Page three of The Watershed vol. 3, no. 2

POND STUDIES/POND SAMPLINGS

Oyster Pond has been the focus of several study groups the past year. Last December, OPET sponsored 2 science projects for Falmouth Academy students Dan Murphy, Misha Strumwasser and Andrew Davies. Under guidance from Dr. Linda Deegan from the MBL, 8th grader Dan Murphy and several OPET Board members learned how to seine a frigid Oyster Pond for fish in the shallows. Dan's project was to determine what kind of fish where to be found in December and whether different kinds preferred different pond locations, i.e. salinities. He found mummychogs, sticklebacks, silversides in all sampling locations, and a few small white perch at Mosquito Creek. Salinity turned out not to differ much in any of the locations.

Juniors Andrew Davies and Misha Strumwasser sampled the deep kettle hole of the southern basin. They found some dissolved oxygen at 6 m in all their samplings, and salinity was between 8 and 15 ppt there. They also found that ammonia and phosphates were high at 6 m depth, and that temperatures there were consistently higher by a few degrees than at the other depths. Andrew and Misha garnered an Honorable Mention for their project at the Falmouth Academy Science Fair.

Salinity and Oxygen - Pond Watchers Study

OPET members Jonathan Davis, John Dowling, Barry Norris, Julia Rankin, Birgit Rose and Marge Zinn took this summer's samples. Dr. Brian Howes' and Dale Goehringer's lab at CMAST, U Mass Dartmouth, analyzed the samples. At a Pond Watchers party, these two scientists honored those Pond Watchers who have been on active duty for at least ten years, with a certificate. 4 OPET members garnered this honor: John Dowling, Barry Norris, Julia Rankin and Marge Zinn. Other prizes were given, too - it was a fun party. Thanks, Brian and Dale!
Here are some of the results: There has been a steady decline in salinity of Oyster Pond from the high salinity levels of 1988-1991, which were 12 - 18 ppt between surface and 4m depth. In 1997 salinity was around 2-2.5 ppt (parts per thousand) all the way down to 4 m - as it was in the 1960s. Even the very bottom layer of water in the deepest kettle hole has lost half its salt content: at 6 m it has dropped from about 22 ppt in 1988-1991 to about 15 ppt in 1997. And at 5.5m it was only about 4.5 ppt compared to 20 or more in the earlier years. The layer of high-salinity bottom water now is only 0.5 m deep compared to the previous 2.5 m and, in addition, its salt content is significantly reduced, making it less heavy. This means there is a better chance that the whole water column gets churned up by storms and become mixed with the top layer. In fact, SEA students sampled the deep kettle hole in early October 1998 after a stretch of very windy days and found a salinity of 1.4 ppt from top to the very bottom: the whole pond had gotten turned over, probably for the first time since hurricane Bob in 1991.

Oxygen now penetrates to deeper waters. It was found this summer at 4 m even in -the northern kettle hole, where in previous years no trace of it had been detectable. The oxygen reaching the deeper layers is bad news for the anaerobic, hydrogen sulfide producing bacteria that can live only in absence of oxygen. This explains the lack of foul smells that used to plague especially the southern end of Oyster Pond during summer months and whenever severe storms blew in. This should be appreciated by the residents at the south end of Oyster Pond.

Trunk River Lagoon, too, has been less smelly. This may be due to less eel grass making its way into the Lagoon during storm tides, presumably because both the silting in of Trunk River and the weir reduce the velocity of tidal inflow up Trunk River. The eel grass sinks to the bottom of the Lagoon where it decays, using up the oxygen. Too much eel grass results in a stagnant, smelly Lagoon.

Nitrates remain at high levels; but at low salinity, phosphates become growth limiting for algae. Sea water has high phosphate content, and sure enough, the first tidal inflow over the weir in September brought a small algae bloom to the part of Oyster Pond just upstream of the weir.

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