Environmental drivers of phytoplankton community succession and cyanobacterial harmful algal blooms along a fluvial-lacustrine continuum

The integrity of freshwater resources has been continually threatened by persistent human influence through increased urbanization, agriculture and the use of chemical fertilizers. One manifestation of anthropogenic influence in aquatic ecosystems in recent decades is an increased prevalence of cyanobacterial harmful algal blooms (cyanoHABs) due to eutrophication. Proliferation of cyanoHABs can alter aquatic food webs and create low oxygen zones, which can be detrimental to freshwater life. These blooms can potentially produce toxins with the most commonly found group of toxins in the lower Great Lakes being microcystins. Surveys within Lake Erie have shown that Microcystis can contribute over 40% of cyanobacterial abundance creating communities of both toxin-producing and non toxin-producing strains. However, toxicity can be difficult to predict, as community composition depends on multiple environmental factors. According to the Paradox of the Plankton, competitive exclusion should result in phytoplankton species out-competing each other for resources and reaching eventual equilibrium; however, it is known that well-mixed bodies of water can support a vast number of species. Phytoplankton communities in the Great Lakes follow seasonal and temporal succession, and limited surveys have found that phytoplankton community structure differs between rivers and their respective lakes. The research that comprises this dissertation was conducted to gain an understanding of the drivers of these temporal and spatial differences by examining nutrients, water quality parameters and phytoplankton community composition along the continuum. Results of microcosm experiments conducted in the Thames River found there was a reduction in overall abundance and diversity of phytoplankton, and changes in community composition with lower light availability. Experimental units with higher light availability were dominated by colonial cyanobacteria, mainly Aphanocapsa. Results from research done along the fluvial-lacustrine continuum found that water quality parameters differ temporally, likely driving the differences in phytoplankton community composition. River sites were generally characterized by higher nutrient concentrations and N:P ratio whereas the lake sites had lower nutrient concentrations and higher temperatures, and the river mouth represented a zone of mixing. Phytoplankton communities follow a seasonal succession, as well, mainly dominated by diatoms in the winter and spring and shifting to cyanobacterial dominance in summer and fall. The composition of cyanoHABs is often attributed to Microcystis, however, we found colonial communities to be more diverse than expected, often co-dominated by several genera. Similar research was conducted in the Sondu River- Lake Victoria continuum as the lake is plagued by cyanoHABS as we see in Lake Erie. Though we obtained only limited results due to a lack of field supplies, they suggest that the river is an important source of nutrients contributing to the degradation of the lake. We found similar water quality characteristics along the continuum as in Ontario. This laid important groundwork for continued research in the Lake Victoria catchment and highlighted the need for continued, place-based studies and investment in African-led research. There is a scarcity of in-depth ecosystem studies conducted in this region, and gaining a better understanding of these systems could inform policy development on freshwater management in African countries. These cumulative findings provide insight into the drivers of phytoplankton community composition along fluvial-lacustrine continuums and supported the notion that cyanoHABS often are more diverse than usually understood.

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