Monday, 26 May 2008

Why oyster reefs are good for estuaries

Oysters are good for estuaries. They improve the quality of the water. Healthy oyster beds attract new predators looking to feed on the oysters themselves, but that is just the headline ecology. An oyster will filter as much as 30 gallons of water a day. The benefits are welcomed by the tiny plankton that live both above and below it. If the oysters are not doing their job, then the balance between zoo and plant plankton shifts with the resulting blooms that will darken and snuff out marine life.

An oyster passes two different kinds of faeces, one a nutrient rich excreta which other algae will feed on, and also a psuedofaeces, the material it does not want to eat, like silt, which it has nevertheless helpfully broken down in the sorting.

Much of the richness of the estuary is born just below the surface where the sun can reach the plankton. An oyster kept on a floating reef at this level will fatten almost twice as fast as one moored on the bed.

The reefs are an important artery in the estuarine food chain and a gauge of the health of the habitat for many other life forms. It is not only the oyster spat that will seek to attach to a reef, but so will mussels, barnacles and other small sessile attachers like tiny tunicates and coral like bryozoans that also filter the waters incessantly. They do not seem to compete with each other for food because each prefers a different kind of plankton. Crabs will use the reefs for shelter. Sea anemones for attachment.

This activity encourages the growth of marine fauna. On one north Carolina oyster reef more than 300 types of plantlife were recorded. The critical salt levels that oysters need, also produce the right conditions for a suite of other desirable estuarine organisms.

Projections in the Chesapeake Bay have shown that building up an estuary oyster population again times 25 would result in a 21 percent increase in bay grasses and remove 11 million pounds of nitrogen.

In Maryland they have been videoing the returning oyster reef and recognising the seemingly obvious point that oysters will flourish more in vertical reefs, than on simple river beds or the flat remnants of an old bed. In trials here, the oysters are growing vertically, not horizontally and quickly creating dynamic and vibrant aquaculture around them.

The reason early settlers in America commented again and again at different bays on the abundance of the fishing was the same reason they cursed the dangers of what the oyster reefs might do to the undersides of their boats. They found a natural ready made resource of easy to catch, abundant fish tempted into the shore by the clean waters and the habitat fostered by and around the oyster. No wonder these colonies managed to get a firm grasp so quickly. Without the oyster the tenuous hold those first settlers had on the continent could have been so much more easily dislodged. One might speculate that uncovering America might have taken decades, even centuries longer, as was the case on the west coast.

For ecologists, the oyster is a keystone species and one much valued because of its influence on the larger eco system around it. Across the marshlands of Mississippi and the glades of Florida, the health of the local oyster has been an early warning system that the local ecology is under threat or unbalanced.

By changing the water flows, even quite badly damaged estuaries can be revived relatively quickly. In the beautifully named Caloosahatchee Estuary (christened after the local Caloosa Indians) in Florida, one reclaiming project was hoping to develop 18 acres of oyster beds into 100 acres in 10-15 years, and exponentially beyond that. So encouraged by the potential are different states, that Virginia alone has invested $40 million trying to develop new reef projects.

The reefs themselves provide important protection from erosion for the estuary banks and also for the reeds, grasses and other vegetation that contribute to the nutrient levels in the waters. These grasses were originally valuable to settlers to feed their newly arrived animals, and also as insulation in their new homes and even now are picked as high grade garden mulch. Studies on marshland grasses have shown up the oyster’s true role in the estuary.