Understanding Environmental History One Diatom at a Time


Cooper

Story posted October 18, 2006

There is something almost Seuss-like about the object of CSC Scholar-in-Residence Sherri Cooper's current research. Perhaps it is because she is studying something so tiny, so varied, and so richly incongruous — a single-celled algae called a diatom.

Consider the diatom's properties: It makes its own food through photosynthesis, but is neither plant, nor animal. It is at the base of the food chain, yet among the most diverse life forms on Earth, with as many as one million species. Although hardly a household word, it produces up to 30 percent of the Earth's oxygen — more than the rainforests.

Most significantly, says Cooper, each diatom prefers a specific habitat, nutrients, light, salinity, temperature and pH-level. When you look at fossil diatoms, as paleoecologist Cooper does, these details matter enormously.

diatom on slide
A common estuarine diatom, Paralia sulcata, as seen through a scanning electron micrograph. Actual size is about ten micrometers in diameter.

"Diatoms have a silica shell or exoskeleton (known as frustules) that remain as fossil evidence in the mud," she says. "In fact, they are one of the most abundant fossils you can find in lake and coastal ecosystems. By identifying fossil diatom assemblages, you can recreate habitat and water quality in that time and place."

Cooper is spending part of the 2006-2007 academic year analyzing fossil diatoms she is finding in sediment cores from the Back River, near Arrowsic. From these samples she hopes to trace back water quality and habitat before the first European settlers made their homes there.

Her work will bring an added perspective to ongoing research on Merrymeeting Bay by John Lichter and Peter Lea, who are studying the estuary's environmental history, including how early mercantile communities influenced the environment.

Her first big challenge is finding what she calls "good mud."

"We're still trying to identify where there are sediments," notes Cooper, who is at Bowdoin during her sabbatical from Bryn Athyn College, outside of Philadelphia. "The estuaries in Maine are much rockier, narrower, more fjord-like, than the mid-Atlantic or Southern coasts, where I've done much of my work."

Lichter Coring
Assistant Professor of Biology John Lichter, with whom Cooper is collaborating on research, takes a core sample of sediments around Merrymeeting Bay.

In September, Cooper examined several samples in the field with Peter Lea and Laboratory Instructor Jaret Reblin, which involved inserting a two-foot metal tube into the mud and pulling out a core of sediment intact. What she saw was promising.

"We found fairly fine mud deep down, usually a good sign," she notes. "And there is shell material deep in the core. What you don't want to see is a sand layer mixed down into it, which means the surface is being reworked."

Cooper returned to the field recently to collect more core samples, this time bringing them back to the lab in plastic tubes. In order to find fossil diatoms, she must separate out other organic and inorganic materials contained in the mud. It is largely a chemical process, involving hydrogen peroxide for eliminating organics and hydrochloric acid to get rid of carbonates. Finally, she performs a size fractionation.

"I usually just let it settle for a short time, pour off the diatoms and leave the heavy stuff," she says, smiling at the simplicity. "Then I make slides. You get mostly diatoms."

Once she locates a fossil diatom, Cooper's real work begins. Within each frustule, a world of information is preserved.

"Algae respond when people clear land and when sediments and nutrients are added to streams," says Cooper. "You can date the sediments, correlating what you see with known historical records. It helps you get a story through layers of time."

"The original impacts were Europeans coming in and clearing the land. There was a real shift in diatom assemblages. With urbanization and industrialization there have been added issues."

Although much of current ecological inquiry focuses on modern environmental issues, Cooper says that many lasting effects actually were caused by the first European settlers.

"The original impacts were Europeans coming in and clearing the land," she says. "There was a real shift in diatom assemblages. With urbanization and industrialization there have been added issues, but an underlying change was already there.

"Of course, we'll never get back to pre-European conditions, but to see what actually caused which changes gives us a really good idea of how to manage the ecosystems today. We can see effects due to land clearance, sedimentation changes, farming, urbanization and industrialization. Which of these has had the most lasting effects? For example, is it organic or inorganic nutrient inputs that cause the kinds of shifts that may be most detrimental to the ecosystem?"

Sherri Cooper in lab
Cooper at her Coastal Studies Center lab, where she is scholar-in-residence.

Paleoecological evidence can also be used to evaluate whether environmental remediation efforts have been successful. After leaded gasoline was banned in the late 1970s, for instance, the amount of lead in estuarine environments declined. With the Clean Water Act, certain heavy metals were reduced.

"In ecology, it's so hard to figure out what's going on right now that if we go back through time and get a sense of the history of the ecosystem and the effects of human impacts, we have a much better idea of what we can do to get it back to where we want it to be," says Cooper.

Cooper will teach an upper-level course in Coastal Ecology this spring jointly in the Environmental Studies and Biology departments. In addition to introducing students to fieldwork in algae, she plans to look at current issues in coastal ecosystems such as wetlands problems around Louisiana, increased storm activity, and sea-level changes.

Cooper's research with John Lichter at Bowdoin is supported, in part, by the Rusack Coastal Studies Project Initiative Fund.

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