The use of stable isotopes to define the extent of incorporation of sewage nitrogen into aquatic foodwebs and to discern differences in habitat suitability within a single estuary

This dissertation uses stable isotopes of nitrogen to determine if sewage nitrogen is incorporated into upper levels of the Delaware River Estuary foodweb and to define the limits of the assimilation of sewage nitrogen into the foodweb of Massachusetts Bay. Processes and other factors that affect the δ 15N of treated sewage effluent have also been identified.; Effluent samples from seven sewage treatment plants located in the Delaware Valley and Boston, MA were obtained in 1994 and 1995 and the δ 15N of ammonium, nitrate, and particulate nitrogen (PN) were determined. It was found that secondary treatment produces isotopically heavier ammonium than primary treatment, probably due to fractionation associated with nitrification and volatilization of ammonia.; In Massachusetts Bay a spatial pattern of sewage ammonium incorporation into the ecosystem's foodweb was identified through the examination of PN δ 15N values. Mean values increased from approximately 1 to 5‰, between 0 to 20 km from the Deer Island sewage outfall, and then were almost level thereafter. This pattern of δ 15N values is attributed to fractionation during the assimilation of sewage ammonium by bay phytoplankton.; To determine if sewage nitrogen is incorporated into upper levels of the Delaware River Estuary foodweb particulate nitrogen (PN) and young of year (YOY) striped bass were sampled in 1993 and 1994. Elevated δ 15N values (means of 20.8‰ for fish; 16.1‰ for PN; and 18.2‰ for sewage ammonium) were found in the central region of the estuary, which receives considerably more sewage than regions to the north and south. It is likely that the high PN and fish δ 15 N values in the central estuary are partly attributable to the incorporation of isotopically heavy sewage nitrogen into several levels of the estuary's foodweb.; Although the dissertation focuses on nitrogen, the use of stable carbon isotopes led to the conclusion that the oligohaline southern region of the estuary provided a qualitatively different habitat for YOY striped bass than the freshwater central and northern regions. However, there was insufficient evidence to conclude that the oligohaline region was a superior habitat for the fish than the other regions.