Learn to love the worm. And, for that matter, clams barnacles and sponges. Because as they go, so goes the Chesapeake.
These spineless creatures are unlikely to find a place on anyone’s wall poster anytime soon but they—and other bottom-dwelling creatures collectively known as benthos—have become leading indicators for the health of the Bay.
“These animals are the thermometer of the quality of the bottom,” said Dan Dauer, a professor of biological sciences at Old Dominion University. “Benthos are used extensively worldwide as indicators of overall environmental health.”
To help read that thermometer, Dauer led an effort to develop a benthic community IBI—Index of Biological Integrity—which describes the Bay bottom from the benthos’ point of view.
The index shows that benthos were not happy in 2003, a year that saw near-record low dissolved oxygen conditions in the Chesapeake. Almost 60 percent of the Chesapeake’s bottom was unsuitable for benthos last year—the worst since the Baywide survey began in the mid-1990s.
Worms, clams and other benthos turn out to be good indicators. Unlike water quality samples, which are just a quick snapshot in time, benthic community health reflects how water quality has impacted a biological community over a period of time.
Most benthos have little ability to move, so they are stuck when things turn bad. Monitoring their condition from year-to-year indicates the amount of environmental stress they’ve been under.
Healthy communities have a wide mix of both long– and short-lived species, as well as pollution-tolerant, and intolerant, species. When scientists find only a few short-lived, pollution-tolerant species in an area, it’s a sign that things are bad.
And a lot of problems can, literally, pile up on benthos.
The Bay’s bottom is the collecting point for almost everything that enters the Chesapeake, from particles of dirt to dying algae to toxic chemicals. Toxic pollution and the smothering effect of sediment take their toll on benthics, especially in certain regions of the Bay.
But the top problem for benthos is the lack of oxygen in the summer. When dead algae sink to the bottom of the Bay, they are consumed by bacteria in a process that depletes the water of oxygen.
Changes in the bottom’s dissolved oxygen concentrations is the leading cause of year-to-year fluctuations in the index, according to Dauer. Last year was a case in point. According to the survey, 59 percent of the Bay’s bottom was unsuitable for healthy benthic communities—the worst score since Baywide monitoring began in 1996.
In Maryland, which traditionally has the most serious dissolved oxygen problem, 65.4 percent of the bottom got a failing score, almost two-thirds of the area, although that was slightly better than 1998 when 68.9 percent failed. In Virginia, 51.6 percent of the bottom flunked the benthos test, the worst ever seen in the state.
In contrast, in 2001, one of the best recent years for dissolved oxygen according to Bay Program figures, 47.8 percent of the Bay’s bottom was unsuitable for benthos, including 56.7 percent of Maryland’s tidal water, and 37.5 percent of Virginia’s.
To come up with the index, scientists rank samples taken from the bottom on a scale of 1–5 (1 is bad) for a series of different parameters, such as overall abundance, species diversity, biomass of pollution tolerant species, and so forth.
Scientists average the parameters to come up with an overall score for a site; anything that scores 3 or better is considered healthy and meets restoration goals.
Anything that scores between 2 .1 and 2.9 has been noticeably altered, while areas scoring less than 2 are ranked as severely degraded, meaning that the existing community has been dramatically changed by human activities. In recent years, about half of the Bay has scored less than 3—or failing— in the benthic IBI.
In Maryland, where dissolved oxygen conditions are typically worse, an average of about 60 percent of the tidal waters were considered unsuitable for benthic communities in recent years. In Virginia, where dissolved oxygen conditions in some areas are moderated by ocean water flowing in, an average of about 40 percent of tidal waters failed.
The annual benthic survey takes samples at many randomly selected sites so it can reflect all of the major regions of the Bay’s mainstem and tidal tributaries in each state.
Benthic organisms are collected from soft bottom habitats, which cover about 99 percent of the Bay bottom. The “benthic grab” devices that scoop samples are designed to collect a mix of infauna, which live in the mud or sand under the surface, along with epifauna, which live on the surface.
The Bay Program kept benthos in mind when it developed new dissolved oxygen criteria for the Chesapeake Bay last year. The criteria set an absolute minimum oxygen requirement of 1 milligram of oxygen per liter of water for all areas of the Bay outside its deep channel (the worst part of the Chesapeake) to protect benthos. With less than that, many types of benthos begin showing stress or dying.
That’s a modest amount of oxygen—a striped bass likes more than 5 mg/L—but as the IBI shows, that’s more than many parts of the Bay have during recent summers.
Virginia and Maryland, which are adopting the criteria as enforceable water quality standards, are also including assessments of the benthic IBI in their standards as a gauge of real-world conditions.
It’s not just the benthos that suffer when Bay conditions are bad. Benthos such as worms and crustaceans are important to the diets of bottom-feeding fish such as spot and croaker. Other benthos are important food for blue crabs and striped bass.
“If you have poor quality mud and poorer quality bottom,” Dauer said, “you will support fewer fishes, crabs, birds, dividing ducks and plovers that feed on the benthos.”
Still other benthic dwellers, such as clams and oysters, are critical because of their role in filtering the Bay’s water—not to mention as food for humans. So when benthic communities suffer, the whole Chesapeake does.