Keala Cummings peers through cutting grass at Jug Bay, where she did summer fieldwork. Credit: Lora Harris.

AN OSPREY SOARS at high altitude above the Patuxent River in southern Maryland. Tucking its mottled brown and white wings, the bird of prey plummets. It dives four times, moving in a straight line upriver, until it emerges clutching a fish. Fighting gravity, the osprey beats its wings and wheels toward golden spires of wild rice on the western bank.

The osprey is one standout in the rich biological community of Jug Bay. This tidal fresh marsh, located just 20 miles southeast of Washington, D.C., is both far enough downstream to experience daily tides and far enough upriver to avoid the infiltration of salt. And without the salinity that limits the biodiversity of saltwater marshes closer to the Atlantic Ocean, a large number of species flourish.

This biodiversity is why Keala Cummings, a rising senior at Scripps College in southern California, spent the summer researching the marsh at Jug Bay Wetlands Sanctuary on the eastern bank of the river. As part of the Research Experiences for Undergraduates (REU) program, administered by Maryland Sea Grant and funded by the National Science Foundation, she conducted a pilot study that will help scientists adapt a predictive model of the relatively simple saltwater marsh to the more complex tidal freshwater marsh. Cummings researched one component of that model — the effect of diverse plant shapes on the capture of free-floating sediment in the river.

"Freshwater marshes are just amazing," says Cummings' mentor Lora Harris, an expert in ecosystem modeling at the UMCES Chesapeake Biological Laboratory in Solomons, Maryland. Unlike the saltwater marsh, where spaghetti-like Spartina patens and stiff, reed-like Spartina alterniflora are the main species, there's a huge diversity of broadleaf plants, she says.

According to Cummings, these plants serve as a "catcher's mitt" for the suspended sediment in the Patuxent River. "As soon as all this free-floating plant debris and organic matter hits the marsh," she says, "it physically hits the plants and gets caught." She adds that the plants slow down the water as well, so that suspended solids sink and settle on the marsh floor.

To find out just how much sediment the plants of Jug Bay capture, Cummings spent four days wading in the marsh. A self-described "hardcore backpacker," she was excited to get her feet wet. She wears her misadventures in the marsh as a badge of pride, not pity.

One day Harris lowered her off the dock at Observation Creek in Jug Bay, and Cummings found herself in the midst of a patch of cutting grass — a plant that can cause a troublesome rash. A leaf blade poked her in the left eye, leaving a burning pain.

A plant with the unlikely name of spadderdock may help to preserve the Jug Bay marsh by capturing silt. Credit: Keala Cummings.

Next time, Harris helped Cummings mount a defense against the plant. "I got this towel, I got a cape and a pole to beat the cutting grass with," says Cummings, her dark brown eyes full of mirth. "I felt like a superhero." After whacking through a 30-foot swath of cutting grass, she emerged in one piece. "It's like backpacking — constantly testing yourself and finding the joy in adversity," she adds.

Cummings braved the cutting grass to collect plant specimens in quarter-meter squares along two lines stretching from the Patuxent River to the upland side of the marsh. She also collected sediment that settled on white tiles beneath the plants. Finally, for comparative purposes, she measured free-floating sediment in each square.

For her lab work, Cummings meticulously washed the sediment from the plants she'd collected, filtering it and measuring its mass. To this she added sediment from the tiles. She used these numbers to calculate a ratio of sediment-capturing efficiency in each square. Finally, she traced each plant to determine its surface area.

Cummings expected that as plant surface area increased, so would the amount of sediment captured. Instead, she found no correlation. On a community level, however, one plant with elongated heart-shaped leaves showed particular promise. Spadderdock, although less dense than other plants in the marsh, collected significantly more sediment. It also happens to occupy the low marsh, the area beside the river channel. "It's possible that the marsh evolved this way to protect the areas most vulnerable to being submerged," she suggests.

Sediment captured at Jug Bay matters because it reduces the amount of suspended solids that the Patuxent River dumps into the Chesapeake Bay, says Harris. There, sediment and organic matter cloud the waters and suppress the growth of underwater grasses. These grasses provide habitat for bottom-dwelling organisms like mature blue crabs. Sediment also smothers the hard substrates on which oysters grow. And toxic metals and chemicals are often bound up in this sediment.

The sediment-capturing function of the tidal freshwater marsh proves especially important in the face of extensive development. Development in the Patuxent River watershed has increased the river's sediment load, explains Mike Lucas, an archaeologist who conducts digs at the Mount Calvert plantation house across the river from Jug Bay Wetland Sanctuary. Sedimentation rates in the marsh in the 20th century averaged about ten times more than before European colonization, according to one study of sediment cores in Jug Bay's marshes by Johns Hopkins University. Lucas suspects that the recent real estate boom has now caused another sediment spike.

Capturing sediments not only protects water quality downstream, it's also how marshes stay afloat. As land in the Chesapeake Bay region slowly sinks and sea level gradually rises, wetlands must capture enough sediment to compensate. On the western side of the river, at Patuxent River Park, senior naturalist Greg Kearns has witnessed an overall reduction of the low marsh, which borders open water. There sedimentation has not kept up with a water level that Kearns says has risen 6 to 9 centimeters (2.4 to 3.5 inches) during his 30 years at the park. Some plants are effectively drowning.

Chris Swarth, who's directed the Jug Bay Wetlands Sanctuary since 1989, acknowledges that he also worries about the long-term sustainability of the marsh at Jug Bay. "We're all concerned that if sea level begins to rise rapidly, we could lose some marshland," he says. For now, though, Swarth says that his part of the marsh is probably keeping pace with sea level pretty nicely. He cites water depth measurements that have remained stable for the last 20 years.

Cummings may have discovered an important line of defense against the threat of rising water levels — the ability of one low-marsh plant species to trap large amounts of sediment. If spadderdock proves adept at capturing sediment, the wetlands of Jug Bay will continue to thrive.

Jonathan Berlin was a communications intern at Maryland Sea Grant this summer. He is currently a senior in the Journalism Department at the University of Maryland, College Park.

Students and Maryland Sea Grant For twenty years, Maryland Sea Grant's Research Experiences for Undergraduates (REU) program has brought college students from around the country to the labs of the University of Maryland Center for Environmental Science (UMCES) to work alongside marine scientists. Funded by a grant from the National Science Foundation, the REU program pairs fourteen students with faculty mentors to conduct 12-week research projects in fields ranging from fisheries to botany to physical oceanography. Find out more at www.mdsg.umd.edu/reu/. In addition to the REU program, Maryland Sea Grant supports a variety of programs in marine and environmental sciences for K-12 students and teachers, graduate students, and the general public. These include workshops, special programs, and interactive web lessons. Maryland Sea Grant also offers fellowships in research and science policy and an internship for undergraduate students in communications.