1996 Citizens Report: Onset Bay, Wareham
Embayment: Onset Bay
Embayment and Watershed Characteristic
Onset Bay is the sixth largest Buzzards Bay embayment. Its watershed has a considerable amount of land in residential land use and with a high basin-wide unit density of 0.65 houses per acre. The watershed is also among the lowest in undeveloped forested areas. Only 4.3% of the land is agricultural. The Bay is shallow, and moderately well flushed, and the flow of low nutrient waters through the Cape Cod Canal into Buzzards Bay probably aids water quality. Eelgrass and shellfish beds are abundant in the estuary, but the uppermost portions of the estuary--Broad Cove, Muddy Cove and Sunset Bay--have degraded water quality as evidenced by shellfish beds mostly closed to shellfishing.
Water Quality monitoring
Onset Bay was one of the better monitored embayments with three regular oxygen monitoring sites in the mainstem and two sites in the Broad Cove and Muddy Cove complex ("East River"). The 1994 oxygen data typified the results of the monitoring program. In that year, the mean of the lowest 33% of oxygen saturations were best in outer Onset Bay (OB2), with a high 91% oxygen saturation, followed by 79% saturation in the inner bay (OB1+OB3), and only 66% saturation at both Broad and Muddy Cove stations (ER1 and BD1). Station OB1 was typical and showed a wide range of high oxygen saturation values between a "low" of 60% (reached only once during the entire 4 year period), with many "supersaturation" values above a healthy 100%. In general there was good agreement between deep (100-200 cm) and shallow (15 cm) oxygen values at OB1, with the deeper sites most often having the slightly lower oxygen saturations. Station BD1 did not show some of the wide swings in oxygen concentrations seen in other poorly flushed sites, with most saturation values between 66% and 86% that year. In some years, oxygen was slightly lower at BD1 than ER1. Typically oxygen saturations at the Onset Bay stations were high compared to most Buzzards Bay embayments. Oxygen data was not available for the outer embayment in 1992, and saturation data from the inner embayment was used. Consequently, the Eutrophication Index score was underestimated in 1992 for the outer embayment (that is, water quality was better that year than shown).Scores for other monitoring parameters was also good, resulting in eutrophication Index scores generally in the "Good to Excellent" range. The drop off in the inner embayment in 1994 and 1995 was partly the result of declines in oxygen saturation, together with moderate increases in phytoplankton and organic nitrogen in the water, which were at very low levels in 1992 and 1993. Total nitrogen concentrations were lowest in 1992 (0.39 ppm), but were elevated considerably in 1994 to 0.49 ppm.
Together these findings suggest that while Onset Bay has good water quality, it may be in
transition to only fair water quality. Unfortunately nutrients were not monitored in the East
River-Broad and Muddy Cove complex, but water quality appears more impaired there.
Eutrophication Index scores, Inner Harbor
Nitrogen Management needs
Nearly 50% of nitrogen inputs to this embayment are derived from residential land use and more than 30% is derived from commercial land use. In 1995, the Buzzards Bay NEP estimated that existing nitrogen loading was only 60% of the Project's recommended limit and future loading 96% of these limits. However, given the uncertainties of this preliminary assessment, the Buzzards Bay NEP recommended more detailed studies of flushing study and land use. To this end, the Town of Wareham gathered last summer the land parcel data for a detailed buildout analysis is underway by the Buzzards Bay NEP. The Buzzards Bay NEP will also fund a flushing study of Onset Bay, including the East River complex. The parcel level land use data will be especially important in characterizing the impacts of commercial development along the Route 6 corridor. The East River complex seems impacted most by nitrogen loading, and the subarea should be the focus of management action. In the future, nutrient data should be collected at station BD1 or ER1 to enable calculation of Eutrophication Index scores for that area.Back