The April 13th Bay Journal has a great OpEd by Tom Horton <http://www.bayjournal.com/article/core_sediments_reveal_when_a_wetter_bay_was_wildly_healthier> about Grace Brush’s research, and beavers, that spells it out:
Core sediments reveal when a wetter Bay was wildly healthier
- By Tom Horton on April 13, 2014
Grace S. Brush of Johns Hopkins University holds a core sample taken from the Blackwater River in 2003. Sediment core samples taken from the bottom of the Bay have revealed how changes on the land have affected water quality. (Dave Harp)
It’s common knowledge that the healthy Chesapeake Bay described by John Smith in 1608 was greener, its forest extending across more than 90 percent of its six-state watershed.Less appreciated is how much soggier, boggier, swampier and wetter that good, green watershed was. Beavers that likely numbered in the low millions controlled the hydrology of hundreds of thousands of streams—damming, impounding and creating a rich and damp mosaic on the landscape.
“Great Shellfish Bay” — American Indians’ name for the Chesapeake — might be amended to include “Great Beaver Bay” if we acknowledge the intimate connections between water and watershed.The wetter, better-beavered landscape checked sediment; converted nitrogen-laden runoff that plagues today’s water quality into harmless gas; restrained the peaks of floods; and recharged the Bay’s rivers during droughts. It created a cleaner, clearer, stable place for life.Bevies of beavers might be the cheapest way to a better Bay; but legions of tree-felling, land-flooding rodents would conflict too heartily with human schemes.
But beaver mimicry is another matter, restoring wetness every way and everywhere we can — from rain gardens throughout suburbia, to strategically placed, engineered wetlands, to retention dams in farm and roadside drainage ditches.Current efforts just scratch the surface of what is possible.
Low-lying, poorly drained, heavily ditched farmland everywhere should be a prime target for purchase and reversion to the wild and wet. Retrofitting every paved surface to let stormwater soak in would advance the cause further.We needn’t achieve the watershed of John Smith to see results. The Bay showed substantial resilience in the face of increasing human pressures for centuries, only to truly fall fatally apart in the 1950s.
All these vital and hopeful links between the once and future Bay are detailed wonderfully in a unique library in the Bay’s bottom opened only recently by scientists like Grace S. Brush of Johns Hopkins University.Her 2009 paper, “Historical Land Use, Nitrogen and Coastal Eutrophication: A Paleoecological Perspective*,” published in Estuaries and Coasts, should be required reading for anyone at any level teaching about Chesapeake Bay (or about beavers).
Essentially, Brush figured how to “read” the Bay’s land use history going back millennia, and how changes in the watershed translated into changes in the water.She did this by extracting yards-long cores of sediment from across the Bay’s bottom. Then, bottom to top, ancient past to present day, she analyzed the seeds and pollens and other data deposited there.At a level deep in the sediments washed from the watershed around A.D. 1000, for example, charcoal from more frequent fires corresponds to a centuries-long medieval warming period.
She can tell when forest clearing for European agriculture began as pollen from the ragweed of open fields increases; and when the Bay’s underwater grass pollens dived sharply in the 1970s after centuries of relative abundance.
This last evidence was critical in disabusing the false hope of some government officials that the seagrass decline was just a natural cycle.Volumes from the library, summarized in Brush’s paper, show how forests began to take a serious hit around the middle 1800s as a growing human population, the invention of the steel plow and markets for timber led to the clearing of all but about 20 percent of the watershed by the early 1900s.
The watershed since then has actually re-greened — to nearly 60 percent forest — as people moved into towns and cities, and farms were able to produce more food on fewer acres. But it was a different sort of green, significantly less wet.
Trapping largely abolished the beavers and their dams by the mid-1750s. Indeed, the sediment cores show a small but significant decrease in nitrogen runoff, a trend that would not turn up until human sewage and commercial fertilizers came on strong in the 1900s. Brush suspects the decline may reflect a decline in beaver poop.In addition, drainage ditching for farms, roads and development further dried out the landscape. “Well-drained” is not as good as it sounds. And increased paving made rain run off hurriedly, no time to soak in.
So it was a different kind of green that returned. Brush’s sediment cores show a big decline in pollens from wetland vegetation and moisture-loving plants in general.
The sediment library documents a most telling shift occurred rapidly in mid-20th century, as human inputs of nitrogen and other pollutants soared. The dominant algae in the Bay shifted dramatically from bottom-dwelling species to surface-floating algae.
The estuary was beginning to “flip,” to lose its bottom — its oysters, its plankton, and by the 1970s, its vital seagrass meadows. The capsize of an incredible ecosystem was complete.It wasn’t just the loss of wetness. Overharvesting, oyster diseases, intensive agriculture, sewage and air pollution were all culprits.
Still, there’s a future for the Chesapeake in its past, as remembered in its sediments, and it’s wet.About Tom Horton
Tom Horton covered the Bay for 33 years for The Sun in Baltimore, and is author of six books about the Chesapeake. He is a freelance writer, splitting his time between Baltimore and Maryland’s Eastern Shore.
Read more articles by Tom Horton
* A Powerpoint of the Grace Brush presentation to MD DNR is available on-line at <http://www.dnr.state.md.us/streams/pdfs/brush29apr09.pdf>