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Bay Blog: benthic

Mar
30
2017

Five signs of resilience in Chesapeake Bay health

Since our formation in 1983, the Chesapeake Bay Program has been leading the effort to reduce pollution and restore ecosystem health across the Chesapeake Bay region. Whether through the Chesapeake Bay Watershed Agreement, signed in 2014, or the Bay’s ‘pollution diet’—i.e., the Chesapeake Bay Total Maximum Daily Load (TMDL)—our partners have continued to work collaboratively toward our shared goal of a healthy Bay.

With decades-worth of environmental data, our scientists are able to study how the health of the nation’s largest estuary is changing over time. Below, learn about a few of the ways the Bay and its rivers have been showing signs of resilience.

A bed of underwater grasses in the Susquehanna Flats near Havre de Grace, Maryland, on July 25, 2016. Between 2014 and 2015, researchers saw the highest amount of underwater grasses ever recorded in their aerial surveys, an increase of 21 percent from the previous year.

Record acreage of underwater grasses

Underwater grasses, also called submerged aquatic vegetation or SAV, grow in the shallow waters of the Chesapeake Bay and its tributaries. They provide food and habitat to wildlife, add oxygen to the water and trap and absorb pollution. Because of their sensitivity to pollution, the abundance of underwater grasses can serve as an indicator of restoration progress.

Between 2014 and 2015, more than 92,000 acres of underwater grasses were observed in the Chesapeake Bay. An increase of 21 percent from the previous year, it marked the highest amount ever recorded in the nearly 30 years the Virginia Institute of Marine Science has conducted their aerial survey. Part of this increase was due to the expansion of widgeon grass—often referred to as a “boom or bust” species because its abundance can rise and fall from year to year—but other species like wild celery and eelgrass also saw a recovery.

A pair of blue crabs in a dip net near Kent Island, Maryland, on August 27, 2010. Last year, the population of adult female crabs in the Chesapeake Bay saw a 92 percent increase from the previous year.

Sustainable blue crab stock

The blue crab is one of the most iconic species in the Chesapeake Bay, supporting commercial and recreational fisheries in the region. But vulnerability to pollution, loss of habitat and harvest pressure have led their abundance to fluctuate over time; in 2014, adult female blue crabs were considered depleted.

Joint management between Maryland, Virginia and the Potomac River Fisheries Commission has helped maintain the Bay’s blue crab stock at sustainable levels. At the start of last year’s crabbing season, there were an estimated 194 million adult female blue crabs in the Bay—a 92 percent increase from the previous year. And according to the 2016 Chesapeake Bay Blue Crab Advisory Report, the blue crab stock was not depleted and overfishing was not occurring.

A school of alewife in the shallows where Deer Creek meets the Susquehanna River in Havre de Grace, Maryland, on April 16, 2015. Underwater animals like fish rely on dissolved oxygen in the water to survive.

Higher levels of dissolved oxygen

Just like humans, crabs, fish and other underwater animals that live in the Bay need oxygen to survive. Dissolved oxygen is a measurement of how much oxygen is present in the water; as dissolved oxygen levels decrease, it becomes more difficult for animals to get the oxygen they need.

As reported in the University of Maryland Center for Environmental Science’s most recent Chesapeake Bay report card, dissolved oxygen levels in many regions of the Bay were frequently in “good condition” (scores of 60 percent or higher) in 2015, and no regions were below “moderately good” levels. Additionally, 2016 marked the second consecutive year that there were no measured anoxic areas—or areas with no dissolved oxygen—in the main portion of the Bay.

Hard clams rest in a basket near the lower portion of the Chesapeake Bay, on March 8, 2016. Clams and other organisms that live at the bottom of the Bay and its rivers are known as "benthos."

Improvements in bottom-dwelling invertebrates

Organisms that live at the bottom of the Bay and its rivers and streams are known as “benthos.” Benthic communities are made up of worms, clams, oysters, shrimp-like crustaceans and other underwater invertebrates, and they provide food for crabs and bottom-feeding fish.

