Page Four of The Watershed, Vol. 5, No. 2, Winter 2000
More Serious Pond Samplings
Salinity and Oxygen
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Of course, OPET monitored salinity and dissolved oxygen during the past year. Due to the still silted-in Trunk River, the pond level has been 6 inches or more too high, and hence there was basically no saltwater inflow into Oyster Pond. Salinity is down to 0.9 to 1 parts per thousand (ppt) throughout the pond and down to 6 m depth. The desired level is
2 - 4 ppt. The low salinity and consequent lack of stratification allowed good oxygenation of the pond for most of the year down to 4 m, even 5m after windy days. Shall we bore you with a graph? Indeed, we will:
Measurements were taken in the mornings between 9 am and noon with a Yellow Springs Instruments Model 85 temperature-conductivity (salinity)-dissolved oxygen probe. The instrument was calibrated before each sampling set, that is for all data on a given day, to 100% dissolved oxygen at sea level and ambient temperature, according to manufacturers instructions. Readings are corrected for temperature.
The stippled line represents the lower limit (2 ppt) of the salinity range of 24 ppt desired by our pond management plan. The line also shows the upper limit of dissolved oxygen levels observed in Oyster Pond at 3 m depth in 1994. Out of 57 measurements during that summer, only 5 had readings at or above 6 mg/I (source: A coastal Pond by K O Emery, Publ. OPET, 1997; p.94). In contrast, since 1998, only 6 readings were below that level and that at greater depth to boot (4m). The better oxygenation of Oyster Pond is due to the pond's reduced salinity (between 1 and 1.5 ppt in above graph, compared to 8-12 in 1994 (source: ibid., p.95)).
Fecal Coliform Bacteria
The Safe Swimming limit is 200 colony counts per 100 ml pond water. We did not find counts of that magnitude in the pond last summer. For shell fishing, the limit is 15 counts per 100 ml. Well, no shell fishing in Oyster Pond anyway. But, just to make you feel good, you should know that the pond did not exceed the shell fishing limit, except after rain storms. How come, since in previous years we quite often had counts in the range of 40-50 and varying widely from location to location? Well, now we sample from a boat and no longer from shore or docks, where ducks, geese and swans may hang out and leave their calling cards. And we found the counts in Oyster Pond water were less than 15 per 100 ml, except for days after rain. Then the highest counts were 185 per 100 ml. The southern basin didn't reach 50 counts even then! It's well known that storm runoff from roads is a major source of such contamination, and is a headache for the shell fish industry that faces state-mandated closings of their shell fish beds when counts are above the 15 per lOOml limit. For this reason, moneys have been available for building storm runoff catchment basins that settle out the bacteria before the runoff reaches pond, stream or bay. The northern portion of Oyster Pond has higher counts than the southern basin, and OPET aims to study whether and where storm runoff impacts the pond (Ransom Rd and Oyster Pond Rd runoff are prime candidates). If so, OPET may consider applying for funds to build such catchment basins. Any students out there who would like to do a science project on storm runoff next summer? OPET will pay for cost of materials and analyses!
WANTED!
The Watershed is looking for a volunteer editor and for any stories and observations, past or present, you might have about Oyster Pond; for photos of the pond and its surroundings; for your comments and opinion about OPETs programs and activities - in short, for anything that might be of interest to our readership.
Please contact us by e-mail at brose@cape.com or by mail at OPET, P.O. Box 496, Woods Hole, MA 02543-0496
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This page updated July 25, 2002