Nutrient Limitation of Phytoplankton and Nitrogen Fixation in Artificially-aerated Lake Carmi, VT

Vermont lakes are changing rapidly in response to interacting climate and landscape disturbance. Oligotrophic lakes in the state have doubled in total phosphorus over the past 40 years, and cyanobacteria blooms are persistent or newly emerging across trophic states. In response, there has been a substantial effort to control watershed phosphorus (P) inputs and remediate internal P loading from lake sediments, but nitrogen (N) has been measured inconsistently across VT lakes and is less understood. Lake Carmi, VT is a eutrophic lake in the northwestern corner of the state and an upstream tributary of Missisquoi Bay, Lake Champlain. An aeration system was deployed in Carmi in July 2019 to oxygenate bottom waters, prevent summer stratification, and control sediment P release, with the goal of mitigating cyanobacteria blooms. These efforts have not effectively reduced cyanobacteria biomass but instead have resulted in increased biomass of nitrogen-fixing taxa and displacement of late-season diatom blooms. Nutrient limitation bioassays conducted in the lake in summer 2018 (pre-aeration) indicated that phytoplankton were N or N and P colimited. Because of this, there is a clear need to understand the role of N in Lake Carmi where P has been the primary focus of management efforts. We assessed seasonal changes as well as the differential roles of inorganic and organic N forms in nutrient limitation using microcosm NO3-N, NH4-N, glycine, and P enrichment experiments in June, August, and September of 2023. In addition to phytoplankton responses, we also assessed nutrient limitation of cyanobacteria. Samples were collected to measure nitrogen fixation rates using acetylene reduction to better understand cyanobacteria-mediated N processing rates. We found nutrient limitation status shifted between dates in all cases and differed between cyanobacteria and total phytoplankton, indicating seasonally dynamic nutrient supply and demand. We found nutrient limitation results to be similar between NO3-N and glycine, but bioassays using NH4-N almost exclusively resulted in no significant limitation effect. Nutrient limitation status determined using bioassays did not align with predicted total N:total P (TN:TP) threshold indicators, although effect size analyses indicated a significant relationship between TN:TP and nutrient limitation status. Nitrogen fixation rates were highest mid-summer and were only significantly positively related to NP colimitation. Results of this work highlight the role of N in cyanobacteria growth in eutrophic Lake Carmi and the need to incorporate N supply and demand effects of management strategies.