Quantifying the importance of marine-derived nutrients atlantic coast streams using stable isotopes : prospects and challenges.

Ecologists have long recognized the importance of essential elements, particularly nitrogen, delivered by anadromous fishes to nutrient poor streams of the Pacific coast. On the Atlantic coast, this process is less widespread but still potentially important. Due to distinct gradients in stable isotope signatures between freshwater and the sea, stable isotope analysis (SIA) offers a mechanism of identifying areas that receive large influxes of marine nutrients in the form of excreta, eggs, carcasses, and young-of-the-year fishes, and the possibility of quantifying such fluxes. However, there are a variety of limitations in using this approach that must be considered. The purpose of this talk will be to describe some of these limitations using original data from three river systems on both sides of the Atlantic Ocean. We present interactions between anadromous fishes (Atlantic salmon, Salmo salar and blueback herring, Alosa aestivalis) and freshwater consumer groups (the sculpins Cottus cognatus and C. gobio, and several benthic invertebrate species). Interpretation of isotope patterns can be confounded by variable baseline signatures, reproductive status of consumers, and spawner densities. The successful use of SIA in determining the importance of marine-nutrients to Atlantic coast streams requires awareness of natural and man-made patterns of isotope ratios, as well as life history characteristics of stream-dwelling organisms