1996 Citizens Report: Buttermilk and Little Buttermilk Bays, Bourne
Embayments: Buttermilk and Little Buttermilk Bays
Embayment and Watershed Characteristic
The drainage basin surrounding Buttermilk and Little Buttermilk Bays is the eighth largest within the Buzzards Bay watershed and encompasses portions of the towns of Wareham, Bourne, and Plymouth. Several creeks and streams discharge to this embayment, but the bulk of fresh water inputs (and nitrogen) enters the Bay via groundwater. Most development in the watershed (largely residential land use) is clustered along shore, but there is considerable dense development in Plymouth in the Big Sandy Pond area. The central watershed is largely undeveloped, and each town has public wells or their recharge areas within the basin. Cranberry Bogs make up most of the agricultural land, which covers about 9% of the watershed.
Because of the shallowness of Buttermilk Bay eelgrass beds covered nearly 40% of the embayment during the 1980s. Anecdotal information suggests there have been recent declines in eelgrass cover. Total salt marsh cover ranked low among Buzzards Bay embayments, and due to the intensive shoreline development along portions of this estuary's shores, several sizeable fringing salt marshes (most notably along Indian Heights) were filled in 30-40 years ago. Several modest size marshes remain along Red Brook, Goat Creek, and within several other covelets.
Little Buttermilk Bay is an inner embayment to Buttermilk Bay. Little Buttermilk Bay was likely formed as a separate freshwater kettle pond adjacent to Buttermilk Bay, which was joined through a narrow constriction as sea-levels rose to flood these systems. Little Buttermilk Bay, although less flushed, also receives a lower nitrogen load than Buttermilk Bay where most development is concentrated, especially in the Indian Mound and Hideaway village areas. These areas, however, were sewered in the early 1990s. However, since the effective nitrogen load to an embayment is the combination of inputs and outputs the restricted inlet and the adjacent nutrient load to Buttermilk Bay indicated the need for separate analysis of Little Buttermilk Bay.
Water Quality monitoring
Due to a lack of volunteers, oxygen concentrations were monitored in big Buttermilk Bay for a full season only in 1992. Little Buttermilk, was monitored for oxygen in all years except 1995. Nutrient data was collected in each bay for 1993 through 1995. Because of the lack of oxygen data for big Buttermilk, Little Buttermilk oxygen data was used to calculated a Eutrophication Index score for that estuary. Because no full summers worth of oxygen was available for either embayment in 1995, a Eutrophication Index score could not be calculated for that year. Because overall Little Buttermilk Bay shows worse water quality than big Buttermilk, the use of Little Buttermilk oxygen saturations for big Buttermilk Eutrophication Index scores represent a "worst case" scenario (the oxygen score represents 25% of the Eutrophication Score).With these limitation in mind, the somewhat higher Buttermilk Bay Eutrophication Index scores 67 for 1993 and 39 for 1994 versus 61 and 38 points respectively for Little Buttermilk Bay) was also corroborated by consistently higher total nitrogen concentrations in Little Buttermilk (0.44, 0.61, and 0.69 ppm for 1993 to 1995) compared to Buttermilk (0.41 ppm, 0.55 ppm, 0.43 ppm respectively). The increasing total nitrogen concentrations in Little Buttermilk were among the more dramatic observed in the monitoring program.
While Little Buttermilk Bay has lower nitrogen loading from its watershed than does Buttermilk Bay, it receives a higher proportional loading in inflowing tidal waters. This higher tidal loading stems from the source waters of Buttermilk Bay being the low nitrogen waters of Buzzards Bay, while Little Buttermilk Bay receives nutrient laden tidal waters from Buttermilk Bay. As a result the nutrient related water quality of Little Buttermilk Bay tends to be poorer than Buttermilk Bay. This inner bay not only has moderately high nitrogen levels but with phytoplankton pigment levels averaging 9.8 for the three year study period (versus 8.6 for Buttermilk Bay) appears to be at the onset of eutrophic conditions. Despite these somewhat pessimistic observations, oxygen levels remain relatively high (>60%) which may be the result of tidal mixing inhibiting stratification within this basin, or high daytime levels of oxygen production by algae. However, oxygen levels are highly variable often ranging 60% and 115% saturation during the summer, which suggests that the system the consumption of oxygen from organic matter decomposition is often greater than oxygen production by photosynthesis or diffusion from the atmosphere. However, as long as oxygen levels remain above 60%, habitat quality for some benthic animals (like shellfish) will be good. Unfortunately, present oxygen levels and possibly decreased light penetration due to algal blooms will likely inhibit eelgrass beds and associated scallop production.
Eutrophication Index scores
Nitrogen Management needs
Buttermilk Bay is the only embayment in Buzzards Bay where the Buzzards Bay Project's nitrogen loading limits have been adopted and where detailed mass loading evaluations have been performed. As a result of Project's efforts, the towns of Wareham, Plymouth, and Bourne "reprogrammed" future growth through changes in zoning to reduce the number of homes in the watershed so that recommended nitrogen limits would not be exceeded. The nitrogen loading targets were also met in part because the towns of Bourne and Wareham sewered several near shore areas, although this was done to minimize health threats associated with failing septic systems, not meet nitrogen goals. The sewering of the nearshore watershed in Buttermilk Bay should result in water quality improvements over the next few years as the septic system groundwater plumes run their course to the bay. While both Buttermilk and Big Buttermilk Bay showed a big decline in the Eutrophication Index between 1993 and 1994, big Buttermilk has shown too much variability from year to year to discern a clear trend. For example, although there is no Eutrophication Index score for 1995, most water quality parameters showed a moderate improvement that year in Buttermilk. On the other hand, Little Buttermilk continued a decline in 1995. Until trends become more apparent, monitoring of water quality in Buttermilk Bay should continue to determine whether additional management action is required. Given the importance of Buttermilk Bay's water quality to Little Buttermilk Bay the nitrogen management of the larger bay may have important positive effects on the adjacent smaller system.Back