This summer I interned at the Marine Biological Laboratories, Woods Hole, Massachusetts, in a research experience for undergraduates (REU) position. I was working on the NISOTREX (nitrogen stable isotope tracer experiment), a project that was looking at the flow of nitrogen through the Rowley River estuary. The field site where this experiment was conducted was located in Rowley, Massachusetts on the north shore of Boston. The Rowley River is one of the major rivers in the Plum Island Sound estuary.

An earlier stable isotope tracer study was conducted in the Parker River, the other main river in the Plum Island Sound estuary, in 1996. The purpose of conducting the same tracer experiment on the Rowley River was to see the impact of bivalves on the system, their connection to the rest of the food chain, and how this might differ from rivers without large populations of bivalves. The Parker River had little to no bivalves present. In other words this project was a food web study that was used to determine all the trophic levels.

Nitrogen-15 was the stable isotope used for this study. The 15N was incorporated into the estuary using a drip that dripped potassium nitrate into the estuary. This inorganic form of nitrogen was taken up as food by the primary producers, which then incorporated the nitrogen into their bodies. These primary producers, phytoplankton, were then eaten by consumers, and the primary consumers were then eaten by other consumers on a higher trophic level. The 15N is incorporated into each of these consumers when they eat an organism that has previously incorporated the 15N. In order to determine the organisms that were getting labeled by this tracer, samples of the organisms were taken, preserved, and then later analyzed for the presence of 15N using a mass spectrophotometer. Samples of nutrients, benthic microalgae, macroalgae, phytoplankton and zooplankton, bivalves and other benthic species, and fish were taken for analysis. Nutrients, phytoplankton and zooplankton were collected at high tide, and all the other samples were collected at low tide. Samples were taken repeatedly throughout the summer in order to determine the rate of labeling and the nitrogen turnover time of the organisms.

This study determined that the benthic food web of the Rowley River system is a lot more important to the net primary production of the estuary than the pelagic food web as was the case in the Parker River. The benthic microalgae were heavily labeled within a short amount of time, indicating that the 15N was taken up quickly by these organisms. The pelagic food web did not show such a dramatic uptake of 15N, indicating its decreased importance to the primary production of the Rowley River.

I was also doing another small independent project that looked at the food choices of two fish species in the river and differences in their choices as the fish moved up river. The two species that I was looking at were the Mummichog, Fundulus heteroclitus, a benthic species and the Atlantic silverside, Menidia menidia, a pelagic species. Literature shows that these two species feed only within their respective habitats, thus silversides only eat pelagic food items and mummichogs eat only benthic species. Nitrogen and carbon stable isoptope data from the previous summer showed a convergence in the stable isotope numbers of these two species as they moved up river, indicating that there could be an overlap in the food choice of these fish up river. As a result of these past data, I hypothesized that up river there is a convergence in the food item choice of these two fish, and that they are not solely benthic or pelagic fish.

To test this hypothesis, I collected fish from stations along the river that were located at approximately 1-kilometer increments. Gut content analysis was then performed in order to determine what the fish were eating. I found that as the fish move up river the Atlantic silversides do in fact include benthic food items in their diet, but mummichogs eat mostly benthic species with few occurrences of pelagic feeding. This finding indicates that Atlantic silversides are not obligate pelagic feeders and they take advantage of abundant food supplies, suggesting that there is an increased importance of the benthic food web for the silversides.

This internship experience was valuable for learning the demanding lifestyle that field research requires, and the kind of time and organization that is also required in order to carry out the study in an effective manner. I also learned a lot about estuarine ecology and the research methods that are utilized for this type of research. Even though I learned a lot about estuarine ecology and coastal marine ecology, I have decided that I do not want to pursue the estuarine ecology field and research of this type. After this experience I have realized that if I do go into research it will be in coral reef ecology with a strong focus on conservation and education. I believe the only way we will be able to conserve natural areas is by educating the public about the importance of these natural areas to the environment as a whole, and the responsibility of the public for protecting them.