https://hdl.handle.net/10388/18396

McAleer, Esther Ranae (2026) Nitrogen and Cyanotoxins: Short-Term Responses in Lakes and Reservoirs, Harvest - Theses & Dissertations, https://hdl.handle.net/10388/18396

Toxins produced by cyanobacteria can constitute a threat to human and animal health. Reducing the risk of cyanobacterial blooms is key to ensure safe waters, yet the best means of doing so is unclear and likely to differ across bloom-affected freshwaters globally. Nitrogens potential to affect blooms and toxins is frequently noted, but there is relatively limited experimentation to assess its influence on cyanotoxin concentrations, with the potential for highly varied responses across phytoplankton communities. This thesis assesses the effect of increased nitrogen concentrations on cyanotoxin concentrations in freshwater lake and reservoir environments. First, we use small scale experiments to test the effects of nitrogen addition (nitrate, ammonium, and urea) on toxins in a lake which serves as a key drinking water source. We then work to integrate these findings into a broader understanding of responses across lakes and reservoirs using a systematic review including experimental studies that have assessed nitrogen effects on cyanotoxins in mixed freshwater communities. Experimental results indicated that nitrogen addition can lead to increases in phytoplankton biomass, changes in community composition, and changes in toxin concentrations. Importantly, many of these changes were affected by nitrogen form, and differential effects on varied toxins were also shown. For example, ammonia addition to experimental microcosms stimulated increases in MC-LR but homoanatoxin-a concentrations declined, while nitrate addition had no significant impact on either toxin. These types of trade-offs in toxin responses have received little attention because of a single-toxin focus of most experimental work. Indeed, the systematic review revealed a surprisingly modest number of community-level experiments assessing N effects (with 16 studies which included 32 experiments meeting our screening criteria, and only three studies assessing multiple toxins). Results of both the systematic review and the experiments demonstrate that nitrogen can induce increases in cyanobacterial biomass and changes in community composition. Effects appear to vary based on factors that may include the time of year, bloom conditions, and the concentration of nitrogen. As with our experimental results, the systematic review illustrated that nitrogen addition can impact toxin production; again, impacts are highly variable and not universally seen. Understanding toxin-related effects of nitrogen addition is challenged by toxin diversity, taxonomic diversity, and complex regulation of toxin production. As a result, there are few generalizable statements possible about the importance of nitrogen.

https://hdl.handle.net/10388/18396