Biogeochemical Impacts of Altered Freshwater Flow to the Mississippi Sound

The Bonnet Carré Spillway (BCS) is a flood control structure on the lower Mississippi River that is periodically opened at peak flow to prevent flooding in New Orleans and other municipalities. Since its construction in 1931, the spillway has only been opened fourteen times. However, in recent years, spillway openings have become more common with four openings since 2016. In 2019, the BCS was opened twice for a combined total of 123 days, sending large amounts of Mississippi River water into the Mississippi Sound resulting in large reductions in salinity, extensive algal blooms and nearly 100% mortality of oyster reefs. Weekly water quality assessments were conducted from June thru August 2019 to measure nutrients, dissolved organic carbon and nitrogen, chlorophyll a, and particulate organic matter as well as the stable isotopic composition of water (δ¹⁸O and δD) for water source tracking. River water samples had high nitrate concentrations (92 µM) and low ammonium and soluble reactive phosphate (SRP) concentrations (<2 µM). Water isotope results show that water from some regions of the Sound was composed of nearly 50% Mississippi River water. Dissolved inorganic nitrogen (DIN) and chlorophyll concentrations increased by an order of magnitude compared to historical measurements and bottom water hypoxia was measured across large portions of the Sound. No significant increases in soluble reactive phosphate concentrations were observed resulting in high DIN:SRP ratios (50 - 70) in the western Sound. Under normal hydrological regimes, DON represents the largest nitrogen pool in the Sound; however, DON concentrations concurrently decreased with DIN increases while the BCS was open. After the BCS closure in late July, nutrient concentrations throughout the Sound returned to low background concentrations within several weeks.