Temporal and Spatial Variability

Seasonal Patterns

The extent and severity of hypoxic masses in the Gulf of Mexico and the Chesapeake varies from season to season due to changes in weather patterns and temperature, and within seasons due to changes in physical conditions (Figure 1). Variation of hypoxic masses in both the Chesapeake Bay and the Gulf of Mexico follow similar seasonal chronologies. Typically, hypoxia develops during springtime, intensifies through midsummer, and then begins to lessen from October through November (Reshetiloff 1995, Rabalais et al. 1999). Increased freshwater input from melting snow and/or frequent rainfall in the spring leads to intense vertical stratification of the water column. Stratification is sustained through summer months due to surface temperature warming. In the fall, surface waters cool rapidly and sink, causing overnight vertical mixing between the oxygenated top layer and hypoxic bottom layer. During the winter, temperature and salinity are relatively constant through the water column. Thus, in cold weather, hypoxia is generally minor to nonexistent because both density and salinity layering do not occur (Reshetiloff 1995).

Figure 1. Seasonal dissolved oxygen trends in the Chesapeake Bay. Source: CBF

Annual Fluctuations

The persistence, extent, and severity of hypoxia through peak summer months can also vary. The layers of a stratified water column can mix due to high winds and rough choppy waters. For instance, hypoxia is typically less severe during and immediately after storm events. However, hypoxic masses return their pre-storm extent after winds subside (Rabalais et al. 1999). In addition, strong shoreward winds, as well as upwelling of oxygen rich water masses from deeper waters can push masses of hypoxia into shallow inshore areas (Breitburg 1990; Rabalais et al. 2002). Often, stressed aquatic organisms are forced along with the hypoxia mass into shallow waters. For fishermen, this occurrence is known as a “jubilee,” as the stunned shrimp, crabs, and fish become easily harvestable in shallow depths. When the organisms forced onshore are already dead due to extreme hypoxia, the event is known as a jubilee “gone bad” (Rabalais et al. 2002). Droughts and floods can also influence hypoxia. When a drought occurs, the input of freshwater is significantly decreased, resulting in significantly weaker density stratification. During droughts, hypoxia is typically minor to completely absent (Rabalais et al. 1999). In contrast, floods generate increased freshwater and nutrient input to saline waters, resulting in greater stratification and eutrophication, and in turn, increased hypoxia.

Hypoxia and Climate Change

In addition, climate change is also thought to have an effect on hypoxia in coastal waters. Justic et al. (1996) predict that if atmospheric carbon dioxide concentrations double, the Gulf of Mexico hypoxic mass will significantly increase in size. With increased carbon dioxide levels and the resultant rise in global temperature, the hydrologic cycle will likely be altered, leading to altered river discharge and more frequent flooding events (Justic et al. 2003). Freshwater river flow will increase, causing higher quantities of nutrients to enter the Gulf of Mexico (Justic et al. 1996; Justic et al. 2003). As a result, there will be greater stratification of the water column, less diffusive transfer of oxygen between surface and bottom waters, and more eutrophication, causing hypoxia to increase in intensity and extent (Justic et al. 1996) (Figures 2 and 3).