Master of Science (MSc)
Geography & Environmental Studies
Faculty of Science
Dr. Jason Venkiteswaran
Guidance, support and overseeing all experiments and writing associated with this thesis
Cyanobacterial blooms are complex phenomenon that are impacted and controlled by various factors such as lake depth, air temperature, wind, and nutrient loading. Nutrients such as phosphorus, nitrogen and iron all have very differing effects on cyanobacterial blooms. While phosphorus and iron are known to drive cyanobacterial biomass, nitrogen has been observed to have a direct impact on toxin synthesis. Nitrogen exists in the environment in many different forms such as atmospheric N2, in inorganic forms such as nitrate and ammonium as well as in organic forms such as urea which can all affect cyanobacterial bloom development differently. As removal of external loading of phosphorus has shown drastic effects in decreasing magnitude of cyanobacterial blooms, phosphorus will still enter waterbodies through internal loading. This leads us to investigate other nutrients such as iron that can act as a limiting agent for cyanobacterial growth. In anoxic environments, iron, which is an essential micronutrient for bloom development, is available as Fe(II) for uptake. In oxic environments, iron remains trapped in the sediment as Fe(III) and unavailable for uptake. By maintaining iron in its oxidized form using oxidizing agents such as nitrate to raise the redox potential at the sediment water interface, we can ensure iron remains trapped in the sediment. In this thesis, two experiments were conducted to understand potential cyanobacterial bloom responses to changing nitrogen in terms of biomass, toxins, and sediment nutrient dynamics. The first experiment focused on looking at the effects of differing forms of nitrogen (ammonium, urea, and nitrate) in combination with phosphorus to assess cyanobacterial biomass response as well as toxin production. We also looked at application differences of pulse vs press to assess whether large, one-time events such as storms compared with recurring, long-term processes such as internal loading yielded toxin and cyanobacterial biomass differences. Nutrient additions of all forms of nitrogen in combination with phosphorus increased chlorophyll-a, with ammonium yielding the highest concentrations, followed by urea and lastly nitrate- all higher than phosphorus alone. The nitrogen treatments also resulted in the highest concentrations of microcystins. When comparing application methods however, the pulse and press methods resulted in similar responses of phytoplankton biomass and toxin (microcystin and cylindrospermopsin) concentrations. The second experiment was a core incubation study where we added nitrate in differing concentrations to quantify how effective nitrate is at sustaining redox above key thresholds to suppress iron release from the sediment and understand associated changes in chemistry. Nitrate was applied in three different treatments, where the high nitrate amendment (20mg N/L) suppressed iron for longer and to lower concentrations than the medium (10mg N/L), low (5mg N/L) and control treatments. This allowed us to see the potential in using redox control to help manage cyanobacterial bloom management via management of iron availability.
Goltz, Catherine, "Nitrogen: Both a potential stimulator and suppressor of cyanobacterial blooms in temperate lakes" (2023). Theses and Dissertations (Comprehensive). 2589.