Chilmark Pond at Its Limit for Nitrogen, Estuaries Report Finds

A report issued by the state Department of Environmental Protection for the town of Chilmark provides a clearer picture of the health of Chilmark Pond, confirming that further nitrogen runoff will impair the estuarys aquatic ecosystem.

The saltwater pond is generally healthy with no serious habitat damage, but nitrogen levels in the pond are at their limit, according to the report from the Massachusetts Estuaries Project.

As a result, the estuary is beyond its ability to assimilate nitrogen without further impairment, the report said.

Ongoing for the past several years, the MEP program aims to identify the factors contributing to nutrient pollution and habitat destruction in 89 estuaries on the Cape and Islands. MEP reports have already been done on the Vineyard for a large number of ponds, including Edgartown Great Pond, Sengekontacket, Lagoon Pond, Farm Pond and Squibnocket Pond.

The major work of the project involves setting a nitrogen-loading threshold for the ponds. The program does not offer specific recommendations for mitigating the problems at each site, but offers a starting point for communities to begin developing plans of their own.

The Chilmark Pond report saw initial discussion among the town selectmen last week.

The pond has no evidence of eel grass, an important aquatic plant in coastal ecosystems, so the study focused on the health of the ponds benthic animal habitat. The MEP recommended that the nitrogen level in the estuary be reduced from about .74 to .5 milligrams per liter to support aquatic animal life.

The pond is part of a complex embayment made up of a large central basin and several smaller basins. The main basin is maintained by periodic mechanical breaching through a single inlet, as scheduled by the town.

According to the report, the estuary is particularly vulnerable to the effects of nutrient enrichment from the watershed, due to its very limited tidal exchange. More frequent or prolonged openings to the sea can lower nitrogen loads in the same way as relieving nitrogen related habitat impairments, the report said.

The MEP said restoring the benthic habitat and achieving the desired nitrogen level could be accomplished by reducing nitrogen loading from the watershed and by breaching the pond three times per year, including a period in the spring when the breach would remain open for eight days.

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Chilmark Pond at Its Limit for Nitrogen, Estuaries Report Finds