Stacy and Caroline conducted short term experiments (5-10 days) in 2-liter bottles. The bottles were incubated either in the pond at about 20 inches depth, or in a growth chamber at pond temperatures and under lights which provided an average light intensity similar to that in the upper 3 feet of the pond. Such small-scale, short-term experiments should be interpreted with care because of limitations and constraints of the experimental design, but short-term bottle experiments have been an important tool that has been used by many investigators (including ourselves) to gain information of the relative importance of one nutrient versus another to primary producers in an aquatic system.

First we ran experiments where ambient water was enriched with either N or P. Ading a bit more of a nutrient and measuring the phytoplankton response indicates whether that nutrient is limiting growth under the pre-existing nutrient conditions. Both experiments showed increased phytoplankton growth after N addition, indicating that N is more limiting. this suggests that the best way to prevent the blooms of algae and noxious plants in Oyster Pond may be to reduce nitrogen inputs to the pond. In one experiment, N and P were also added together, resulting in a much greater increase in phytoplankton growth than that of N alone. This indicates that once N limitation is overcome, then P can quickly become limiting to further growth, and so effective management may require reductions of both nutrients.

Another set of experiments examined the importance of salinity. Pond water that contained phytoplankton was diluted with deionized water or with filtered seawater from Vineyard Sound to provide salinities of 0.2, 2.3 and 5.0 o/oo. In one of these experiments, both N and P were added to determine whether

In another experiment with varying salinities, the bottles were enriched only with P. The purpose of this experiment was to examine whether nitrogen-fixing cyanobacteria ("bluegreen algae") present in Oyster Pond would respond more or less strongly to different salinities. These organisms are able to take the gaseous form of N present in the atmosphere and dissolved in water and convert it into biologically available nitrogen (the process of nitrogen fixation), which has been shown to aggravate nutrient pollution problems in freshwater. We had predicted that the cyanobacteria would grow better and fix more nitrogen at the lowest salinity (0.2 o/oo). However, our preliminary analysis of these results shows that the greatest increase in cyanobacteria occurred at the ambient salinity (2.3 o/oo), suggesting that they are well adapted to the ambient Oyster Pond salinity.

Management Implications:

The experiments with nutrient enrichments of ambient water from Oyster Pond indicate that control nitrogen inputs to the Pond may be necessary to reduce phytoplankton growth and associated eutrophication problems. As is now generally recommended for estuaries, it is probably also prudent to consider and control phosphorus inputs, especially as these inputs might change over time.

Would altering the salinity of the Pond have an affect on algal production and europhication? The 2002 summer experiments were too short in duration and do not encompass enough of the potential responses of the ecosystem to provide a clear answer to this question. For example, changing the salinity is likely to alter the net exchange of nutrients with bottom sediments, which may have a profound effect on eutrophication.

Nonetheless, the experiment did show the presence of nitrogen-fixing cyanobacteria. These organisms can aggravate nutrient pollution problems by fixing nitrogen when it is

Opportunities and needs for further research on nutrient pollution in Oyster Pond:

These experiments provided some interesting preliminary information on how Oyster Pond might respond in the future to nutrient inputs from septic systems, road runoff, and lawn fertilizer and to changes in salinity. To study how salinity changes might interact with nutrient pollution in Oyster Pond requires experiments of longer duration and larger size than those conducted by the summer students. We believe such experiments could be performed in "limnocorrals," or larger plastic enclosures (usually between about 1.5 and 4 feet in diameter) within Oyster Pond during several weeks to months. A critical question would be the response of the nitrogen-fixing cyanobacteria to the salinity change. Provided that the residents are interested in seeing this type of research conducted in Oyster Pond, we are interested in seeking funding from the National Science Foundation or other granting agencies to carry out such studies.

by

Roxanne Marino, PH.D. (The Ecosystems Center, MBL, Woods Hole, MA 02543),

Robert Howarth, Ph.D. (David R. Atkinson Professor of Ecology and Environmental Biology, Cornell University, Ithaca, NY 14853),

Eric Davidson, Ph.D. (Woods Hole Research Center, Woods Hole, MA 02543)