A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

Abstract

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.

Quantifying the Relative Importance of Riverine and Open‐Ocean Nitrogen Sources for Hypoxia Formation in the Northern Gulf of Mexico

“…The large N and P loads stimulate high rates of net primary production (NPP) on the shelf (Lohrenz et al, ), which fuels microbial respiration during organic matter decomposition. Vertical density stratification—a prerequisite for hypoxia development—is pronounced in spring and summer in parts of the NGoM, limiting the resupply of O 2 to bottom waters (Yu, Fennel, & Laurent ). This strong stratification is largely due to the riverine FW inputs.…”

N and P as ultimate and proximate limiting nutrients in the northern Gulf of Mexico: Implications for hypoxia reduction strategies

“…In our previous studies, we have used several, qualitatively different parameterizations for the interaction between sediment and overlying water column: an instant remineralization (IR) parameterization, which assumes that all organic matter is remineralized immediately upon reaching the sediment , an empirical parameterization where sediment oxygen 30 consumption and nutrient efflux from the sediment depend on bottom water temperature and oxygen (e.g., Yu et al, 2015a), and sediment flux parameterizations based on a metamodel analysis of a diagenetic model . The simple empirical parameterization, in particular, has proven useful as a computationally efficient and accurate bottom boundary layer Biogeosciences Discuss., https://doi.org /10.5194/bg-2017-470 Manuscript under review for journal Biogeosciences Discussion started: 6 November 2017 c Author(s) 2017.…”

Impact of Satellite Remote Sensing Data on Simulations of Coastal Circulation and Hypoxia on the Louisiana Continental Shelf

“…The net water column respiration term is constant in time and in depth, but varies horizontally with water depth. Inshore of the 20-m isobath, net water column respiration is negative implying an oxygen source; the largest positive net water column respiration representing an oxygen sink occurs between the 20-and 30-m isobaths [4].…”

“…A simple oxygen model described by [4,[30][31][32] was used to assess the impact of Kd and rainfall from satellites on the bottom water hypoxia. This simple oxygen model consists of a parameterization for air-sea oxygen exchange of Wanninkhof [30], an empirically derived parameterization of net water column respiration based on observations by Murrel et al [31], and an empirical oxygen-and temperature-dependent parameterization of sediment oxygen consumption or SOC described by Hetland and DiMarco [32].…”

“…Hydrodynamic models can improve understanding of a wide variety of processes; including heat transfer, advection, mixing, and material transport. Further, coupling hydrodynamics with biogeochemical observations and models that characterize nutrient, carbon, and oxygen dynamics and food web interactions can provide products for specific applications, such as formulation of nutrient and carbon budgets or distribution maps of dissolved oxygen [2][3][4].…”