Benthic organisms such as worms, insect larvae, and crustaceans live at the bottom of water bodies or burrow into the sediments. These organisms are an important link in the estuarine food web, consuming plant debris or plankton and in turn are food sources for fish. Assessing a benthic community's diversity, population densities and distribution is another way to measure the health of an estuarine ecosystem.
Sampling revealed a consistent pattern of moderate to significant impairment throughout the basins of Oyster Pond. Oyster Pond supports both fresh and estuarine invertebrate populations due to its brackish water, but supports only a few species (only 3 per sample overall, as compared to 30 species per sample found in nearby healthy systems) and only a few of those in significant numbers. The distribution of these animals was also low. Altogether, these are indicators of an impoverished community showing the signs of nutrient related stress and low dissolved oxygen levels.
Nitrogen enrichment of estuarive waters impacts oxygen levels by increasing the amount of phytoplankton and algae growing in the water. As the plant and alga matter decompose they promote low dissolved oxygen conditions in the water column. This can cause stratification of the water column,
These problems are magnified in a system such as Oyster Pond that already experiences stratification due to differences in salinity. The level of nitrogen enrichment inputs in Oyster Poond is relatively low compared to other systems, but due to its unique nature, this is enough to cause periodic oxygen depletion in waters below 4 meters in the summer.
In order to determine how much nitrogen is causing impairment of Oyster Pond's benthic community, a sentinel site was selected. The sentinel site is a representative location where if nutrient threshold concentrations in the water column are achieved habitat quality throughout the pond may occur.
Bottom water dissolved oxygen levels measured by Falmouth Pond Watch during July and August, determined from grab samples and Winkler Titrations (to 0.5 mg/L). Note that OP 1, 4 m (within the enclosed northern basin) and OP 3, 6 meters (within the deep southern basin) periodically go anoxic. The benchmark for the threshold analysis is OP-3 at 4 meters, this site had a D.O. >3 mg/L on 96% of sampling dates.
Source: MEP Oyster Pond Report, Jan. 2006
The OP-3 station, located in the southern basin and also a Pond Watchers and OPET monthly sampling site, was selected. Depths from 0-4 m were targeted for setting the nitrogen threshold levels, as this water depth is impaired the most by nutrient enrichment but if improved, offers the most potentially useable benthic habitat.
Results form the Linked Watershed Model show that to achieve the targeted dissolved oxygen concentrations at the sentinel location,
This translates into removing 82% of the septic load without any reductions in fertilizer or stormwater runoff to achieve a minimum dissolved oxygen level of 6 mg/l for waters 0 to 4 meters deep. It should be noted that below 5 mg/l, many aquatic organism are at stress and fish kills can occur at oxygen concentrations less than 2 mg/l.
Reducing septic inputs is just one method for reducing nitrogen inputs. Load reductions can be achieved in many different ways including reducing or removing any or all sources of nitrogen either by engineered removals or via natural attentuation within the freshwater portions of the pond system. The load reductions presented in the MEP report for Oyster Pond represent only one of a suite of options that need to be evaluated by the community.
OPET members are now armed with an understaning of how much nitrogen load should be removed from the Oyster Pond watershed system under one specific scenario to achieve ecological health. Now the next phase of questioning - how do we achieve it?
-by Wendi Buesseler