Does a bottom-up mechanism trigger hypoxia in the Mississippi Bight?

Abstract:

The Mississippi Bight, east of the Mississippi River, is a complex coastal ecosystem that, like the better-known Louisiana Shelf to the west, experiences seasonal bottom water hypoxia. However, input of allochthonous nutrients from the Mississippi River to the Mississippi Bight appears to be limited, begging the question of what drives seasonal hypoxia in this system. Prior research has suggested submarine groundwater discharge (SGD) could be an overlooked component of the Mississippi Bight biogeochemical system. We thus examined the hypothesis that SGD provides a “bottom up” driver for seasonal hypoxia in this area. We used a multi-tracer approach based on known SGD indicators (dissolved Ra, Ba, Si, methane) to i) demonstrate the presence of SGD as a constituent contributor to Bight bottom waters, ii) constrain the SGD flux of macronutrients, and, iii) investigate the hypoxia-SGD linkage. We found excess SGD tracers in saline bottom waters relative to surface waters, suggesting a bottom source. Examination of sources for the constituent enrichments besides SGD (e.g., produced waters from oil rigs) appear inadequate to close the bottom water chemical mass balances. Additionally, inverse correlations between DO and SGD indicators in bottom waters support a common mechanism supplying dissolved Ra, Ba, and Si, and decreasing DO concentrations in these waters. Two different approaches to modeling the bottom water Ra distribution both suggest a seepage rate of ~0.04 m³/m²/d, in line with previous estimates in similar systems. SGD appears in some cases to be the dominant contributor of nutrients to Bight bottom waters. Likewise, the potential oxygen demand of reduced substances within SGD is shown to be able to contribute significantly to the development of seasonal hypoxia in Bight bottom waters. The bottom-up influence of SGD on the Mississippi Bight thus appears to be a significant and overlooked aspect of this and possibly other coastal systems.