Ecological impacts of microcystins on biodiversity structure and ecosystem functioning in freshwater systems

Microcystin, a prominent cyanotoxin produced by cyanobacteria during harmful blooms, has been widely reported from freshwaters worldwide, affecting humans and causing high mortalities and reproductive defects among fish, amphibians, and zooplankton. However, the ecological implications of microcystin exposure on structure and functions in freshwater ecosystems remain unclear. In this thesis, to evaluate the ecological impacts of microcystin on survival and ecosystem functions among freshwater species, populations, and communities, I used a scaling up experimental approach to test a set of explicit hypotheses. First, using a literature-based approach, I illustrated that environmentally relevant microcystin concentrations (0.01-10.0 µg·L-1) used in this study were comparable to the range of safe levels recommended by the WHO for human health in freshwaters. Second, I used a suite of standard ecotoxicological assays to test the sensitivity of five key food web components (namely, Scenedesmus quadricauda (phytoplankton), Daphnia magna and D. pulex (zooplankton grazers), Gammarus pulex and Dikerogammarus villosus (native and invasive amphipod shredders) to the purified microcystin LR (MC-LR) and crude extract of Microcystis aeruginosa. Purified MC-LR significantly inhibited S. quadricauda growth by ca. 22% and slightly elevated its photosynthetic pigment contents at low concentrations compared to the control, suggesting hormesis is potentially a physiological response associated with the putative allelopathic role of microcystin during harmful blooms. In contrast, increased crude extract concentrations significantly reduced survival of D. magna by ca. 70%, suggesting the effect of low environmental concentrations may vary across taxa, biological endpoints, and treatments. Third, using a combination of sublethal and chronic toxicity tests, I evaluated the effects of two microcystin treatments on survival, feeding, growth and reproduction among three ecologically important freshwater species. Individual survival was unaffected at low exposure concentrations, however, feeding, growth and fecundity were significantly altered, thereby stimulating the population growth rate. Fourth, using a multiple stressor approach, I tested the individual and combined effects of two microcystin treatments and three water temperatures (15, 20 and 25°C) on survival and ecological processes among key freshwater taxa. Purified MC-LR had a higher growth inhibitory effect on S. quadricauda compared to the crude extract, while increasing microcystin concentrations in the crude extract reduced the grazing rate of D. pulex and survival of Ischuran elegans. Reduced prey handling time and higher predation rate induced by warming in I. elegans were offset by a 50% reduction in survival caused by the synergistic interaction between temperature and microcystin, suggesting complex effects on survival and ecological processes of species across multiple trophic levels. Finally, using a set of community microcosms, I report for the first-time complex effects of sublethal microcystin concentrations on structure and functioning in freshwater ecosystems. The results showed indirect effects of sublethal microcystin exposure on community composition and organic matter decomposition may be compounded by changes in abiotic factors such as water pH. Taken together, these studies suggest strongly that microcystin concentrations considered safe for human drinking and recreation may still have significant but subtle effects on the structure and functioning of aquatic communities.