US Long-Term Ecological Research Network
Stable isotope diagrams of freshwater food webs
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Stable carbon and nitrogen isotopes are valuable tracers in ecological research (Runde! et al. 1988). One use of isotope measurements is to rapidly survey how organic matter is cycled in different ecosystems. For example, a few measurements of isotopic compositions of dissolved nutrients, aquatic plants, and animals establish a chemical outline of food web structure (Fry and Sherr 1984, Minagawa and Wada 1984). These tracer results can be used to check conventional ideas about trophic structure in well-studied systems, and to test how food web structure varies in other systems where conventional visual studies of trophic relations are difficult or lacking (Rau 1980). A dual-isotope approach is often useful in these studies, for example, with nitrogen isotope measurements functioning as trophic level indicators, and carbon isotope measurements indicating which plants are important sources of nutrition for consumers (Peterson and Fry 1987). Ecologists from Long Term Ecological Research (LTER) sites in the United States and Puerto Rico recently used isotope analysis to test ideas about aquatic food-web structure and terrestrial nitrogen cycling. This note reports an overview of the food-web results discussed at a workshop in September 1989, and includes a ll’5N survey of terrestrial materials that contribute to freshwater food webs as allochthonous organic matter. A list of isotopic analyses from each site is available.