Name
at least 2 reasons why there's so much more sediment in the Chesapeake
Bay today than 200 years ago?
Part of this answer has to do with what we put into the Chesapeake Bay
and part has to do with what we take out. During the first ¾ of
the 20th century, nearly 40,000 acres of the Bay's shoreline (in Maryland
and Virginia) were lost to erosion. More than 15 million people now live
in the lands that drain into the Chesapeake Bay - our Nations' largest
estuary. This is perhaps 300 times more than the native population at
the time Captain John Smith landed at Jamestown, almost 400 years ago.
Until the first European settlers arrived, there was very little farming,
and thus very little agricultural erosion. The Bay's capacity to cleanse
itself via tidal flushing and evenly spread rains was soon compromised
as the new arrivals "tamed" the land. In addition, the waters'
of the Chesapeake, which more or less translates to "great shellfish
bay," was a land of plenty for the colonists. Catching fish that
crawled or pretty much stayed put proved easy pickings.
Oysters are a type of shellfish known as a bivalve - clams and scallops are also bivalves. It should come as no surprise that bivalves have two shells -- held in place by a powerful adductor muscle. Oysters are terrific filterers. Over a century ago, when American oysters were still profoundly abundant, these miniature vacuum cleaners were capable of filtering all of the Bay's water every 3.3 days. As Oysters remove algae and other nutrients from the water, making it noticeably cleaner, sunlight is able to penetrate to the Bay floor, permitting sea grasses and other wetland plants to be part of a healthy and diverse ecosystem.
Though Native Americans harvested Bay oysters for thousands of years, without damaging its ecosystem, Colonial settlers changed this equation by employing more harvesters and more sophisticated harvesting techniques. By the mid 1800s, oystering and crabbing was a major industry in the Middle Atlantic States. By the late 1880s, more than 10 million bushels of oysters were removed from the Chesapeake Bay every year (the world's leader), and one fifth of all Americans employed in fishing harvested oysters in the Chesapeake! Even as late as 1980, 500 million pounds of shellfish and finfish were annually pulled from the Bay, and a National Geographic article referred to the Chesapeake as " a very fecund place."
As a result of over-fishing, water pollution (including raw sewage, pesticides and herbicides and industrial metals), sediment loading (from soil erosion and organic solids), and nutrients (nitrogen and phosphorus) attributable to increased development around the Bay, the once prolific numbers of shellfish began a precipitous and predictable decline. Nitrogen and phosphorus act to nourish blooms of algae during the summer months, and when the algae die off, they decompose, and by doing so, rob the Bay's waters of oxygen (hypoxia). Since oysters and other shellfish filterers live on the bottom of the Bay, where oxygen levels are particularly low, they readily succumb to hypoxia. It's really a double whammy - the oysters are lost and therefore the Bay's self-cleaning ability vanishes too.
Furthermore, as the health of the oysters fell off, they were more susceptible to parasitic invaders. Two such marauders ravished what remained of the Bay's diminished oyster supplies. Oyster populations bottomed out in 2002 when 99 percent of the oysters that once lived in the Bay disappeared. Whereas 150 years ago, the Chesapeake Bay could be filtered in 3.3 days, it now takes the remnant oysters more than a year to do the job!
Add to this the fact that clearing of the land for homes, agriculture and industry allows rain and snow to be more quickly directed to stream channels. As a result, not as much rainfall infiltrates into aquifers and too much is ushered into normally brackish tidal catchments.
Of course, it needs to be mentioned that not all of the Bay's problems result from human excesses. Oysters live in salty to brackish waters but can tolerate brief influxes of fresh water. However, during Tropical Storm Agnes in June of 1972, so much fresh water entered the Chesapeake Bay drainage system (covering more than 64,000 square miles or 164,000 km and parts of 7 states) that an estimated two million bushels of oysters perished - in some locales they were totally decimated.
In the end, the Bay will lose out. Estuaries are short lived in the geologic scheme of things. Like all estuaries, even the grand Chesapeake will eventually fill in. It's likely to be fully choked with sediment, both natural and human induced, in another 15,000 years or so. Over the last century, accelerated storm runoff into the Chesapeake Bay drainage system, loss of the once prodigious oyster populations, and increased erosion from agriculture and housing developments have lessened the Bay's productiveness and increased its sordidness.
This being said, a number of steps have been and are being taken in an effort to mitigate sediment loading in the Bay. Agricultural run-off, sewage treatment and urban run-off are all being addressed. For example, the Maryland state legislature passed the flush-tax in 2004 -- monthly fees on sewer bills and septic system owners. This tax is used to upgrade the States 66 major sewage treatment plants, to improve septic systems and to help fund a cover crop program, which will help reduce agricultural run-off into the Bay. Moreover, retention ponds retain silt and sediment from new housing starts and are required in many jurisdictions having streams feeding into the Bay watershed. Of course, a lot more needs to be done and vigilance is mandatory, but the Bay water is cleaner now than it was two decades ago, and a few species once prominent in the Bay, shad and bald eagles for instance, are making welcome comebacks.
Here are two Landsat images of the Chesapeake. The first one shows the Chesapeake Bay drainage basin and the second one shows sediment (tan/pink colors) entering the Bay system.
For more about this see:
The National Geographic (October 1980 and September 1964)
The
Washington Post (May 8, 2006, A-7)
This week's question comes from Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Two years ago, Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.