Monday, 26 May 2008

The plankton cocktail

In the Chesapeake, vast underwater fields of eelgrass, widgeon grass, coontail and many other species flourished and protected sediments from being resuspended and coastlines from eroding. Leaves swaying in the water dissipated wave energy and slowed water currents. Plant seeds go into the plankton cocktail. Losing these submerged grasses, as has happened in Chesapeake, has meant they can no longer act as protector against wave and current damage that tears up smaller seedlings.

These grasses are also home to microscopic plants and animals which if they are not grazed by small fish build up in layers on the surface and starve the grasses of sunlight.

The combined effects of more sediment in the water and the plants encrusted with micro plants is literally to muddy the waters and stop the oxygen from getting through to the estuary floor thereby breaking the cycle of life.

Photosynthesis in an acre of Chesapeake Bay salt marsh can surpass the productivity of the most fertile land farm. These decaying grasses are a food source for the tiny zooplanton in the water that we can only see with a microscope.

This is the organic material at the base of the detritus food pyramid. The marsh grasses and the underwater grasses, usually abbreviated to SAV from the cumbersome Submerged Aquatic Vegetation, are essential to the health of the bay. The submerged grasses create especially valuable habitat for snails, small fish, birds, and larger fish that feed on each other in a hierarchy of life and death.

There was natural band of SAV surrounding the points of land and islands, but little SAV on the bottom of most of the Bay. The depths at which submerged grasses will grow in the tidal waters of Virginia and Maryland is limited by wave action in the shallow end - the roots can't hold the plants in one place where the water is sloshing around - and by light penetration in the deep end. Light can only go through several feet of water naturally before being absorbed. The loss of the oyster reefs has taken away the wave breaks. Pollution has meanwhile poured piles of darkening algae in to the bay where there are now too few oysters left to digest them.

To encourage the plants again, the water has to be cleaned. Enter the oyster, again.

The sway of the evidence has suddenly found ecologists demanding new oyster reefs not for harvesting for food, or to save the local fishing industry, but just for the benign effect the oyster creates for other species. Scientists are not asking for small sanctuaries either, but reefs at least half the depth of the water and demanding that the natural cycle be left untouched by fishermen to allow the estuary to revive. In Barnstaple one third of all the new cultivation paid for out of public money is currently prohibited to fishermen.