From: Allison Winter, ENN
Published July 9, 2013 01:37 PM

Chesapeake Bay "Dead Zones" Reduce Diversity and Abundance of Near-bottom Species

The Chesapeake Bay is the largest estuary in the United States and even though President Obama has declared it a "national treasure" in 2009, this watershed is becoming emptier with fewer shellfish and fish populations mainly due to upstream pollution.


Consequently, a 10-year research study conducted by researchers at the Virginia Institute of Marine Science (VIMS) indicates that low oxygen levels reduce diversity and catch rates of species that live and feed near the Bay bottom. 

Low-oxygen areas of the Bay are identified as "dead zones". Dead zones are caused mainly by excessive nutrient pollution from human activities. Nitrogen and phosphorus, which come from fertilizers and sewage, enter streams and rivers that eventually run into the Bay. These pollutants feed algal blooms that block sunlight to underwater plants and suck up oxygen when the plants die and decompose. 

Research author Andre Buchheister says "This is the first study to document that chronically low levels of dissolved oxygen in Chesapeake Bay can reduce the number and catch rates of demersal fish species on a large scale." 

"Demersal" fishes in the Bay include the Atlantic croaker, white perch, spot, striped bass, and summer flounder. 

Buchheister comments: "The drastic decline we saw in species richness, species diversity, and catch rate under low-oxygen conditions is consistent with work from other systems. 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." (Normal coastal waters contain about 7-8 milligrams of oxygen per liter.)

On the other hand, there are some positive effects caused by low-oxygen conditions as nutrient-rich waters that encourage dead-zone formation also fuel algal growth, thus "turbocharging the base of a food web that ultimately supports fish and other predators" in mid and surface waters.

The research also shows that salinity is the most important factor affecting the distribution of Bay fishes, whether they live near the bottom or towards the surface.

Buchheister says "The saltier waters of the lower Bay and the fresher waters of the upper Bay generally have a more diverse and dynamic fish fauna than the middle Bay, whose brackish waters place physiological demands on many aquatic organisms."

Buchheister's VIMS colleague and research author Dr Rob Latour, stated "Continued monitoring will be critical for detecting how the Bay ecosystem responds to continued stresses from fishing, development, and climate change. It's an essential component to a successful management strategy for the marine resources of Chesapeake Bay and the coastal Atlantic."

The study is published in the May issue of Marine Ecology Progress Series.

Read more at the Virginia Institute of Marine Science.

Chesapeake Bay image via Shutterstock.

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