Eelgrass Distribution and Water Quality in West Falmouth Harbor, MA

George Hampson, Brian Howes, and Kirsten Smith

The Problem:

West Falmouth Harbor is a coastal embayment opening into the eastern waters of Buzzards Bay, which historically has maintained high water quality. The Harbor remains an important habitat for quahogs, soft-shell clams, and oysters. Scallops, which depend upon eelgrass beds, are still present in the outer Harbor. However, the West Falmouth Harbor System appears to be currently undergoing changes due to a rapid increase in watershed nutrient loading primarily from recent entry (1994-95) of a groundwater nitrogen plume created by effluent discharge from the Town’s Wastewater Treatment Facility (WWTF). One of the major ecological issues concerns the effect of rising nutrient loads on the eelgrass community which provides the fundamental ecological structure to the Harbor System.

The Approach:

Eelgrass is sensitive to overall water quality conditions and particularly nitrogen levels, typically inhabiting areas of low nutrient loading and organic matter inputs. To assess ecological shifts within West Falmouth Harbor temporal changes in eelgrass distribution were determined. The present and historical distribution of eelgrass beds within the Harbor were obtained from detailed visual mapping (Hampson, 1999) and aerial photography with on-site verification (Costa in 1979, Costello in 1996-97). Changes in distribution were assessed relative to summer nitrogen levels in Harbor waters collected by the Falmouth Pondwatch and CBB Baywatch Programs.

The Results:

There has been a clear loss of eelgrass from the inner basins of West Falmouth Harbor. Total eelgrass coverage within the Harbor has declined by about 50%. These losses began to occur prior to 1996 (WWTF plume entered in 1994-95) and appear to have rapidly increased to 1999. Additional historical data constrains the onset of eelgrass loss to between 1985 and 1996. During this interval watercolumn nitrogen levels have also increased as have nitrate discharges to Snug Harbor. Eelgrass beds areas are currently being colonized by macroalgae, primarily Ulva which is typical of high nitrate areas. It appears that the Harbor is currently above its threshold for tolerating N inputs and that additional loading will cause increased degradation.

The Projection

The WWTF currently accounts for two-thirds of the total watershed N load to the Harbor and its N load will nearly double over the next 6 years as the more heavily N loaded regions of the upgradient portion of the existing plume discharge to the Harbor. The projection for the next 6-8 years is a continued loss of eelgrass and expansion of the macroalgal areas. Recovery of the eelgrass community will only follow N source reductions within the West Falmouth Harbor watershed.