For decades, water quality and the health of Nantucket’s Harbors have been issues of concern for the island’s residents and seasonal visitors for very good reason. The harbor is an important recreation resource for our tourist-based economy, a key economic part of the local fishing and shellfish economy and a source of natural beauty enjoyed by all. The health of Nantucket’s harbors is vitally important to the island, making efforts to protect and restore its estuaries and aquatic resources critically important.
The Foundation’s Windswept Cranberry Bog operates within the watershed of Polpis Harbor, a small embayment along the southeastern side of Nantucket Harbor. The cranberry bog was constructed in the early 1900’s and includes a complex system of dikes and flumes that utilize gravity to move water from surrounding wetlands in the watershed through the cranberry bog farm system and towards a single culvert under the Polpis Road that eventually drains into Polpis Harbor. The operation of the cranberry bog, combined with leaching from nearby septic systems, lawn fertilizers and other land uses in the entire watershed, undoubtedly contributes increased nutrients to Polpis Harbor that may impact water quality.
Although the Windswept Cranberry Bog has operated organically for many years, organic fertilizers still contribute increased nutrients to the surrounding watershed. To address this, we operate the cranberry bog based on best management practices in consultation with the UMass Extension Cranberry Research Station. This means that we collect data on length and density of cranberry vine uprights, conduct soil and tissue nutrient tests and keep detailed weather records. All of this information is used to accurately estimate the exact amount of fertilizer necessary for cranberry productivity to limit runoff of excess nutrients.
The Foundation has been working with the Town of Nantucket Department of Natural Resources to gain a better understanding of how water moves into the Windswept cranberry bog from the surrounding watershed and determine the potential for mitigating high nutrient loading at key locations. We have recently completed a detailed survey of the cranberry bog property to better understand inputs, outflows and the way that water circulates while it is within the cranberry bog farm system. We identified a total of 10 sites where a culvert or pipe underneath the Polpis Road drains water from surrounding areas, which includes developed land, small scale agriculture and other extensive wetlands, directly into the cranberry bog. We have initiated water quality testing at these potential point source locations, particularly for nitrogen and phosphorus. Combining this information with currently sampling of the cranberry bog outflow to Polpis Harbor will help us gain a better understanding of the sources of excess nutrients and identify potential mitigation measures to reduce or eliminate inputs to the harbor.