Impact of Seabed Resuspension on Oxygen and Nitrogen Dynamics in the Northern Gulf of Mexico: A Numerical Modeling Study

Resuspension affects water quality in coastal environments by entraining seabed organic matter into the water column, which can increase remineralization, alter seabed fluxes, decrease water clarity, and affect oxygen and nutrient dynamics. Nearly all numerical models of water column biogeochemistry, however, simplify seabed and bottom boundary layer processes and neglect resuspension. Here we implemented HydroBioSed, a coupled hydrodynamic-sediment transport-biogeochemical model to examine the role of resuspension in regulating oxygen and nitrogen dynamics on timescales of a day to a month. The model was implemented for the northern Gulf of Mexico, where the extent of summertime hypoxia is sensitive to seabed and bottom boundary layer processes. Results indicated that particulate organic matter remineralization in the bottom water column increased by an order of magnitude during resuspension events. This increased sediment oxygen consumption and ammonium production, which were defined as the sum of seabed fluxes of oxygen and ammonium, plus oxygen consumption and ammonium production in the water column due to resuspended organic matter. The increases in remineralization impacted biogeochemical dynamics to a greater extent than resuspension-induced seabed fluxes and oxidation of reduced chemical species. The effect of resuspension on bottom water biogeochemistry increased with particulate organic matter availability, which was modulated by sediment transport patterns. Overall, when averaged over the shelf and on timescales of a month in the numerical model, cycles of erosion and deposition accounted for about two thirds of sediment oxygen consumption and almost all of the sediment ammonium production. In coastal waters, oxygen and nitrogen levels affect the health of fish and other organisms. In the Gulf of Mexico, for example, low-oxygen regions called hypoxic areas or dead zones form in the summertime near the seabed in bottom water. It can be difficult to understand and quantify variations in bottom water oxygen and nitrogen levels, however, because: (1) water quality there is affected by many different physical and biological processes; and (2) observational studies are limited by cost, safety and technological advances. To complement previous observational studies, this paper used a new numerical modeling approach that accounts for many physical and biological processes in the seabed and water. Specifically, we used the model to evaluate how resuspension, especially the entrainment of organic matter from the seabed into the water, affected oxygen and nitrogen levels in the Northern Gulf of Mexico. Model results indicated that resuspension increased the decomposition of organic matter, decreasing oxygen levels and increasing ammonium (a form of nitrogen) levels in bottom water. This effect was largest in regions with abundant seabed organic matter and frequent resuspension. These modeling results can help scientists and environmental managers understand how resuspension affects oxygen and nitrogen levels in bottom waters.