In 2015, almost two-thirds of the bottom habitat in the tidal Bay was home to a healthy community of benthic organisms, an increase from 59 percent in 2014. In addition, the area of degraded and severely degraded habitat—or areas that are home to more pollution-tolerant species, fewer species overall, fewer large organisms deep in the sediment and a lower total mass of organisms—was the lowest it has been since 1996. Scientists attributed this improvement to increases in dissolved oxygen.

David Brower from the U.S. Geological Survey conducts water sampling on a bridge over the Patuxent River near Bowie, Maryland, on April 2, 2015. In 2015, nutrient and sediment loads to the Bay were below the long-term average.

Declines in nutrient and sediment pollution

While some nutrients and sediment are a natural part of the Chesapeake Bay ecosystem, too much can be harmful to fish, shellfish and other underwater life. Excess nitrogen and phosphorus can fuel the growth of algae blooms, which can lead to low-oxygen dead zones that suffocate marine life. Sediment can cloud the water, preventing sunlight from reaching underwater plants and smothering bottom-dwelling species.

According to data from the Bay Program and U.S. Geological Survey (USGS), nitrogen, phosphorus and sediment loads to the Bay were below the long-term average in 2015. Additionally, since 1985, long-term trends in nitrogen pollution have improved at six of the nine monitoring stations located along the biggest rivers that feed into the Bay, including at stations in the region’s four largest rivers: the Susquehanna, Potomac, James and Rappahannock.

 

Want to learn more about our work toward achieving the goals of the Chesapeake Bay Watershed Agreement? Visit ChesapeakeProgress.

Stephanie Smith's avatar
About Stephanie Smith - Stephanie is the Web Content Manager at the Chesapeake Bay Program. A native of the Midwest, she received her Bachelor’s in Professional Writing from Purdue University and Master of Science degree from the University of Michigan. Stephanie’s lifelong love of nature motivates her to explore solutions to environmental problems and teach others what they can do to help.



Jul
16
2013

Summertime dead zones displace Chesapeake Bay fish

Dead zones are impacting the distribution and abundance of fish that live and feed near the bottom of the Chesapeake Bay, according to new research from the Virginia Institute of Marine Science (VIMS).

Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fed algae blooms die and decompose, and are most pronounced in the deep waters of the Bay’s mainstem during warm summer months. During a decade-long study of the bottom-feeding fish that inhabit this portion of the Bay’s water column, scientists noticed drastic declines in species richness, diversity and catch rate as dead zones restricted habitat and displaced the fish toward more hospitable waters.

So-called “demersal” fish—which include Atlantic croaker, white perch, spot, striped bass and summer flounder—avoid dead zones because a lack of oxygen can place stress on their respiratory and metabolic systems. While the fish often return to their former habitat when oxygen levels improve, dead zones can also wreak havoc on their forage grounds, stressing or killing the bottom-dwelling invertebrates the fish need for food.

“Once oxygen levels go up, we do see the average catch rate go up,” said Andre Buchheister, Ph.D. student and author of the VIMS study. “That’s a good sign. It indicates that once those waters are re-oxygenated, it’s possible for fish to move back in. But the availability of food is compromised, and studies have shown that the productivity of benthic biomass—or the critters that live in and on the bottom of the Bay—is stressed.”

The impact that demersal fish displacement could have on Bay fisheries is unclear, Buchheister said. Commercial fishermen who work outside of the mainstem might not be affected. But recreational anglers searching for striped bass could struggle if their forced move out of cool, deep waters is shown to contribute to poor health among the population.

In June, a forecast from researchers at the University of Maryland Center for Environmental Science (UMCES) and the University of Michigan predicted a smaller than average dead zone for the coming summer, thanks to lower than average nutrient loads that entered the Bay last spring. But to return the Bay’s mainstem to its former health, “one or two good summers won’t make that much of a difference,” said Buchheister. Instead, benthic habitat must be rebuilt, as long-term improvements boost Bay health from the bottom up.

Images courtesy Virginia Institute of Marine Science (VIMS)



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