The Effect of Breaching on Coastal Pond Water Quality

Abstract

Coastal pond breaching is used as a management strategy to improve water quality and reduce the frequency of harmful algal blooms (HABs) in many New England coastal ponds. At Sesachacha Pond, on Nantucket Island, MA, breaching currently is conducted twice annually with the goal of flushing eutrophic pond water and maintaining a brackish ecosystem through ocean input. Recent breaches have been too short to allow for significant water exchange, raising questions about the effectiveness of breaching as a water quality management method because the total nitrogen (TN) concentration remains four times in exceedance of the total maximum daily load (TMDL). The high level of eutrophication in the pond has led to anoxic and hypoxic conditions, resulting in an increase in HABs that poses a danger to the ecosystem and those interacting with it. Observations collected during the December 2024 breach using nutrient sampling and spectral data suggest that even brief connections may lower TN concentrations, but greater and sustained improvements to pond water quality likely depend on extending breach duration to enable greater water exchange.

Field observations identified the marsh culvert stream and Cains Pond as nutrient sources and potential sites of algal growth. Modest decreases in TN and reflectance at 703 nm indicate that more effective breaches may limit nutrient availability and help mitigate HAB formation. With procedural adjustments and the potential addition of a third breach with greater timing flexibility, it may be possible to reduce nitrogen levels further and restore healthier pond conditions. Although recent breaches have not achieved lasting improvements, this appears to reflect logistical limitations rather than a failure of the breaching process itself. The findings provide a framework for improving breach execution and informing future coastal pond water quality management strategies.