1996 Citizens Report: Mattapoisett Harbor
Embayment: Mattapoisett Harbor
Embayment and Watershed Characteristic
The Mattapoisett Harbor drainage basin is the fifth largest in Buzzards Bay and is among the least developed. It is in the top third of all basins in terms of agricultural use. In this watershed corn and other crops rather than cranberry bog account for most of the agricultural activity. The Harbor is among the larger, deeper and better flushed embayments in Buzzards Bay. There are more than 650 moorings and slips in the harbor, seven town beach areas and eight private beaches. Eelgrass is abundant along the periphery of the estuary, but because the center of the Bay is deep, eelgrass cover is less than 30% of the entire bay area.Despite the size of the watershed, total loading is low compared to other watersheds of similar
size. This, coupled with a large bay volume and rapid flushing time, place the Harbor at only
17% of the critical nitrogen loading limit for the estuary, and among the least loaded systems
studied.
Water Quality monitoring
Water quality monitoring in Mattapoisett Harbor supports the conclusions of the Buzzards Bay Project's subwatershed evaluation. Eutrophication Index scores in the outer harbor were between 74 and 92 points, and scores in the inner Harbor were between 60 and 74 points, the best scores for any of the large river subwatersheds in Buzzards Bay.In calculating Eutrophication Index scores, it is worth noting that values for the "inner" harbor were based on oxygen collected at station MH3, the River mouth. Data for station MH3 was unavailable for 1994, hence data from station MH4 (town pier) was substituted. Similarly calculations for the "outer" embayment were based on oxygen data from station MH4 in 1994 and 1995. This station was not monitored in 1992 or 1993, hence we used the data from MH3 in those. The oxygen data of course represents 20% of the Eutrophication score. These substitutions mean that water quality in 1993 may have been slightly worse for the inner bay than shown on the centerpiece water quality map. Similarly we may have underestimated water quality for 1992 and 1993 in the "outer" bay eutrophication Index scores, relative to 1994 and 1995. Because oxygen station MH4 is not really an outer embayment station, water quality is slightly better in the outer harbor than shown.
The poorer water quality in the inner Harbor reflect somewhat higher chlorophyll concentrations, higher organic nitrogen, and poorer water transparency than the outer embayment. Water quality in both areas showed a small but steady decline in the inner harbor during the 4 years of study, with Eutrophication Index scores dropping to only "fair" conditions in 1994 and 1995. The worse water quality was observed near the mouth of the Mattapoisett River, at station MH3. Oxygen was measured in 1992, 1993, and 1995 (too few samples were taken in 1994 to make interpretation of that year meaningful). In those years, mean oxygen saturation values were 71%, 64%, and 33% respectively for the mean of the lowest third of observations. Generally this station has oxygen in the range of 60% to 90% saturation, but in 1995 a third of the observations were below 50% saturation. Other parameters like chlorophyll and organic nitrogen also showed increases, which also helped drop the Eutrophication Index.
Eutrophication Index scores using nutrient data near the river (not shown in the center panel
map) showed an even greater decline during the study period. In 1992, the Eutrophication Index
score for this site was 74, in 1994 it was 52, and in 1995 it was only 34, the big drop being
largely the result of the very low oxygen saturation values observed at this site in 1995. Inorganic
nitrogen concentrations appear to have increased in the river in 1994, but declined again in 1995.
What is causing the decline in water quality in Mattapoisett Harbor is not clear, but because the
trend for declining water quality is most pronounced near the mouth of the River, it is reasonable
to conclude that there has been increased pollutant loading to the River. More frequent
monitoring of nitrogen along the River should be a future priority so that nitrogen sources and
transport in the watershed is better understood.
Nitrogen Management needs
Residential and commercial land and roads account for 60% of the inputs to the Bay whereas cropland accounts for 28% of the Bay's discharges. Most of the nitrogen inputs to Mattapoisett Harbor are focused in two areas: the cove at the mouth of the Mattapoisett River and Eel Pond. Both these areas are closed to shellfishing because of fecal coliform inputs and both these ecosystems may also be degraded due to nitrogen inputs. Because of the considerable volume and degree of flushing in Mattapoisett Harbor, this embayment has one of the highest loading limits in Buzzards Bay. Using a SA water quality goal, the embayment does not now and will not in the future exceed recommended loading limits and for this reason, the nitrogen management for the Harbor as a whole was not recommended as a high priority. As noted on the following pages, however, those portions of the watershed draining into Eel Pond do require management action to restore water quality.Back