Nitrogen bioremediation through hard clam culture in Sengekontacket Pond
In recent decades, the water quality in the Island’s ponds has declined significantly. Recent results from studies conducted under the Massachusetts Estuaries Program (MEP) in Sengekontacket Pond confirm that the major cause for this environmental decline is excess nitrogen entering the ponds primarily from residential septic systems (80% of controlled sources). The excess nitrogen causes microscopic and macroscopic algal blooms which in turn set in motion a cascade of damaging impacts including decreased dissolved oxygen, increased turbidity, loss of eelgrass, siltation and eventually stressed and dying fish and shellfish.
Bivalve mollusks like the hard shell clams found in Sengekontacket are filter feeders. They clear excess particles and microscopic algae from waters at rates of 1 to 4 L/h, and healthy populations of shellfish can filter a substantial fraction of the water in coastal ponds on a daily basis. Actively growing shellfish incorporate nitrogen and other nutrients into their tissues as they grow. On average, 16.8 g of nitrogen is removed from estuaries for every kilogram of shellfish meat harvested. In addition to removal of nutrients through shellfish aquaculture, shellfish beds may act to promote removal of nitrogen from estuaries by increasing organic nitrogen deposition to the sediments that stimulate denitrification processes. It is suggested that shellfish restoration projects and establishment of small-scale molluscan shellfish aquaculture operations may mitigate the effects of coastal housing development or other activities that promote excessive coastal eutrophication.
In Sengekontacket, the hard clams grown by the towns of Edgartown and Oak Bluffs not only render ecological services to the pond and its inhabitants, but also bring a steady income to the towns through recreational and commercial fishing. Under the existing propagation program, the qhahog seed produced at the hatchery facility each spring is transferred to floating sand boxes in Sengekontacket Pond during the month of June. The sand boxes act as sheltered nursery grounds for the clams to grow away from predators. While the clams grow they filter microalgae and uptake nitrogen. In the fall, the sand boxes are emptied, and the clams planted in Sengekontacket and Katama Bay. Once planted, the clams have to fend for themselves and are more vulnerable to predators. Mortality increases. The larger the clams are at planting, the less they are susceptible to predators.
The friends of Sengekontacket have expressed interest in a quahog relay approach to improve water quality. In a quahog relay, adult clams from and off-island contaminated area (Taunton River estuary) are imported into our ponds to depurate meanwhile filtering algae. It has been shown that tissue growth of clams in actively growing shellfish beds is an effective means of removing nitrogen from the water (Rice, 1999). However, this is not true for bivalves that are mature and not actively growing. In a mature state, clams are generally in a state of nitrogen balance in which the organic nitrogen ingested is equal to the nitrogen excreted as ammonia (Rice, 2001). Therefore, a quahog relay would not achieve the degree of bioremediation sought. Increasing the number of actively growing shellfish seed on the other hand would.
The Martha's Vineyard Shellfish Group, Inc. is a non-profit organization comprised of the shellfish departments the six Island towns. For over 35 years, the Group's community-based resource management program has sought to preserve and expand the Island's traditional shellfisheries. Key to this effort has been the operation of a solar-assisted shellfish hatchery, the application of innovative aquaculture technology and a continuing vigilance to improve and maintain the good surface water quality crucial to a viable shellfish industry.
In this project, the Martha’s Vineyard Shellfish Group proposes to increase its bioremediation effort in Sengekontacket by holding 400,000 clams for overwintering in bottom nursery systems in Sengekontacket therefore increasing survival and nitrogen uptake before their transfer to Katama Bay and Lagoon Pond. To achieve this goal, funding is needed for the construction of 16 bottom nursery sand boxes, labor to grade the clams and fill the boxes in the fall, tend to the nursery systems during the winter and spring and empty the systems and plant seed in the spring.