Silvia Newell (Advisor), Mark McCarthy (Committee Member), Chad Hammerschmidt (Committee Member), Morgan Steffen (Committee Member), Wayne Gardner (Committee Member)
Doctor of Philosophy (PhD)
Non-point source nitrogen (N) from agriculture is a main driver of eutrophication in aquatic systems, which often manifests as toxin producing cyanobacterial harmful algal blooms (cyanoHABs). Non-N2 fixing cyanobacteria, such as Microcystis, thrive on chemically reduced N forms (e.g., ammonium (NH4+) and urea) used as the main N form in fertilizer. NH4+ turnover rates are important components of the aquatic N cycle in eutrophic lakes affected by cyanoHABs. Regeneration of NH4+ can contribute to the internal cycling of NH4+, which can sustain cyanoHABs when external loads are low. Additionally, NH4+ uptake by cyanobacteria competes directly with nitrification, another important pathway in the N cycle. This dissertation research focuses on NH4+ dynamics (uptake, regeneration, and nitrification) in hypereutrophic Lake Taihu (China), Lake Okeechobee (LO) and St. Lucie Estuary (SLE; Florida), and Sandusky Bay (Lake Erie) via a combination of geochemical rate measurements and molecular biology techniques. Additionally, the study in LO and SLE also focuses on the effects of Hurricane Irma on N cycling and cyanoHABs. In Taihu, nitrification rates varied seasonally, corresponding with cyanobacterial bloom progression, suggesting that nitrifiers were poor competitors for NH4+ during the bloom. Ammonia oxidizing archaea (AOA) amoA gene copies were more abundant than ammonia oxidizing bacteria (AOB) gene copies at all times; however, only abundance of AOB amoA was correlated with nitrification rates for all stations and all seasons (p < 0.005). Regeneration results suggested that cyanobacteria relied extensively on regenerated NH4+ to sustain the bloom. Internal NH4+ regeneration exceeded external N loading to the lake by a factor of 2 but was ultimately fueled by external N loads. Our results thus support the growing literature calling for watershed N loading reductions in concert with existing management of P loads. In Sandusky Bay, during the summer Planktothrix bloom in 2017, potential NH4+ uptake (light and dark) and regeneration rates increased from June to August at all stations. NH4+ uptake kinetics during a Planktothrix-dominated bloom in Sandusky Bay and a Microcystis-dominated bloom in Maumee Bay were also compared. The highest half saturation constant (Km) in Sandusky Bay was measured in June and decreased throughout the season. In contrast, Km values in Maumee Bay were lowest at the beginning of summer and increased in October. Similar to Taihu, during late summer in Sandusky Bay (when ambient N concentrations were low) cyanoHABs relied mostly on regenerated NH4+ to support growth and toxin production. LO and SLE were sampled during a massive Microcystis bloom in July 2016, a smaller bloom in August 2017 (two weeks before Hurricane Irma), and 10 days after Hurricane Irma landfall. Hurricane Irma traversed the Florida peninsula on Sept 10, 2017, with sustained winds exceeding 160 km h-1 and generating torrential rains over LO and SLE. Extreme natural events, such as hurricanes, can dramatically alter N cycling and amplify preexisting eutrophication and cyanoHABs, which are prevalent in LO and SLE. We show that NH4+ regeneration and potential uptake rates and Microcystis abundance decreased significantly after the hurricane in both LO and SLE. Although regeneration decreased post-hurricane, measured rates exceeded external loading by a factor of 15, suggesting that, even after major disturbances, phytoplankton rely heavily on internal N. In contrast, nitrification rates in LO and SLE were orders of magnitude greater after the hurricane. AOA exceeded AOB at all times in LO and SLE. After Irma, the abundance of AOA increased in both LO and SLE, while AOB remained similar in LO and increased in SLE. These data show that storm events can shift major processes in the N cycle and alter microbial community structure. These results highlight the importance of internal regeneration of chemically reduced forms of N in cyanoHAB-impaired systems and can also lead to improvements in nutrient management practices that are necessary for cyanoHABs mitigation. These studies support the recent calls for dual nutrient (N + P) management strategies and highlight the importance of N removal through nitrification and denitrification, specifically in HAB-impaired systems.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
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