Nitrogen isotopes in Zostera marina: tracking anthropogenic nitrogen in Casco Bay, Maine.

An influx of nutrients into any aquatic system can disturb its ecological balance and cause eutrophication. This is a process that involves a rapid increase in the population of certain primary producers which causes a reduction in available oxygen to be used by other organisms. Previous studies have used stable nitrogen isotope ratios (δ 15N) in eelgrass, Zostera marina, to better monitor the distribution of anthropogenic nutrients in Casco Bay, Maine. This paper investigates the δ 15N of eelgrass growing at three different sites within Casco Bay (East End Beach, Clapboard Island, Chebeague Island) each at increasing distance from the East End wastewater treatment facility in Portland, Maine, an area of high human activity. There is no statistically significant difference among δ 15N values of eelgrass from all three sites and the average δ 15N values is 6.38 ± 0.40. These values are relatively low and suggest minimal nitrogen input from the East End wastewater treatment plant. Furthermore, there appears to have been a ~1.8‰ decrease in δ 15N values of eelgrass in the south Casco Bay area since 2011 perhaps reflecting that the East End wastewater treatment plant reduced its nitrogen load by ~60-70% in 2017 (A. Brewer; Levin et al., 2019). There is a positive correlation between DIN and chlorophyll concentrations in water from surrounding rivers. Similar research could be used to easily identify areas receiving anthropogenic nitrogen and thus implement changes to prevent the damaging effects of eutrophication and nutrient loading