Contributions to Hypoxia Development in Mississippi Bight Waters as Revealed by Tracer Distributions During a 4-year Time Series

A transect across the Mississippi Sound (MS) and Bight (MB) from 2008-11 revealed hypoxia occurred in the MB during late spring and summer, except for 2009. Riverine sources to this region include the Mississippi River (MR), the Pearl River (PR) and St. Louis Bay (SLB). Two distinct freshwater signals in April 2008 and May 2011 were caused by the opening of the Bonnet Carre Spillway which redirects MR water into the MS and MB. Oxygen isotope data (d¹⁸O) showed that MB waters were regulated by mixing between MR water and seawater during hypoxic periods when strong stratification persisted. High Cs levels (5-99 nM) were observed in MS waters due to the release of Cs into SLB from a point source. Distributions of d¹⁸O and Cs in surface waters suggest that the influence of water from SLB was limited to the MS and not responsible for the development of MB hypoxia. Abnormally high dissolved Ba and nutrients were observed in MB bottom water in August 2009 (no hypoxia), suggesting the presence of submarine groundwater discharge (SGD) in this region. During hypoxia, enriched bottom water Ba (89-360 nM) was also observed, as was high dissolved Mn (90-11,356 nM). The Ba enrichment, in particular, is again suggestive of an SGD influence in the bottom waters. Overall, this comprehensive data set suggests that MR water, through stratification and possibly nutrient contributions, is a major contributor to hypoxia formation in the MB. Moreover, the trace metal data reveals SGD to be a non-negligible presence in MB waters.