The dynamics of biological nitrogen fixation in prairie lakes
Nutrient-rich lakes are highly susceptible to cyanobacterial blooms. Nutrient pollution can lead to worsening blooms and the loss of important ecosystem services. A key approach to managing aquatic health has been nutrient control. Although current technologies can remove a large proportion of nutrients from wastewater prior to discharge, agricultural nutrients are difficult to control, as are in-lake sources of nutrients, such as biological nitrogen fixation. The consumption of dissolved inorganic nutrients can be rapid and potentially lead to the depletion of nitrogen stores in highly productive lakes. Reductions in nitrogen loads can be compensated for through nitrogen fixation by select cyanobacteria (diazotrophic taxa) to a variable extent. The goal of this work is to support a better understanding of potential management options for eutrophication and for blooms by better understanding the uniquity of cyanobacterial blooms and the associated roles of nitrogen fixation in time and space. This study is the first to use 15N2-calibrated acetylene reduction bottle assays to measure gross rates of nitrogen fixation in lakes located in southern Saskatchewan, Canada. Nutrient-rich lakes with differing nutrient sources were sampled over the course of one summer. Results indicated that upstream lakes receiving nutrients pulses from land-surface runoff and internal cycling experienced fluctuations in total biomass, predominantly cyanobacteria, and a mid-bloom shift in taxa dominance from diazotrophic to non-diazotrophic taxa. Meanwhile, downstream wastewater-impacted lakes receiving more constant nutrient supplies (in addition to nutrient pulses) were able to maintain more consistent levels of biomass, which were dominated by diazotrophic cyanobacteria. Nitrogen fixation rates were greatest in lakes with fluctuations in biomass, while the lakes with more consistent biomass dominated by diazotrophs actually exhibited relatively low rates of nitrogen fixation. A more intensive study of one upstream lake found that cyanobacteria rapidly increased with elevated water temperatures and increasing PAR. At the onset of the bloom, key nitrogen fixation events co-occurred with transient stratification in limnetic zones. Interestingly, some of the highest seasonal rates of nitrogen fixation occurred in surface scum formations, which were consistently dominated by diazotrophs, suggesting that fixation is important to scums. Although existing work in the region found that nitrogen fixation provides a relatively small proportion nitrogen compared to external and internal cycling, this thesis work shows that diazotrophic cyanobacteria may provide short-term relief from nitrogen depletion. Nitrogen fixation can help support high densities of cyanobacteria, and potentially toxin-producing taxa.
nitrogen fixation, nitrogen, phosphorus, nutrients, cyanobacterial blooms, cyanobacteria, eutrophication, eutrophic lakes, prairie lakes
Master of Environment and Sustainability (M.E.S.)
School of Environment and Sustainability
Environment and Sustainability