Eelgrass Distribution and Water Quality in West Falmouth Harbor, MA
George Hampson, Brian Howes, and Kirsten Smith
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.
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.
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 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.