What Lies Beneath?

A Salt Marsh Keeps its Dirty Secrets Hidden Below the Surface
by Tracey Crago, WHOI Sea Grant

To the casual observer, West Falmouth's Wild Harbor appears to be a pristine and highly productive estuary. Tucked off Cape Cod's Buzzards Bay coastline, its lush green marshes are bordered by thick stands of trees. It is a setting often captured in photographs and paintings. But beauty, in this case, is only centimeters deep. In 1969, the barge Florida spilled 185,000 gallons of No. 2 fuel oil into the bay, and much of the oil that washed ashore ended up in Wild Harbor. Large kills of fish, crustaceans, worms, mollusks, and other invertebrates resulted, along with large areas of dead Spartina grasses.

Jennifer Culbertson (center), along with her Boston University Marine Program advisor Ivan Valiela (right) and fellow student Ylva Olsen, transplant Spartina grasses in the Wild Harbor salt marsh on Cape Cod, site of a 1969 oil spill.

Only seven miles from Woods Hole, the spill presented marine biologists and chemists with a unique opportunity to begin research into the fate and effects of spilled oil within hours of the disaster.

The resulting studies, published in the 1970s and 1980s, have helped shape the research of Jennifer Culbertson, a Ph.D. student in marine biology at the Boston University Marine Program (BUMP) in Woods Hole. Nearly 40 years after the Florida spill, Culbertson is one of a new generation of scientists studying Wild Harbor and the long-term ecological effects of the spill.

Another, Chris Reddy, is a chemist at the Woods Hole Oceanographic Institution (WHOI). In 2000, Reddy collected vertical profiles of marsh sediments and found that, at depths of 12-16 cm, the concentrations of oil are close to those measured at the surface in 1973. His detailed analyses of the oil, using advanced gas chromatography techniques, show that the oil is only moderately degraded: the more persistent compounds have remained in the sediment since 1969.

Reddy's results, says Culbertson, beg the question: does this subsurface reservoir of petroleum hydrocarbons still have potent biological effects?

With funding from Woods Hole Sea Grant, Culbertson and her advisor, BUMP professor Ivan Valiela, designed a three-pronged—and three species—approach to examine population and behavioral effects of residual oil on prominent species present in the oiled habitats of the Wild Harbor estuary.

After two growing seasons, Culbertson will measure the filtration rates of transplanted ribbed mussels as a way of determining potential impairment related to hydrocarbon exposure.

Much of Culbertson's work to date has focused on behavioral responses in fiddler crabs (Uca pugnax). Her experiments have been based on work done in the mid-1970s by Charlie Krebs, Kathy Burns, her advisor, and others, in order to extend the original work. Culbertson's fiddler crab experiments examined feeding rates, escape response, and burrowing depth. To study feeding rates, Culbertson devised a "reciprocal treatment" study to show whether crab response is affected by location, sediment type, or sediment depth. She collected 60 crabs from the Wild Harbor (WH) site and 60 from Great Sippewissett Marsh (GSM), a clean site located between Woods Hole and Wild Harbor. WH crabs were fed WH sediment taken at three different depths, including "oiled" sediment from 12-16 cm depth, where Reddy's analyses had shown the highest concentration of oil. Another group of WH crabs were fed GSM sediments collected at the same three depths. She duplicated this feeding protocol for the GSM crabs. Over the course of the experiment, Culbertson collected crab fecal pellets, a proxy for feeding rate. "Regardless of where the crabs came from, those fed on oiled sediments showed a lower feeding rate," says Culbertson, "and the Wild Harbor crabs had a lower feeding rate overall."

Culbertson used the same crabs to look at escape response. She placed the crabs into an aquarium, one at a time, positioned inside a circle drawn on the bottom of the tank. A weighted cardboard square at the end of a pendulum was suspend-ed over the tank and put into motion. The crabs were timed as they tried to escape the swinging box. Two trials, five days apart, were performed for each crab.

Culbertson found that crabs fed on WH's oiled sediment exhibited lower escape response rates—around 18 seconds—when compared to all the other crabs, whose escape rates averaged just over 8 seconds for crabs fed non-oiled WH sediments, and just over 3 seconds for crabs fed GSM sediment. "We expected to see a slow escape response for crabs from both locations when they were fed oiled Wild Harbor sediments," says Culbertson. But what she found—and didn't expect—was that the Wild Harbor crabs were so much slower in general, even when fed their own sediment. "We thought they would have acclimated to the oiled sediments over time," explains Culbertson, "but this wasn't the case."

To examine burrowing depth, Culbertson made plaster casts of fiddler crab burrows in both study sites and compared depths from oiled and non-oiled areas. Her results showed significant differences between sites: the mean burrow depth at oiled sites in WH was 6.4 cm. In contrast, burrows from non-oiled WH sites were 16.2 cm deep. In GSM, the mean burrow depth was 13.7 cm.

In photographs of the plaster casts, burrows from oiled WH locations are easily recognizable: they are vertical to a depth of about 10-12 cm, at which point they turn horizontally. "We think [the crabs] may sense the oiled sediments and try to avoid them," says Culbertson. Indeed, extensive fieldwork has shown that most of the oil resides at a depth of 6-16 cm. Krebs and Burns, who did a similar study in the mid-1970s, attributed the shallow burrows they recorded in WH to ‘locomotor impairment.' "It's an interesting debate," says Culbertson. "Are the crabs avoiding the oil, or are they affected by oil in their tissues, which causes them to burrow abnormally?"

The fiddler crab results will constitute the first chapter of Culbertson's Ph.D. thesis, which she is busy writing up for publication this spring. Work on the two remaining species, salt marsh grasses (Spartina alterniflora and Spartina patens) and the ribbed mussel (Geukensia demissa), is underway.

Now in her third year in the BUMP program, Culbertson says she has benefited greatly from the work of others who studied the spill—some whose work was done before she was born. "Having access to so many studies of this one area, we can look at long-term ecological effects," she says, something not ordinarily possible for a student who has only five or six years to work on a project. "Plus," she adds, "there are still people available to talk to." Valiela, being one of them, isn't sure how that makes him feel. "I realize that I'm one of a few left who are still actively working in the field. Or," he quips, "alive, for that matter."

For Culbertson and her predecessors, digging up secrets hidden within the Wild Harbor marsh has revealed the fragile nature of coastal ecosystems, while proving that beauty is only part of the picture.