Report to the Friends of Sengekontacket 2005 Annual Meeting

W. Wilcox, Water Resource Planner,

Martha’s Vineyard Commission

2 September 2005

During the summer, 9 stations in Sengekontacket and 1 in Trapp’s pond were sampled four times (3 in Trapp’s).  David Grunden, Oak Bluffs Shellfish constable assisted with the data collection and provided a boat for the sampling.  Emma Green-Beach, UMass Dartmouth Senior, was project field assistant and processed samples for analyses for dissolved nutrients, chlorophyll a and particulate carbon and nitrogen before shipment to the UMass Coastal Systems Group lab where all analyses were run.  Blind duplicate samples were sent with each batch of samples allowing an evaluation of the accuracy of the results.  This data is not yet available for evaluation.  This discussion is based solely on field data collected with a YSI 85 multi-parameter meter, Secchi disk and field observations.

Sampling was carried out 29 June, 13 July, 27 July and 10 August.  In general water clarity as determined with a Secchi disk was good during the four rounds being in excess of 2 meters throughout the course of the study.  This indicates that light penetration to the bottom is probably adequate for growth of eelgrass over a considerable portion of the pond because it is so shallow.  Light penetration for growth of phytoplankton and large algae that have much lower light requirements is also adequate.

Dissolved oxygen saturation was over 60% in the deeper water (1 meter or more) throughout the survey.  The lowest values were found off the Boulevard near the Town landing, at the north end of the Pond off Brush Island and inner Major’s Cove where circulation is reduced and where nutrient loading from wastewater and other sources is higher.

The excessive growth of the large algae (also called drift or wrack algae) has been observed at the bottom in Major’s Cove, at the north end of the Pond and the embayments that are found on the landward side of the Pond south of the main inlet to the Pond.  These algae can impact water resources by drifting into eelgrass beds where they intercept the light or into deeper areas where they remove the oxygen from the water as they decay.  This year the extent of the drift algae was observed more closely than in previous years while diving at the locations of eelgrass seeding stations from 2004 and 2005.  At four of the locations, the bottom was covered by 1 to 2 feet of drift algae.  This was a widespread occurrence.  The cause may be related to the excessive spring rain adding nutrients to the Pond that these large algae are effective at absorbing.  On the other hand, it could be related to the lack of rainfall during July and August that would reduce the amount of phytoplankton in the water column whose absence might have allowed greater light penetration stimulating these algae.  Water quality analyses may help us to better understand this problem by indicating the amount of phytoplankton and nutrients in the water column.