Forty percent of U.S. estuaries are characterized as severely eutrophic; as illustrated by hypoxic waters, fish kills, harmful algal blooms, loss of sea grass beds and decreased biodiversity. Increased nitrogen loading is at the source of these problems. According to the Long Island Sound Study, more than 60% of nitrogen entering the Sound comes from sewage treatment plants; while non-point sources, including atmospheric deposition, fertilizers, animal waste, and soil microbial nitrification contribute the remaining 40%. In order to effectively manage this problem it is necessary to know the relative importance of various non-point sources.
My doctoral research looks to apply isotopic signatures to the various land covers within a watershed. Recent studies have shown that sources of NO3 can be differentiated based on their isotopic signature using both ä150 and ä180. Currently, the range of isotopic values in the literature for nitrate sources is too large to clearly differentiate sources. In order for this method to be successful, local source values are needed.
I will use a stratified sampling plan to assess the feasibility of using this stable isotope approach to determine the relative importance of various nitrogen inputs in a limited geographic region. I will examine how these isotope signatures vary spatially across different watersheds and how much sampling (spatially and temporally) is needed to accurately assess the delivery of nitrogen to a given estuary. Small watersheds throughout the Long Island Sound catchment will be sampled on a variety of scales.
At the smallest scale, first and second order forested, agricultural, and urban catchments will be sampled to identify source signatures of these landscapes. Tributaries, mostly third order streams, will be sampled to gain additional information about the relationship between land cover and isotopic signatures. At the largest scale, longitudinal sampling will be done along the main stems to assess the relative contributions of different tributaries and to provide further information regarding nitrogen cycling within these systems. Various chemical indicators of nitrogen loading will also be measured to assess their use in conjunction with the stable isotopes. This suite of measurements will help determine the best strategy (in terms of both time and cost) for detecting watershed nitrogen loading to estuaries.