phosphorus and iron concentrations in the water column trigger the dominance of cyanobacterial biomass in the phytoplankton community. In contrast, high phosphorus and iron concentrations in the water column trigger high microcystin production. Nevertheless, the correlations between the cyanobacterial and microcystin dynamics and environmental factors are site-specific. In addition to nutrients, changes in the structure of phytoplankton community are also significantly correlated to the dynamics of microcystin production. Under high nutrient concentrations, cyanobacteria may not dominate the phytoplankton community. Higher microcystin production associated with increased nutrient concentrations and the dominance of other phytoplankton groups in the systems supports the hypothesis of allelopathic interaction in cyanobacteria.

The results presented in this thesis will provide a new basis for the improvement of the assessment strategies used to determine the risk of the presence of cyanobacterial blooms and their associated toxins in water systems to humans and the environment.