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

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 other sources for the constituent enrichments besides SGD (e. g., rivers, produced waters from oil wells) 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 similar to 0.055 m(3) m(-2) d(-1), in line with previous estimates in similar systems. Our more complex model, involving four mass balances, suggests that as much as 10-20% of the bottom water in the Bight circulates through the underlying permeable sediments on a time scale of similar to 10 days. This circulated water emerges as SGD with completely altered chemistry. More specifically, SGD appears in some cases to be the dominant contributor of nutrients to Bight bottom waters. Additionally, the potential oxygen demand of reduced species within SGD likely contributes significantly to the development of seasonal hypoxia in Bight bottom waters. Further work is needed to better resolve sources of nutrients and additional reduced species within the Mississippi Bight SGD as well as the variability and pathways of this supply. Nonetheless, the bottom-up influence of SGD on the Mississippi Bight appears to be a significant and overlooked aspect of this system. We suggest that such a bottom-up influence may be a generally important feature of coastal ecosystems.

Automated summary

This study examines the potential impact of submarine groundwater discharge (SGD) on seasonal hypoxia in the Mississippi Bight biogeochemical system and suggests that SGD may be a significant source of nutrients for bottom waters in the area.