Detection of eutrophication in aquatic ecosystems: Nitrogen stable isotopes in macrophytes and groundwater

Increased nitrogen loading to coastal systems, from atmospheric deposition, use of fertilizer, and disposal of wastewater, is changing aquatic environments world-wide. The resulting eutrophication can lead to blooms of phytoplankton and macroalgae, and to loss of important estuarine habitats and commercial shell- and fin-fish species. In this dissertation I assessed the use of N stable isotopic signatures (expressed as δ15N in ‰) to identify wastewater N in groundwater, phytoplankton, and macrophytes in freshwater ponds and estuaries. Due to processing in septic systems, wastewater has a heavier δ15N signature than other N sources, therefore N stable isotopes have the potential to be a powerful tool to trace anthropogenic N from watersheds into aquatic food webs.; I assessed how spatial heterogeneity of nitrogen concentrations and δ 15N in groundwater entering freshwater ponds and estuaries was affected by watershed land use. Nutrient concentrations varied greatly in groundwater samples taken within 200 meters of each other. Land use within each watershed affected NO₃ concentrations and δ15N in the groundwater. NO₃ load and δ15N values increased with residential development of watersheds, due to increased wastewater delivery. NO₃ load was inversely related to natural vegetation cover.; In Cape Cod, δ15N values of macrophytes in freshwater ponds and estuaries, related significantly to wastewater as a percent of the total land-derived N load. Particulate organic matter (POM) δ 15N was also significantly related to % wastewater. The lack of a temporal pattern in macrophyte δ15N, the relative ease of collection, and the relatively low cost of analysis make this method useful for both researchers and managers in freshwater and estuarine systems.; I expanded, across a wide geographic range, the assessment of the use of δ15N in macrophytes and POM to track N inputs. I found that δ15N of macrophytes and POM increased with N load, water column DIN, and wastewater as a percent of total N load. Macrophyte δ 15N was used to predict the percentage of wastewater inputs to aquatic systems where δ15N values of macrophytes were known, but no quantitative information on N inputs existed. The wastewater predictions fit well with qualitative information known about each site.