Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: Lizotte R.

Intensive row-crop agriculture in the Mississippi Delta has increased eutrophication and cyanobacteria blooms in freshwater systems in western Mississippi. Two study lakes, Beasley Lake and Roundaway Lake, are shallow riverine lakes influenced by agricultural activity typically found in the Mississippi Delta. During 2018, both lakes were monitored for nutrients and cyanobacteria (measured as phycocyanin concentration). Both monitored lakes indicated eutrophic to hypereutrophic conditions. Total nitrogen (TN) ranged from 0.7 to 2.1 mg/L (average: 1.3 mg/L) and 1.1 to 3.6 mg/L (average: 1.8 mg/L) in Beasley and Roundaway, respectively. Total phosphorus (TP) ranged from 0.05 to 0.49 mg/L (average: 0.18 mg/L) and 0.08 to 0.46 mg/L (average: 0.22 mg/L) in Beasley and Roundaway, respectively. Cyanobacteria blooms ranged from 5 (March) to 112 (June) µg phycocyanin/L and 4 (April) to 210 (June) µg phycocyanin/L in Beasley and Roundaway, respectively. Cyanobacteria nutrient limitation was assessed during summer and fall (June-November 2018). Cyanobacteria nutrient sensitivity was measured in October 2018 when ambient nutrient concentrations were lowest to ascertain nutrient concentrations eliciting cyanobacteria blooms. Results indicate that summer and fall cyanobacteria biomass were primarily nitrogen + phosphorus co-limited in both lakes. Nutrient sensitivities indicated that lake cyanobacteria blooms occurred when: NO3-N > 0.15 to 0.28 mg/L; TN > 0.38 to 0.48 mg/L; PO4-3 > 0.03 to 0.06 mg/L; and TP > 0.04 to 0.07 mg/L. Regression analysis showed that summer to fall increased lake cyanobacteria nitrogen + phosphorus co-limitation responses occurred with elevated PO4-3 and decreased NH4-N during summer and fall (adjusted R2 0.42, p = 0.036, N = 12). Monitored Roundaway Lake summer to fall cyanobacteria biomass increased with elevated TN and decreased NO3-N (adjusted R2 0.82, p < 0.001, N = 14). Monitored Beasley Lake summer to fall cyanobacteria biomass increased with elevated NH4-N and decreased NO3-N (adjusted R2 0.45, p = 0.021, N = 13). This study provides insight and understanding of necessary nutrient controls to better manage and mitigate cyanobacteria blooms and improve water quality in Mississippi Delta agricultural lake watersheds.