STABLE ISOTOPES IN ZOSTERA MARINA AS A PROXY FOR ANTHROPOGENIC NUTRIENT LOADING IN CASCO BAY, MAINE

Common eelgrass, Zostera marina, is the most abundant and productive seagrass in near shore coastal environments of the Gulf of Maine (GoM). It provides crucial habitat for commercially important species and contributes to marine nutrient cycles and sedimentation processes. Nitrogen is the primary limiting nutrient in marine systems; thus any large increase in its concentrations can have a profound effect on marine environments, particularly through the process of eutrophication which can shade out eelgrass beds to their acute light sensitivity. Stable nitrogen isotope analysis of eelgrass, provides a method for tracking nutrient flow in and around eelgrass beds. The purpose of this study is to determine the extent to which stable isotopes in eelgrass can be used as a proxy for anthropogenic nitrogen loading in Casco Bay, an estuary in the GoM.

Samples of water, sediment and eelgrass, were collected in two different areas of Casco Bay in Fall, 2009. One area is located near Portland, ME at the mouth of the Presumpscot River where eelgrass biomass is relatively low and anthropogenic loading of nitrogen is expected to be relatively high. The other site is Maquoit Bay, 25 miles north of Portland, where eelgrass biomass is high and anthropogenic loading of nitrogen is expected to be low. The isotopic composition of nitrate in the water column, nitrate extracted from the sediments, and nitrogen in eelgrass and sediments are in the process of being analyzed. Additionally, the waters are being analyzed for total nitrogen concentrations. Preliminary results indicate that the d¹⁵N values of eelgrass from Portland are significantly enriched in ¹⁵N relative to eelgrass from Maquoit Bay. These trends may reflect higher nitrogen loading and/or higher levels of ¹⁵N enriched nitrogen available for uptake in Portland relative to Maquoit Bay.