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摘要
Patterns of temporal and spatial variability in hypoxia ( < 2 mg O(2)l(-1)) on the inner Louisiana-upper Texas (LaTex) shelf were examined using FVCOM LaTex, an unstructured grid, three-dimensional, hydrodynamic-water quality model. Dynamics of dissolved oxygen were modeled using an expanded and revised version of the Water Analysis Simulation Program (WASP) that was fully coupled to a Finite Volume Coastal Ocean Model (FVCOM). The coupled model was driven by surface wind forcing, tidal forcing, offshore remote forcing, heat fluxes, oxygen exchanges at the air-sea interface, solar radiation, and freshwater and nutrient (nitrogen and phosphorus) fluxes from the Mississippi and Atchafalaya Rivers. The model simulations were carried out over a 9-month period, from January 1 to October 4, 2002, and the model skill was assessed using multiple sets of observational data that included time series of dissolved oxygen concentrations from a station within the core of the Gulf hypoxic zone (C6), dissolved oxygen measurements collected during the mid-summer shelfwide cruise, and vertical dissolved oxygen profiles through the year. The model results indicate that hypoxia originates in bottom waters on the mid-continental shelf, where isolated pockets of hypoxic water develop during early spring and later join into a larger continuous hypoxic zone. The model accurately described the seasonal cycle of hypoxia at station C6, including the episodes of intermittent hypoxia during May and June, persistent hypoxia during July and August, and dissipation of hypoxia during September. The onset of hypoxia coincided with high stability of the water column (i.e., Richardson number values > 1) and the initial transition from normoxia (i.e., 6 mg O(2)l(-1)) to hypoxia lasted about three weeks. The model results point to a significant short-term variability in the extent of hypoxic bottom waters, indicating that the size of the mid-summer hypoxic zone cannot be adequately captured by a single shelfwide cruise. The dynamics of bottom-water hypoxia is clearly influenced by the bathymetric features of the LaTex shelf, namely the presence of three shallow shoals ( < 5 m) in the Atchafalaya Bay region and several deeper shoals ( < 10 m) in the northwestern section of the study area. Lastly, the model results support the view that dynamics of hypoxia on the LaTex shelf is strongly modulated by the frequency and intensity of cold fronts and tropical storms. High winds associated with these events disturb stratification, causing partial or complete breakdown of hypoxia. However, cold fronts and tropical storms also cause significant sediment resuspension that fuels respiration in the lower water column, and in this manner promote redevelopment of hypoxia.
- 出版日期2014-1-1
