Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Lizotte R.E., Yasarer L., Locke M.A.

Intensive row-crop agriculture in the Mississippi Delta have well-documented impacts on lakes in the region. Eutrophication resulting from nutrient loads (nitrogen and phosphorus) in runoff directly affect lake productivity and dissolved oxygen. Agricultural best management practices (BMPs) supported by the USDA Natural Resources Conservation Service (NRCS) can be implemented by land users to help protect and improve water quality. Although NRCS supported BMPs are primarily used to control soil loss, erosion and associated sediment loads in runoff, these same practices have the added benefit of reducing nutrient loads. The current study attempts to assess the effects of BMPs on lake eutrophication and associated oxygen stress in two Mississippi Delta lake watersheds. Beasley Lake watershed (BL) in Sunflower County has multiple integrated BMPs (16.9% watershed acreage) including edge-of-field vegetated buffers, conservation reserve areas and constructed wetland habitats. In contrast, Roundaway Lake watershed in Coahoma County has a few isolated BMPs (2.3% watershed acreage) including conservation reserve areas and a constructed wetland habitat. During May-September 2018, biweekly water quality variables comprising soluble orthophosphate (PO₄-P), total phosphorus (TP), nitrate nitrogen (NO₃-N), total Kjeldahl nitrogen (TKN), chlorophyll a (total algal biomass), phycocyanin (cyanobacteria biomass), and weekly diel surface dissolved oxygen (DO, 0.3 m at 15 minute intervals) were measured. Nutrient data showed BL had significantly lower concentrations of TKN (p < 0.001) and TP (p = 0.003) than RL. Mean summer TKN in BL and RL were 1.098 and 1.581 mg/L, respectively, and mean summer TP in BL and RL were 0.079 and 0.147 mg/L, respectively. However, mean summer NO₃-N (p = 0.416) and PO₄-P (p = 0.836) concentrations were not significantly different between BL (0.024 mg NO₃-N/L and 0.021 mg PO₄-P/L) and RL (0.061 mg NO₃-N/L and 0.023 mg PO₄-P/L). Mean summer total algal biomass (36-58 g/L) and cyanobacteria biomass (30-102 g/L) were both significantly lower in BL relative to RL (p < 0.010). Concomitantly, frequency of summer DO stress (as DO < 4 mg/L in hours per week) was significantly lower (p = 0.010) in BL (3.4 hours per week) than RL (37.6 hours per week). Correlation and regression analyses indicated associations with increased BMPs, decreased total nutrient inputs, corresponding decreased algal biomass, and decreased DO stress during summer of 2018. Results clearly indicated BL, in the presence of more intensive BMPs, was significantly less eutrophic and less oxygen stressed than RL during summer conditions.