Food resources of stream macroinvertebrates determined by natural-abundance stable C and N isotopes and a 15N tracer addition

Trophic relationships were examined using natural-abundance C-13 and N-15 analyses and a N-15-tracer addition experiment in Walker Branch a Ist-order forested stream in eastern Tennessee. In the N-15-tracer addition experiment we added (NH4)-N-15 to stream water over a 6-wk period in early spring, and measured N-15:N-14 ratios in different taxa and biomass compartments over distance and time. Samples collected from a station upstream from the N-15 addition provided data on natural-abundance C-13:C-12 and N-15:N-14 ratios. The natural abundance N-15 analysis proved to be of Limited value in identifying food resources of macroinvertebrates because N-15 values were not greatly different among food resources. In general, the natural-abundance stable isotope approach was most useful for determining whether epilithon or detritus were important food resources for organisms that may use both (e.g., the snail Elimia clavaeformis), and to provide corroborative evidence of food resources of tara for which the N-15 tracer results were not definitive. The N-15 tracer results showed that the mayflies Stenonema spp. and Baetis spp. assimilated primarily epilithon, although Baetis appeared to assimilate a portion of the epilithon (e.g., algal cells) with more rapid N turnover than the bulk pool sampled. Although Elimia did not reach isotopic equilibrium during the tracer experiment, application of a N-turnover model to the field data suggested that it assimilated a combination of epilithon and detritus. The amphipod Gammarus minus appeared to depend mostly on fine benthic organic matter (FBOM), and the coleopteran Anchytarsus bicolor on epixylon. The caddisfly Diplectrona modesta appeared to assimilate primarily a fast N-turnover portion of the FBOM pool, and Simuliidae a fast N-turnover component of the suspended particulate organic matter pool rather than the bulk pool sampled. Together, the natural-abundance stable C and N isotope analyses and the experimental N-15 tracer approach proved to be very useful tools for identifying food resources in this stream ecosystem.