1996 Citizens Report: Hen Cove, Bourne
Embayment: Hen Cove
Embayment and Watershed Characteristic
Hen Cove is the smallest embayment assessed and was assumed to have one of the smallest watersheds of all the estuaries studied (revisions to the Buzzards Bay watershed delineation by the Cape Cod Commission need to be carefully examined). More than 300 boats are moored in the harbor, and shellfish resources are ranked good, although the upper Cove is permanently closed to shellfishing. The shoreline is extensively developed and dwelling unit density is among the highest in the Buzzards Bay however (1.1 units per acre overall). In addition, Hen Cove is part of the Pocasset Harbor and Red Brook Harbor complex created by Bassetts Island. Hen Cove's location within the complex results in it receiving its source water after it transits either of these adjacent systems which increase the nitrogen levels. The combined effect of land-use and circulation patterns has been to produce an existing effective annual nutrient load of concern to managers. Nutrient management, however, must rely on the impact of these nutrient inputs on water quality.
Water Quality monitoring
Like Pocasset Harbor, water quality measures of Hen Cove showed a steady decline during the four years of study, with the Eutrophication index dropping nearly 37 points, one of the largest drops observed in the baywide study (see centerfold map). Still water quality in 1995 was only somewhat below the baywide average and remains in water the authors consider "fair" conditions. The declining Eutrophication Index scores were largely driven by increasing phytoplankton pigment levels (going from 3.3 to 8.1 ppb during the study period) and organic nitrogen concentrations in the water. Mean summertime total nitrogen concentrations showed more year to year variability, but clearly were elevated by the end of the study. Although development has greatly increased in the watershed during the past 10 years and nitrogen from septic systems may be reaching the bay only now, the somewhat poorer than expected water quality in the Cove could also be the result of unaccounted for sources (see above) or declining source water quality from either Red Brook Harbor or Pocasset Harbor. In general, the water quality of Hen Cove is relatively good compared with other small embayments to Buzzards Bay, but is the worse of the three estuary complex. The declining water quality in Hens Cove appears to be related to increasing nitrogen and chlorophyll levels causing depletion in dissolved oxygen concentrations during the summer months. Although variable, the degree of oxygen excursion appears to be increasing in parallel with nutrient increases. These are classic signs of eutrophication.Eutrophication Index scores
Nitrogen Management needs
The Buzzards Bay NEP estimated that existing nitrogen loadings are 23% over Project recommended limits. Nearly 70% of existing nitrogen inputs are derived from residential land use (i.e. septic systems). The preliminary flushing estimate methodology used for Hen Cove may not be appropriate. A more rapid flushing rate may greatly affect recommended loading rates and prioritization for action. The citizen's water quality monitoring program suggests that the loading analysis has either underestimated nitrogen inputs or that flushing is overestimated. The Buzzards Bay NEP has not redone its loading analysis using the redone subwatershed delineation by the Cape Cod Commission. The new delineation shows a much larger Hen Cove subwatershed, extending much farther inland, and containing many other residential and commercial sources. These newly identified sources may help to explain the observed water quality in Hen Cove. However, the decline in water quality over the study period suggest significant increased nitrogen inputs the source of which is currently unclear. Like Pocasset River, later this year the Buzzards Bay NEP will fund a flushing study and buildout analysis for the town of Bourne to determine appropriate management goals for Hen Cove. Revisions to the drainage basin by the Cape Cod Commission will be examined to determine if nitrogen sources were not accounted for in the Buzzards Bay NEP's loading evaluation.Back