Observed nitrogen accumulation during salinity and temperature surges in a coastal dam controlled lake

In this study, we tested the hypothesis that bidirectional dam operations modulate Total Nitrogen (TN) accumulation through changes in salinity, temperature, and hydrodynamic flushing in Guana Lake, Florida. We conducted one year of monthly surveys, complemented by continuous and discrete measurements of TN and key water quality parameters (salinity, temperature, pH, chlorophyll-a, and dissolved oxygen). During summer 2024, management actions increased saltwater inflow to suppress freshwater vegetation, which reduced net outflow and tidal flushing. Together with high evaporation, these actions coincided with elevated salinity and temperature, exceeding values in the adjacent estuary. These conditions were associated with increased organic nitrogen and ammonium concentrations, elevated chlorophyll-a, and reduced dissolved oxygen, consistent with altered nitrogen cycling pathways. Nitrate and nitrite remained below detection limits throughout the study period. Our results indicate that episodic salinity and temperature increases were associated with TN accumulation, particularly in reduced and organic forms, under the observed management regime. However, this study did not directly quantify process rates or sediment fluxes, and mechanistic interpretations are based on observed correlations and established biogeochemical frameworks. The findings suggest that dam operations that modify residence time and flushing may influence nitrogen accumulation patterns, but quantitative evaluation of alternative management scenarios requires further hydrodynamic and biogeochemical modeling. This study provides observational evidence of coupled management, salinity, and nitrogen dynamics in a dam-managed coastal lake. While system-specific hydrologic and geomorphic conditions must be considered, the conceptual framework may inform future investigations of nitrogen dynamics in similar managed estuarine systems.