Shellfish aquaculture can clean waterways
provided by University of Rhode Island
s environmental engineers, regula- tors and government agencies seek ways to clean the waters of local bays, harbors and other coastal areas, a University of Rhode Island researcher advocates using a natural cleaning system that provides economic benefits at the same time shellfish aquaculture.
Michael Rice, professor and chairman of the URI Department of Fisheries, Animal and Veterinary Science, says that the natural feeding method of bivalves like clams, mussels, oysters and quahogs serves as an estuarine filter, increasing the clarity of coastal waters and facilitating the removal of nitrogen and other nutrients. By increasing the quantity of shellfish in local waterways through the establishment of aquaculture facilities, these waters can be continually cleaned naturally at little or no cost to the state or community.
All of the economically important bivalve shellfish derive most of their nutritional needs by filtering particles from the water. They actively sort particles according to their nutritional value, ingesting food particles and depositing rejected particles into the seabed. When the shellfish are harvested, the nutrients absorbed in their tissues and shells are permanently removed, too.
Healthy assemblages of shellfish can filter a substantial fraction of the water in coastal estuaries on a daily basis, said Rice. Aquaculture operations may mitigate the effects of coastal housing developments or other activities that promote excessive coastal eutrophication. Eutrophic waters have low levels of dissolved oxygen and increased plant life, especially algae, at the expense of other life forms.
Rice estimates that the 26,400 metric tons of northern quahogs living in the Providence River section of Narragansett Bay filter 21.3 percent of the water at that location during each complete tide cycle. On average, 16.8 grams of nitrogen is removed from the water for every kilogram of shellfish meats harvested. If just ten percent of the standing stock of quahogs were harvested annually, eight metric tons of nitrogen would be removed as well (though this is a small percentage of the 4,580 metric tons of nitrogen discharged into the Providence River annually). The remaining quahogs deposit large quantities of particulate matter from the water on the bottom where chemical processes in the sediments may convert the nitrogen and other nutrients to less biologically active forms.
The input of inorganic nutrients like nitrogen and phosphorous into coastal waters is seen as a significant environmental problem that is expected to intensify as coastal development continues. These nutrients come primarily from sewage treatment facilities, individual sewage disposal systems, lawn fertilizers and various agricultural practices. The result is eutrophication: reduced dissolved oxygen levels, increased algae blooms, and generally poor water quality and clarity.
In addition to advocating increased private aquaculture development, Rice said that publicly funded shellfish restoration projects may be a cost-effective complement to coastal wastewater management strategies and other projects designed to mitigate the impacts of coastal eutrophication.
Since shellfish beds filter particulates from the water column and increase water clarity, projects to restore eelgrass and other submerged aquatic vegetation may be improved if there were parallel efforts to restore shellfish in adjacent areas, he said. Bivalve aquaculture combined with shellfish fishery enhancement may in fact be an effective mitigation practice to counteract the effects of excessive nutrient enrichment.