The role of environmental perturbations, such as hypoxia (<2 mg O2/L) and anoxia (0 mg O2/L), in determining the outcome of food-web dynamics is poorly known. Hypoxic zones in marine systems are increasing in areal extent and duration due to heightened anthropogenic stresses such as eutrophication, particularly in estuarine systems such as Chesapeake Bay, Long Island Sound, and the Gulf of Mexico. Worldwide, there are more than 50 dead zones due to hypoxia. Further worsening of environmental conditions due to global warming or escalating anthropogenic insults may alter the productivity base for food webs and their respective fisheries because many oxygen-stressed systems appear close to a threshold.
Hypoxia is generally thought to be detrimental because of the observed reductions in benthic faunal abundance associated with persistent severe hypoxia. However, transfer of benthic production to higher trophic levels may be facilitated in hypoxic areas because of the vertical migration of infauna to shallower depths where they are more susceptible to epibenthic predators when hypoxia is not severe. In contrast, where hypoxia is chronic and severe, epibenthic predators such as fish and crabs may not be able to enter hypoxic areas to exploit benthic prey.
We are currently measuring dissolved oxygen levels and faunal responses at fine spatial scales across a gradient encompassing normoxic to anoxic conditions (shallow shoals to deep channels); these are compared with responses at normoxic control sites. The study is conducted in two tributaries (York and Patuxent Rivers) of Chesapeake Bay that differ fundamentally in the severity and duration of hypoxia, and are therefore expected to have contrasting impacts on trophic dynamics. Impacts on a major epibenthic predator (blue crab, Callinectes sapidus) and its chief prey (Baltic clam, Macoma balthica), as well as other infaunal prey such as polychaetes, are quantified concurrently.
A demonstration of the impact of hypoxia on trophic transfer within the Chesapeake Bay benthic system can serve as a model for the other estuarine systems worldwide.
This work is supported by Maryland Sea Grant.