Sensitivity of hypoxia predictions for the northern Gulf of Mexico to sediment oxygen consumption and model nesting

Abstract

[1] Every summer, a large area (15,000 km 2 on average) over the Texas-Louisiana shelf in the northern Gulf of Mexico turns hypoxic due to decay of organic matter that is primarily derived from nutrient inputs from the Mississippi/Atchafalaya River System. Interannual variability in the size of the hypoxic zone is large. The 2008 Action Plan put forth by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an alliance of multiple state and federal agencies and tribes, calls for a reduction of the size of the hypoxic zone through nutrient management in the watershed. Comprehensive models help build mechanistic understanding of the processes underlying hypoxia formation and variability and are thus indispensable tools for devising efficient nutrient reduction strategies and for building reasonable expectations as to what responses can be expected for a given nutrient reduction.Here we present such a model, evaluate its hypoxia simulations against monitoring observations, and assess the sensitivity of the hypoxia simulations to model resolution, variations in sediment oxygen consumption, and choice of physical horizontal boundary conditions. We find that hypoxia simulations on the shelf are very sensitive to the parameterization of sediment oxygen consumption, a result of the fact that hypoxic conditions are restricted to a relatively thin layer above the bottom over most of the shelf. We show that the strength of vertical stratification is an important predictor of dissolved oxygen concentration in bottom waters and that modification of physical horizontal boundary conditions can have a large effect on hypoxia simulations because it can affect stratification strength.Citation: Fennel, K., J. Hu, A. Laurent, M. Marta-Almeida, and R. Hetland (2013), Sensitivity of hypoxia predictions for the northern Gulf of Mexico to sediment oxygen consumption and model nesting,

Parameterization of biogeochemical sediment–water fluxes using in situ measurements and a diagenetic model

“…However, sediment-water flux parameterizations are a coarse representation of sedimentwater interaction, and typically do not capture nonlinearities in nutrient fluxes that occur under hypoxic/anoxic conditions. Moreover, the choice of parameterization can have a significant effect on model results as shown in Fennel et al (2013) where different parameterizations of SOC led to dramatically different regions of hypoxia.…”

“…3). The simulation is described in Fennel et al (2013;case B20clim) and covers the period from 2004 to 2009. The same simulation was used to prescribe POM depositional fluxes during the parameter optimization.…”

“…The same simulation was used to prescribe POM depositional fluxes during the parameter optimization. For details on the model set up and validation we refer the reader to Fennel et al (2013). We included only those grid cells on the Louisiana Shelf (z < 50 m) and west of the Mississippi River delta.…”

“…The other, referred to as M&L (Eq. 8), is from Murrell and Lehrter (2011), with a temperature dependence added by Fennel et al (2013). Sediment-water O 2 fluxes are formulated as follows:…”

The sensitivity of estuarine aragonite saturation state and pH to the carbonate chemistry of a freshet-dominated river

“…In the Congo River for example, TA is flow-dependent but DIC is persistently high year-round (Wang et al, 2013). On top of natural variability, many estuaries also experience heavy anthropogenic pressure (Frankignoulle et al, 1996;Zhai et al, 2007;Cai et al, 2017), making them particularly vulnerable to ocean acidification (Cai et al, 2011 and in some cases the subject of intensive management and policy initiatives (Fennel et al, 2013).…”