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1996 Citizens Report: Pocassett River, Bourne

Embayment: Pocasset River

Embayment and Watershed Characteristic

The original evaluation of land use in the Pocasset River drainage basin by the Buzzards Bay Project included errors in total loading rates due to the inclusion of land area actually contributing to the waters of Phinneys Harbor behind Tobys Island. Because of this and because these watershed areas were redelineated last year by the Cape Cod Commission using more up to date groundwater data, the Buzzards Bay NEP will need to complete a need loading evaluation. This basin and embayment is among the smaller studied. Pocasset River is well utilized as a mooring area. One public beach is available, and four public access points. In the Buzzards Bay NEP's watershed evaluation, shellfish resources were ranked as poor, but overall Cove resources were ranked medium. The Pocasset River estuary has significant areas of fringing salt marshes. Most of the adjacent upland is forested (81%) with residential development dominating the terrestrial nitrogen loading, although the housing density remains relatively low, 0.2 units per acres.

Water Quality monitoring

Pocasset River water quality, as indicated by the Eutrophication Index scores (center map) have shown a steady gradual decline to only fair conditions since 1992. Although chlorophyll levels remain low (a four year mean of only 3.7 ppb), nitrogen levels are intermediate (a four year mean of 0.43 ppm) compared to other embayments monitored. The major factor in determining the ecological health of this system is dissolved oxygen levels. Throughout the study, oxygen has periodically dropped below 70% of saturation. However, declines below 60% occurred in 3 of 4 years with 1995 recording frequent levels approaching 30% of saturation. The most recent data indicates a system of low habitat quality to benthic animals. At present the low chlorophyll yet low oxygen concentrations suggest that potential interactions with bordering wetlands (possibly organic matter imports) may be involved in the organic matter-oxygen dynamics. Accumulated algae and organic matter in sediments may also be accounting for these low oxygen levels. Ongoing work should focus upon determining the cause of the observed low oxygen conditions. However, it appears that like adjacent systems (see below), additional nutrient inputs to Pocasset River to the extent that they result in additional organic matter production are likely to result in even more extreme oxygen depletions.

Eutrophication Index scores

Nitrogen Management needs

The Buzzards Bay NEP completed an evaluation of nutrient loading to the Pocasset River in 1994 as part of the Buzzards Bay subwatershed evaluation. Two thirds of existing nitrogen inputs are derived from residential land use (i.e. principally septic systems). Existing nitrogen inputs are 46% of recommended limits, but at buildout will be 63% above recommended limits. The embayment's present water quality is close to the average of embayments monitored. Existing loads are much lower than the predicted assumed acceptable limits yet the water quality within the system is already moderately impacted. Consequently it appears that the estimates of acceptable nitrogen loading may be too high, possibly because of an overestimate of flushing or omissions in the assumed loading rate. Better flushing estimates are required to better determine recommended loading limits to Pocasset River before nitrogen management strategies can be considered. In response to this, 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 the Pocasset River. 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.

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