Abstract

Hypoxic waters with critically low levels of dissolved oxygen are well established in coastal regions in the Northern Gulf of Mexico (NGOM). These dead zones, largely associated with massive freshwater outflows from river systems such as the Mississippi and Atchafalaya deltas, have the potential to disrupt biological systems including some of the na-tion?s most productive and important fisheries. Hypoxia was discovered East of the Mississippi in the NGOM in 2011; however, much of the spatial and temporal extent of this dead zone has yet to be fully quantified. In July of 2012 and 2013 we recorded surface and bottom dissolved oxygen, temperature, and salinity values using a SeaBird CTD (conductivity, temperature, depth instrument) at a subset of 56 total stations located East of the Mississippi River. Despite not finding hypoxic conditions (< 2 mg/l) in 2012 and 2013, the lowest dissolved oxygen (3.10 mg/l) and salin-ity (20.26 psu) levels were recorded at stations proximal to Biloxi Marsh, Mississippi. Preliminary analysis indicate that the 2011 opening of the Bonnet Carre spillway may have enabled hypoxic conditions to develop because of a substantial influx of nutrient rich freshwater into the Biloxi Marsh Area. We tracked this influx by using satellite imagery to analyze changes in chlorophyll concentrations and surface salinity levels. Cumulatively, these data indicate that elevat-ed nutrient levels and water column stratification from the spillway outflow in addition to high summer surface tem-peratures potentially caused the 2011 dead zone east of the Mississippi.

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