Ecological Effects

Species-level Responses

The effects of hypoxia on organisms residing in both the Gulf of Mexico and the Chesapeake Bay are similar. Overall, hypoxia influences the distribution and abundance of most marine organisms. Individual responses to hypoxic conditions vary depending on the physiological characteristics of organisms and dissolved oxygen concentrations within the hypoxic area.

One reaction to hypoxic conditions is escape behavior. Mature mobile organisms are able to tolerate, to some extent, exposure to low dissolved oxygen levels when hypoxia develops, but then escape to more oxygenated waters (Breitburg 2002). Mobile benthos, or bottom-dwelling organisms, such as fish, crabs, shrimp, and squid, are examples of species that flee low hypoxic conditions (Rabalais, 2002). Similarly, the most common reaction exhibited by pelagic organisms, or species living in open water away from the bottom, is also displacement. Pelagics have the ability to swim freely throughout the water column, thus can flee an area of hypoxia when low dissolved oxygen conditions are detected.

The Chesapeake blue crab (Callinectes sapidus) and the brown shrimp (Penaeus aztecus) in Louisiana are two commercially important organisms that react to hypoxia with escape behavior. As a result, in the case of brown shrimp, a negative relationship between catch and the size of the hypoxic zone has been observed (Rabalais et al. 2002). In addition, catch per unit effort also has decreased during recent spatial expansion of the Gulf of Mexico hypoxic zone. However, these data may not be reliable, as in general, the implications of hypoxia for commercial fisheries are difficult to substantively determine due to poor fisheries data, natural variability of fish populations, and climate changes.

Blue crab, or Callinectes sapidus. Photo: Dickinson College Environmental Studies Department

Source: Mississippi State University

Brown shrimp, Penaeus aztecus. Source: South Carolina DNR

Ecosystem-level Responses

Depending on severity, extent, and duration of a hypoxic mass, the community dynamics of an ecosystem can potentially be altered (Breitburg 1992). As hypoxia is often episodic, marine communities experience periods of decline and recovery in reaction to changes in dissolved oxygen levels. However, when hypoxic conditions persist for prolonged periods of time, community structure can be permanently altered. Mortality and emigration caused by low dissolved oxygen conditions can result in reduced diversity, abundance, and production of organisms within hypoxic areas (Breitburg 2002).

For example, prolonged exposure to low dissolved oxygen levels affects the structure of macrobenthic communities (Flemer et al. 1999). Hypoxia can influence the distribution, abundance, and taxa richness of organisms residing in substrate. Immobile invertebrates typically die at dissolved oxygen levels below 1.5 mg l-1 and infaunal invertebrates display stressed behavior below 1 mg l-1 (Rabalais et al. 2001). Prolonged exposure to hypoxic conditions lead to reduced abundance, species richness, and biomass of bottom-dwelling populations. Community structure during and after extended periods of hypoxia consists of limited taxa, resistant fauna such as polychaetes and sipunculans, decreased species richness, severely reduced species abundance, and low biomass. Following the abatement of a prolonged hypoxic event, the recovery of remaining species is often limited.

Bristle worm, a polychaete in the Chesapeake Bay. Source: chesapeakebay.net

In addition, predator-prey relationships can be altered. Some organisms are believed to be able to develop competitive advantage as a result of stressed behaviors displayed by vulnerable organisms. In the Chesapeake Bay, cnidarians and ctenophores, or jellyfish, have been observed to survive at dissolved concentrations of less than 2 mg l-1 , while the prey of jellyfish, including larvae of the naked goby (Gobiosoma bosc) and the bay anchovy (Anchoa mitchilli) are less tolerant of hypoxic conditions (Purcell et al. 1994). Jellyfish can potentially experience greater ease of predation when their prey displays stressed behavior (Rabalais et al. 2002). As a result, the size and abundance of predator species can increase and the abundance of prey species will be reduced (Breitburg 2002).

Naked goby, Gobiosoma bosc. Source: Gobioid Research Institute

Bay anchovy, Anchoa mitchelli