Much more than hors d'oeuvres

Tom Horton & William M. Eichbaum

June 13, 1991|By Tom Horton & William M. Eichbaum

The Chesapeake Bay Foundation yesterday released "Turning the Tide: Saving the Chesapeake Bay." Written by Tom Horton and William M. Eichbaum, underwritten by the Abell Foundation and published by Island Press, the book amounts to a status report on the bay, with recommendations for rescuing the estuary from the human onslaught. This is the second of three excerpts. IT IS on the shallow bay bottom, along with the grass beds, that we find yet another regulatory system on the order of the forests, marshes and grasses. You may know this elegant mechanism of homeostasis as the old, gray oyster (variously rendered as "aryster," "oistuh," "erster" and "arschture."

The oyster is a good example of how we sometimes underestimate the importance of bay creatures by considering them mainly as they relate to our bellies and our commerce. For more than a century, dating from the 1860s, our huge harvests of the tasty bivalve were synony mous with Chesapeake Bay's incredible bounty.

In retrospect, the towering peaks of oyster production during the late 19th century were not sustainable. Rather, they simply represented a short-term mining of the wealth accrued on the bay's bottom. The edible oyster meat harvested in Maryland waters alone during the peak of that exploitation was equivalent to the yield from 160,000 head of prime steers. A testament to the resilience of the estuary and the oyster is that harvests could remain at world-class levels (1 to 3 million bushels a year) almost until the last decade of the 20th century. Overfishing, combined with diseases, mismanagement and pollution, has now reduced the oysters in the bay to an estimated 1 percent of their numbers before heavy harvesting began after the Civil War.

One percent! Think of that. And think of what it would be like to live in your house if you had sold off 99 percent of the plumbing, or 99 percent of the heating and air conditioning system, or 99 percent of the roof. Because in taking too many of the bay's oysters, we have been losing much more than a plentiful supply of appetizers or a significant portion of our seafood economy. We have been destroying a vital filter, a recycler, a habitat for other creatures and a banker of food energy like the underwater grasses.

After their free-floating larvae attach to other oyster shells and become spat in the first weeks of life, oysters never move. They have traditionally been superabundant in the bay because its vigorous circulation brings them plenty of food, and because their food is phytoplankton, which grows so well in the sunlit shallows. (The bulk of oysters grow in water 30 feet deep or less.) They are superb filterers, feeding by gaping their shells slightly and pumping bay water through their gills at rates up to 2 gallons per hour.

In addition to growing fat oysters, this process has at least two other important consequences for the estuary as a system. By sucking in sediment that clouds the bay's waters, and depositing it on the bay bottom as compacted fecal matter, the oyster clears the water, thereby helping sunlight penetrate and grow more plankton and more underwater grasses. One estimate is that the pre-1870 stocks of oysters in the bay had the potential to filter a volume equal to that of the entire Chesapeake every few days -- compared with a "filtration time" of nearly a year for today's diminished stocks.

Moreover the oyster, like the grasses, seems to have been a "banker" and recycler of the huge pulses of nutrients that surged off the watershed in the wet springtime. It filters the lush, nutrient-fertilized spring plankton bloom through its gills, using some of it for growth but also depositing nutrient-rich feces on the bay's bottom. On the bottom, which is never far from the top, these nutrient packages can be recycled into production again. But just as the bay's oysters no longer vacuum much sediment from the water, so has their capacity to bank and package food for recycling declined drastically. Estimates are that the pre-1870 oyster population could have removed 23 to 41 percent of the plankton blooms. Now they are thought to remove about 0.4 percent -- a drop of fiftyfold or more. Where does the unfiltered plankton go nowadays? It appears that it goes to the bottom, where its decomposition can intensify problems with low oxygen.


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