Ecological Implications

Physical Characteristics of Hypoxic Zone Including Spatial and Temporal Distribution in the Gulf of Mexico

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In earlier discussions it was outlined that hypoxia in coastal water occurs as a result of two main factors: density stratification which impedes reoxygenation from surface water, and the decomposition of organic material which ultimately depletes oxygen to lethally low levels. The intensity of both of these factors depends greatly on annual occurrences in the Gulf of Mexico. Inputs from the Mississippi River dominate the oceanographic properties of the Louisiana inner shelf where the dead zone occurs. On average, the Mississippi River basin drains approximately 635 km2 of freshwater to the shelf annually, and also contributes heavy discharges of both suspended sediments and nutrients (Wiseman et. al., 1995). Hypoxic waters in the Gulf are most prevalent from the late spring months when nutrient discharge is peaking through late summer, and the expansiveness and persistance of the dead zone can vary from year to year (Rabalais et al., 1999). Hypoxic waters are typically found between 5 and 30 meters in near shore areas, occurring mostly in the lower half to two-thirds of the water column. There is a fairly predictable succession of events that take place in the affected waters of the Gulf. First there is a gradual decline in oxygen in the spring whcih can be occasionally interruptioned by wind-mixing events. This is followed by persistent hypoxia for much of May through September with occasional summer upwellings of oxygenated water from the outer shelf. Finally the seasonal disruption of low oxygen in the fall due to tropical depressions and cold fronts cause the body of water to destratify and end hypoxic conditions for that year (Rabalais et. al., 1999).

It is believed that hypoxia was not a prominent feature of the shallow continental shelf before the 1950's (Rabalais et al., 1996). However, much has changed within the Mississippi River basin and so has the extent of hypoxia. Most of the freshwater entering the west Louisiana shelf stays confined to the coastline and flows westward as a narrow current, known as the Louisiana Coastal Current (Wiseman and Kelly, 1994). The seasonal changes in runoff to the Louisiana Coastal Current alter the stratification which increases hypoxic events (Wiseman et al., 1986). The result of changing climate patterns coupled with runoff changes is to maximize the extent of stratification, and thus hypoxia (Wiseman et al., 1995).

Spatially, the mid-summer bottom areal extent of the hypoxic zone in 1985-1992 averaged 8,000 to 9,000 km2; in 1998 this was estimated to have increased to an area of 12,500 km2 (Rabalais et al., 1999). The present distribution of the dead zone includes the inner to mid continental shelf of the Gulf of Mexico, from the Mississippi River birdfoot delta westward to the upper Texas coast (Wiseman et al., 1997). Temporally, hypoxia is ephemeral in March, April, and May and becomes more extensive in the summer months of June, July, and August (Rabalais, Turner, and Wiseman, 1995). The severity and persistence of the dead zone in the fall depends mainly on climatic shifts.

For further information, see the online report "NOAA Gulf of Mexico Ecosystem and Hypoxia Assessment."

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