Dead zones occurring in the Chesapeake Bay are disrupting the distribution and number of many of the area's fish, according to a 10-year study by researchers at the Virginia Institute of Marine Science (VIMS).
As the first quantitative evidence on a bay-wide scale that these low-oxygen zones are impacting demersal fish (fish that live or feed near the bottom seafloor), the study warns that the affected species represent a key part of the Chesapeake Bay ecosystem and support important commercial and recreational fisheries.
Low-oxygen conditions can form when excessive loads of nitrogen from fertilizers, sewage and other sources feed algal blooms in coastal waters. When these algae die, they provide a rich food source for bacteria, which take up dissolved oxygen from nearby waters as they decompose.
In Chesapeake Bay, low-oxygen conditions are the most pronounced in mid-summer and in the deep waters of the Bay's middle reaches.
"The drastic decline we saw in species richness, species diversity, and catch rate under low-oxygen conditions is consistent with work from other systems," Andre Buchheister, a Ph.D. student in William & Mary's School of Marine Science at VIMS, said in a press release. "It suggests that demersal fishes begin to avoid an area when levels of dissolved oxygen drop below about 4 milligrams per liter, as they start to suffer physiological stress."
This response is interesting, says Buchheister, "because it occurs at levels greater than the 2 milligrams per liter that scientists formally use to define hypoxia."
Normal coastal waters contain between 7 and 8 milligrams of oxygen per liter.
Previous research suggests that oxygen-poor waters can stress fish through increased respiration and elevated metabolism, as well as by affecting their prey.
"Low levels of dissolved oxygen stress or kill the bottom-dwelling invertebrates that demersal fishes rely on for food," Buchheister explained. "Prolonged exposure of these invertebrates to hypoxic conditions in the mid-Bay represents a substantial reduction in the habitat available for foraging by demersal fishes baywide, and could reduce the quality of foraging habitat even after bottom waters become re-oxygenated."
In contrast, however, the limits on fish abundance and distribution brought on by these conditions are somewhat balanced by the positive effects that nutrients have on the production of mid-water and surface-dwelling fish elsewhere in the Bay, according to the scientists. The nutrient-rich waters that encourage dead-zone formation also fuel algal growth, they explain, thus turbocharging the base of a food web that ultimately supports fish and other predators.
Going forward, the researchers explain that, in order to fully understand the effects of the many factors affecting the Bay, a watchful eye is needed.
"Continued monitoring will be critical for detecting how the Bay ecosystem responds to continued stresses from fishing, development, and climate change," said Rob Latour of the VIMS. "It's an essential component to a successful management strategy for the marine resources of Chesapeake Bay and the coastal Atlantic."