Preliminary Stable Isotope Tracer Study of Sewage Nitrogen Inputs to Massachusetts Bay

Objectives:The primary objective of this project is to use stable isotope measurements to characterize the movement of sewage nitrogen into the planktonic ecosystem of Massachusetts Bay. This proposed research is organized around a series of hypothesis drive objectives:1. To test whether the isotope composition of nitrogen in sewage is significantly different from the isotopic composition of marine nitrogen.2. To test whether the combination of different source signatures and isotopic fractionation during primary production creates a spatial gradient in the nitrogen isotopic composition of suspended particles in the vicinity of a sewage outfall.3. To test whether zooplankton near a sewage outfall have a distinct nitrogen isotopic signature reflecting the movement of sewage nitrogen through the food web.In addition to explicitly testing these hypotheses, this project will also allow an evaluation of the spatial extent of the influence of sewage nitrogen on planktonic primary production, and the subsequent spread of sewage nitrogen into the planktonic food web in Massachusetts Bay.Methodology:This project involves a combination of laboratory and field measurements designed to quantify the movement of sewage nitrogen into the planktonic biota of Massachusetts Bay. The major innovation in this study is to use the stable isotopes of nitrogen as a natural tracer of the movement of nitrogen through the ecosystem. In brief, the methods to be used are:1. Measurement of the isotopic composition (&15N) of sewage nitrogen and marine nitrogen.2. Samples of suspended particles, inorganic nitrogen (NH4+ and NO3-), and common zooplankton will be collected on monitoring cruises in Massachusetts Bay. The &15N of these samples will be measured and the spatial distribution of &15N values around the sources of the sewage nitrogen will be used to calculate the contribution of sewage nitrogen to suspended particles (primary producers) and zooplankton (heterotrophs) in different parts of the Bay. 3. In the context provided by ancillary measurements of hydrographic parameters, nutrient and plankton concentrations, and biological rate measurements, these isotopic data will be used to evaluate the spread of sewage nitrogen within the Massachusetts Bay ecosystem.Rationale:Sewage inputs to Boston Harbor have had a significant negative impact on an economically and historically important coastal environment. This project involves a preliminary set of field measurements designed to quantify the isotopic signature of sewage and marine nitrogen, and to begin to trace the movement of sewage nitrogen into the planktonic food web. The results of this study will be helpful to regulators and members of the general public interested in assessing the effects of sewage on Massachusetts Bay ecosystem as a whole.