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Ecology of Great Salt Pond, Block Island
Hale, S. Ecology of Great Salt Pond, Block Island. Presented at New England Estuarine Research Society, October 11 - 13, 2012.
Bring awareness of the unique ecology of this relatively healthy salt pond estuary that is under increasing threats from nitrogen and fecal coliform runoff and invasive species.
Great Salt Pond is an island of estuarine water on Block Island, which sits in the middle of the Northwest Atlantic Continental Shelf. When the last continental glaciers retreated, they left a high spot on a terminal moraine. The rising sea from melting glaciers formed two islands. Later, currents and longshore drift connected the two islands with sandy beaches that enclosed the Pond. Rain falling on the upland parts of the island created a salinity gradient between the fresher water in Harbor Pond and Trim’s Pond in the southeast corner of the Pond and shelf-level saltwater elsewhere. That freshwater supports stands of native Phragmites, as well as the invasive one. There’s not another estuary on the island. Prior to the construction of the permanent breachway in 1895, the western beach was periodically battered open by storms, then closed again by longshore drift of sand. The mix of species changed with the salinity changes. The Pond (255 ha) is relatively deep for a coastal lagoon: 50% of area > 4m, 17m maximum; the deeper waters may provide a winter refuge for species that, on the mainland, move out of salt ponds during the winter. The Pond’s watershed (858 ha) covers about one-third of the island. Present-day saltmarshes are post-1938 hurricane. Eelgrass, formerly abundant, apparently has disappeared from the Pond. While new forces— invasive species on boat dock pilings, areas of elevated nitrogen and fecal bacteria from the watershed, the effects of 20,000 boat-days during the summer season—are driving new changes to the species mix, the Pond, compared to mainland Rhode Island salt ponds, is relatively healthy.