Soil under the Chesapeake details the dynamic history of an ancient ecosystem

November 27, 1993|By TOM HORTON

Grace Brush doesn't look at all ancient, but my, how she does go on, sitting in her office at the Johns Hopkins University, recalling the old days around the Chesapeake Bay.

With familiarity, she can describe the bay country that greeted early colonists: coastal plain canopied by black gum, sweet gum and huge old oaks and hickories, with tiny orchids blooming in the understory.

The colonists must have thought those great, ancient woods had been there forever.

But Dr. Brush knows they dated only to around the birth of Christ.

Before then, endless tracts of hemlock had reigned supreme in the bay region -- but only back to about 8000 B.C. Before then, a spruce-fir forest, scattered with alder and birch, had been the norm, probably since the northward retreat of the glaciers 18,000 years ago.

With the authority of one who's been there, Dr. Brush explains that 17th century settlers encountered a bay that was both very clear and very salty.

Also, the climate was more hospitable than at any time during many previous centuries. All in all, says Dr. Brush, "It must have been a pretty neat place."

In the past decade or so, the Hopkins paleoecologist and her colleagues have found new and better libraries of the ancient Chesapeake than anyone suspected existed. The researchers are using them to sketch the history of an ecosystem that turns out to be more dynamic than imagined, complicating notions of what is "pristine" and "natural."

The history reveals a system extraordinarily sensitive in its waters to changes on the lands of its drainage basin, or watershed.

Trends that would profoundly alter the bay can be seen from almost the first land-clearing for Colonial agriculture -- and changes have accelerated dramatically in the past 50 years.

Beginning in the 1970s, Dr. Brush began extracting sediment cores -- plugs of silt, sand and clays more than 15 feet long -- from the bottom of the bay and its tributaries, and from surrounding marshes.

The cores consist of soils washing off the land and settling to the bottom.

Centimeter by centimeter, they lay down a record covering thousands of years that can be read literally year by year in the best cores.

Trapped and preserved in the layers of mud are volumes of information that can show us how and why the bay changed throughout time, and guide our response to current changes.

For example, a debate raged in the 1970s over the widespread disappearance of underwater grasses: Was it a natural cycle, or something unprecedented, caused by humans?

Reading the preserved pollens of bay grasses through thousands of years in the cores, Dr. Brush found that grasses always tended to shift dramatically, disappearing in one place and reappearing in another.

But she could find nothing like the 1970s disappearance.

As a result, re-establishment of the grasses is now one of the major goals driving the Chesapeake restoration program.

Pollen in the cores also tells Dr. Brush that areas like the upper Severn and Chester rivers once grew underwater vegetation that now exists only farther downstream, where the water is saltier.

It is evidence, she says, that clearing forests has fundamentally shifted the system, producing more freshwater runoff than ever existed before colonization.

The preserved pollens of trees and shrubs show nicely how the vegetation on land changed in response to a well-documented period of global warming that occurred about A.D. 1000.

It was a time of heat and drought and frequent fires. A core from the Eastern Shore's rural Nanticoke River shows levels of metals during this period more in keeping with latter-day Baltimore Harbor -- apparently released from soils and vegetation by forest fires.

A mini-Ice Age

Following this so-called Medieval Warming Period, the core pollens reflect more changes on the land in response to a worldwide cooling, a mini-Ice Age that affected the world for the next few centuries.

"You have to wonder, if such changes occurred today, would we blame ourselves, or nature?" Dr. Brush says.

During the 1700s a shift from oak to ragweed pollen corresponds with the advent of Colonial land clearing. In the 1930s, the absence of chestnut pollen in the upper layers of cores records the great blight that nearly wiped out the tree that used to dominate many bay forests.

As its watershed changed from forest to farms, and later to cities and suburbs, the Chesapeake began a transformation that would prove as profound as the conversion of the Great Plains from prairie to corn, and from buffalo to hogs.

The cores have recently revealed this history dramatically, through the preserved skeletons of diatoms, species of tiny algae.

In the pre-17th century bay, the occurrence of "centric" diatoms, ones that swam near the surface, was just slightly greater (1.3 times) than that of "pennate," or bottom-dwelling diatoms.

With development came sediment and fertilizers running into the Chesapeake.

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