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Figure 3: Plots of the PEB index (%PEB) in sediment cores from the Louisiana shelf. Higher values of the PEB index indicate lower dissolved oxygen contents in bottom waters. Taken from Osterman et al. (2005).
Oxygen budget: general considerations
The oxygen budget on the NGOM shelf is influenced by several sink and source terms. Oxygen (O2) concentration in the bottom layer will decrease and possibly become hypoxic or even anoxic when the export and consumption of oxygen by respiration exceed the import or production of “new” oxygenated water by photosynthesis. Mathematically, this relationship can be expressed in its simplest form by the following oxygen balance equation:
in which the left-hand term represents the change of oxygen concentration with time; term (1) on the right represents the horizontal advection by across-shelf currents, u; term (2) represents the horizontal advection by along-shelf currents, v; term (3) represents vertical transport by upwelling or downwelling; term (4) represents vertical mixing and Kz (x,y,z) is the vertical eddy diffusivity; term (5) represents horizontal diffusion and KH (x,y,z) is the horizontal eddy diffusivity; term (6) is oxygen flux across the air-sea interface; term (7) is the non-conservative sink (i.e., oxygen consumption); and term (8) refers to in situ production of oxygen by photosynthesis. The horizontal advection terms may reflect contributions from tides, wind stress, buoyancy, and momentum input from rivers, large-scale and mesoscale eddies, or topographically trapped shelf waves. Three-dimensional hydrodynamic models are required to adequately account for these contributions (Morey et al., 2003a, 2003b; Hetland and DiMarco, 2007). The respiration term (7) relates directly to organic matter mineralization and must be understood in the context of water column and sediment biogeochemical processes described in later sections. As depicted in equation 1, the change in oxygen concentration with time at any point in the water column is affected by sources and sinks of oxygen at and below the surface. Term 6 (oxygen flux across the air-sea interface) represents a surface source and sink, while term 8 (photosynthesis) is a source of oxygen in waters beneath the air-sea interface. Although equation 1 above suggests that alongshore and cross-shore dispersion coefficients are of equal magnitude, the Panel notes that this has not been demonstrated. The effects of cross-shore dispersion processes must be parameterized and additional research on lateral mixing processes must be completed before such parameterization can be performed with confidence.
Vertical mixing as a function of stratification and vertical shear
Over the Louisiana-Texas shelf, the vertical mixing term (4) plays a key role in the local oxygen balance. Its magnitude depends on the value of vertical eddy diffusivity Kz, which is highly variable in both space and time and depends on the gradient Richardson number Ri (MacKinnon and Gregg, 2005), defined by
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