Introduction

Hypoxia has been an issue of growing concern over the past few decades as the size and severity of hypoxic zones throughout the world have been increasing. The implications of this are potentially serious for estuaries and coastal ecosystems experiencing hypoxic events. Two areas of particular concern in the U.S. are the Gulf of Mexico and the Chesapeake Bay. In this site, you will find a variety of topics describing the scientific phenomenon of hypoxia. Comparisons between hypoxia in the Gulf of Mexico and the Chesapeake Bay are examined in the context of a general description of hypoxia, causes of hypoxia, temporal and spatial variability, historical trends, and ecological effects. In addition, I have also included a page providing ideas for further research with links to web resources on additional topics.

What is Hypoxia?

Hypoxia, or oxygen-depleted water, is a common feature of many estuaries and coastal ecosystems.  The term, hypoxia, is typically used to describe waters in which low dissolved oxygen levels produce stressful responses in marine organisms (Rabalais et al. 2002).  Although organism responses in hypoxic environments vary depending on physiological characteristics and level of dissolved oxygen, the most sensitive fish typically display stressed behavior, such as avoidance and reduced growth, when oxygen levels reach 2 mg/l, or parts per million (ppm).  Thus, water is considered hypoxic when the level of dissolved oxygen is less than or equal to 2 mg/l.  Although hypoxic zones are not completely devoid of life, the popular term, “"Dead Zone,"”refers to the lack of organisms caught with bottom trawlers in hypoxic areas (Rabalais et al. 2001).

Where does hypoxia occur?

Throughout the world, hypoxia occurs both naturally and as a result of human activities (Figure 1). For instance, the Gulf of Trieste in the Northern Adriatic Sea has experienced increasingly severe hypoxia over the last 25 years due to the correlation between population growth, increased nutrient input and increased productivity in the water column (Diaz 2001). In contrast, hypoxia often occurs in fjords and the Black Sea basin as a result of naturally prolonged lack of oxygen mixing between layers in the water column. In the U.S., hypoxic areas are also present in the Long Island Sound and the Sarasota Bay (USEPA 2004). However, the two U.S. “Dead Zones” receiving perhaps the most attention are those of the Gulf of Mexico and the Chesapeake Bay.

Figure 1. Occurrences of hypoxia throughout the world. (Source:Earth Policy Institute)

The Gulf of Mexico Watershed and Hypoxic Zone

The Gulf of Mexico is a large, nearly landlocked body of water that spans approximately 1.6 million square km (Gore 1992). The Mississippi is the primary river that discharges into the Gulf; however nearly all runoff between the Rockies and the Appalachians eventually flows into the Gulf of Mexico. Immense in size, the Mississippi River drainage basin covers a land area of 3,225,000 square km, incorporating 31 states, and 2 Canada provinces, with major tributaries including the Illinois, Chippewa, Black Wisconsin, Saint Croix, Iowa, Des Moines, and Rock Rivers (Figure 2). Approximately 38% of the Gulf consists of shallow and intertidal areas that are less than 20 m deep. The continental shelf and slope (180-3,000 m deep) encompass 42%, and abyssal areas deeper than 3,000 m represent 20%. Mean depth of the Gulf of Mexico is 1,615 m, and the total tidal volume is 2,434,000 cubic km. The total of the area of the Gulf of Mexico is 1.6 million square km and the mid summer extent of the Gulf hypoxic zone covers an area of 18,000 square km along the coast within the boundaries of the continental shelf (Figure 3).

Figure 2. Mississippi River Watershed. Source: World Resources Institute

Figure 3. Size and general location of the Gulf of Mexico hypoxic zone. Source: NOAA

Chesapeake Bay Watershed and Hypoxic Zone

The Chesapeake Bay and the Chesapeake drainage basin are considerably smaller compared to the Gulf of Mexico, covering areas of approximately 11,000 square km and 167,000 square km respectively (Reshetiloff 1995). The Chesapeake watershed encompasses only 6 states; however the ratio of watershed land area to tidal water volume in the Chesapeake is the greater than all other coastal bodies of water in the world (CBF 2005). Average depth in the Chesapeake Bay is 27 m, and the total tidal volume is approximately 51 cubic km. Major tributaries include the Susquehanna, James, Potomac, Patuxent, Choptank, Rappahannock, York, and Tuckahoe Rivers. During peak summer months, the Chesapeake hypoxic zone has grown to encompass a volume of 11 billion cubic m (Figure 4).

Figure 4. The Chesapeake bay watershed and hypoxic zone. . Source: http://intl-jeq.scijournals.org/content/vol30/issue2/images/large/303f1.jpeg