1996 Citizens Report: Phinneys Harbor, Bourne
Embayment: Phinneys Harbor
Embayment and Watershed Characteristic
Phinneys Harbor is a well flushed shallow embayment with good shellfishing resources and abundant eelgrass beds. The harbor is relatively open which enhances both tidal and wind driven water exchange. Much of the nitrogen in the watershed discharges to Phinneys Harbor through the upper estuary in the areas known as Back River and Eel Pond. Nitrogen enters harbor waters almost entirely through groundwater flows, although small surface water flows enter the inner Back River/Eel Pond system. Most of the watershed directly adjacent the harbor contains residential development but the associated nitrogen load is low due to the number of residences. Much of the outer harbor watershed is associated with the peninsula which represents a small contributing area and nitrogen load relative to the volume of the harbor.
Water Quality monitoring
Phinneys Harbor showed good to excellent water quality the three years that the system was monitored (1992, 1993, and 1995) as a result of its comparatively low nitrogen loading and high rate of water exchange. The harbor had consistently high oxygen levels, typically over 80% of saturation during the study. Similarly, total nitrogen concentrations, with a three year average of only 0.36 ppm, was the best for any "inner" embayment area, and more typical of offshore Buzzards Bay levels. Phytoplankton and inorganic nitrogen concentrations were somewhat variable, but when phytoplankton pigments were high inorganic nitrogen concentrations tended to be low, and vice versa, so that a good score on one offset a poorer score on the other. Consequently the 3 year Eutrophication Index score was a high 76 points, again the best score for any "inner" embayment area. As would be expected from the pigment data, water clarity was generally very good. Barring radical changes in land-use, the outlook for a continued high water quality within the harbor is good.Eutrophication Index scores
Nitrogen Management needs
Residential development accounts for nearly three-quarters of embayment loadings of nitrogen and the watershed is approximately two thirds of the way towards full buildout. Yet almost two-thirds of the watershed remains forested. Currently the nitrogen loading is only an estimated 14% of the Project's recommended limit and at full buildout the total nitrogen load is expected to be only be 20% of the recommended load. The results of the water quality monitoring program support the current conditions loading assessment, and for these reasons nitrogen management actions are not advised for Phinneys Harbor. The upper harbor areas--Back River and Eel Pond--appear impacted and suggest that management action is required for those portions of the Phinneys Harbor drainage basin contributing to those waters. The Cape Cod Commission has updated the subwatershed for the Phinneys Harbor, and the Buzzards Bay NEP needs to update its nitrogen loading assessment for the estuary. Nonetheless, it appears that management of Phinneys Harbor should probably focus upon the limited seasonal shellfish bed closure in the Harbor and other direct management of harbor resources.Back