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Home  >   Technical Data  >   Citizens Monitoring Program  >   1996 Water Quality Report

1996 Citizens Report: Aucoot Cove, Marion

Embayment: Aucoot Cove

Embayment and Watershed Characteristic

Aucoot Cove has important shellfish resources, eelgrass habit, and other important wildlife habitat, and for this reason, the Buzzards Bay NEP recommended the adoption of an outstanding resource waters water quality designation. The Cove is deep and well flushed, and consequently the estuary has the ability to assimilate a large load of nitrogen. The Cove has a relatively small watershed, and the principal source of nitrogen is the Marion Sewage Treatment Facility (more than 60% of the watershed load) which discharges to the Cove via a small creek ("Effluent Creek") at the head of the embayment.

It is worth noting that portions of the upper Cove is also closed to shellfishing. Most of this closure is due to mandatory closure around the Effluent Creek as a precautionary measure, but also because high fecal coliforms are sometimes observed there. Some of the closures on the Mattapoisett side of the Cove may also be the result of contributions from failed septic systems nearshore.

Water Quality monitoring

Aucoot Cove, especially the central and outer portions of the Cove, exhibit very good water quality, a reflection of the fact that the total nitrogen load is small compared to the volume and flushing of the bay [note outer Hiller Cove was included as part of outer Aucoot Cove in this study]. Indicators of water quality drop off markedly however at the mouth of Effluent Creek (AC1) and at a small cove with a boatyard on the Mattapoisett side (AC2). In most years, AC1 had distinctly lower oxygen saturation (summer time lows hovering around 40% oxygen saturation) than AC2, which had summertime low oxygen saturation values around 55%. AC2, the boatyard, showed a slight decline in oxygen during the study period. Declines in water quality was also apparent for dissolved inorganic nitrogen, total nitrogen result in a dropoff in the eutrophication index in 1994 and 1995 for both inner and outer sites. Station AC8, Aucoot Creek (not shown), was sampled only in 1994 and 1995, so it is impossible to say whether or not that declines in 1994 and 1995 were related to increased nitrogen loadings on this stream. In both years, total nitrogen was consistently at 0.75 ppm.

Total nitrogen concentrations in inner Aucoot Cove were reasonably good for 1992 and 1993--less than 0.35 ppm, but increased substantially in 1994 and 1995 (above 0.5 and 0.4 ppm respectively). The increase in both those years reflected largely increased dissolved inorganic nitrogen, but dissolved inorganic nitrogen and particulate nitrogen also showed similar percentage increases. In 1994, dissolved inorganic nitrogen concentrations in the outer harbor were nearly triple the previous years.

The mean of the lowest third of oxygen saturation values in Hiller Cove were fairly consistent at 69%, 60%, 59%, and 66% for the four years of study. These values are reasonably good, and other parameters of water quality such as total nitrogen and chlorophyll also indicate good water quality. The Eutrophication Index for inner Hillers Cove could be calculated only for 1993 and 1994, but the values--63 points and 74 points respectively--are similar to outer Aucoot Cove scores. Total nitrogen values, for 1993, 1994, and 1995 in Hiller Cove were 0.33, 0.36, and 0.38 ppm, very low values and similar to central "offshore" values for Buzzards Bay.

Eutrophication Index scores

Nitrogen Management needs

Although Aucoot Cove is smaller than most Buzzards Bay embayments, it is also among the deepest and best flushed embayments in Buzzards Bay. Consequently, it has a far greater recommended nitrogen loading limit compared to other embayments of similar area. Existing nitrogen loading is only a fourth of the Buzzards Bay NEP's recommended limits. A considerable amount of this drainage basin consists of forested wetland which will limit watershed buildout. Even if full buildout occurs, potential future loadings to the Cove are also expected to below the recommended limits unless there is any sizable expansion of the sewage treatment facility. Hence management of nitrogen inputs on that basis may not be warranted. However, nitrogen concentrations are elevated in the creek which appears degraded, and management should focus on improving the quality of the sewage treatment facility discharge to the creek. The town of Marion has planned improvements to the sewage treatment facility such as aeration of its sewage ponds and construction of an additional lagoon. Both these improvements will help reduce nitrogen loadings of the existing discharge volumes.

In 1990, large masses of algae accumulated on the bottom of the covelet, and large sheets of Ulva (sea lettuce) were observed in the Effluent Creek. Dissolved oxygen percent saturations and nitrogen and chlorophyll concentrations are elevated at both sites. For related reasons, fecal coliform levels in both of these areas are often elevated. Thus, because these two confined areas are the initial receptacles for nitrogen discharges to the Cove, they appear to be impacted even though outer portions of the Cove have good water quality and is well below recommended nitrogen loading limits.

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