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Modeling and Hypoxia

Numerical modeling is an important element of this project.

images of modeling
The objective of our numerical modeling effort is to build a coupled physical, biological and geochemical model for the Texas/Louisiana continental shelf with the ultimate goal of identifying and quantifying the relative contributions of physical, biological and geochemical factors influencing hypoxia (e.g., nutrient and freshwater loads, biogeochemical feedbacks and hydrographic conditions). This project is novel because it combines the interactions of all these components in the model. Notably, the coupling between the biological-chemical module and the sediment transport module has never been undertaken before. We are presently testing a prototype that links the sediment transport and biochemical modules within the framework of the hydrodynamic (physical) model. We believe that this linkage is important because near bottom and sediment biochemistry is an important process in the formation of hypoxia and sediments have been shown to be dynamic over the shelf, with tens of centimeters being deposited and remobilized by tropical cyclones, for example (Dail et al. 2007). Thus we will be able to examine through the model fractions of carbon and nutrient transport carried within the water column versus that modified within the sediments.

The basis for our numerical simulations is the Regional Ocean Modeling System (ROMS), a state-of-the-art hydrodynamic model (see Haidvogel et al 2008). ROMS is open source, has a large international user community and is arguably the standard for coastal numerical modeling. This is because of its numerical efficiency and robustness, and the continuous development of new features including biological modules, a sediment transport model and machinery for variational assimilation and atmosphere-ocean coupling (Haidvogel et al. 2008). ROMS was chosen as the basis for the Community Sediment Transport Modeling System (CSTMS), and thus has a very mature sediment transport module (Warner et al. 2008). Of its many features, perhaps the most relevant to this project is the availability of high-order numerical schemes for advection in ROMS. Hetland (2005) demonstrated that the use of a high-order numerical advection scheme is critical for accurate representation of river plume dynamics.

Biological and chemical processes in the water column are described using a Fasham-type biological model coupled with dynamic representations of phosphate and dissolved oxygen. The biological model describes a simplified nitrogen cycle with the state variables nitrate, ammonium, small and large particle detrital pools, phytoplankton, chlorophyll and zooplankton (Fennel et al. 2006). It has been coupled successfully with an inorganic carbon module and used to derive nitrogen and carbon budgets including the air-sea flux of CO2 for the US east coast continental shelf (Fennel et al. in press). These previous modeling studies have shown the importance of benthic-pelagic coupling on continental shelves and the power of these biogeochemical ROMS models in quantitatively enhancing the interpretation of observations.

Suspended and seabed sediment impact the biogeochemical processes relevant to hypoxia by limiting light penetration, redistributing sediment-associated organic matter both within the water column and on the bed and burying and eroding geochemical constituents on the seafloor.

Numerical modeling objectives
Preliminary animations
Idealized modeling
Realistic modeling
Biological modeling runs
Surface salinity
Extended Grid 2000-2010