Abstract
Trophic relationships were examined using natural-abundance
13C and
15N analyses and a
15N-tracer addition experiment in Walker Branch, a 1st-order forested stream in eastern Tennessee. In the
15N-tracer addition experiment, we added
15NH
4, to stream water over a 6-wk period In early spring, and measured
15N:
14N ratios in different taxa and biomass compartments over distance and time. Samples collected from a station upstream from the
15N addition provided data on natural-abundance
13C:
12C and
15N:
14N ratios. The natural-abundance
15N analysis proved to be of limited value in identifying food resources of macroinvertebrates because
15N 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 taxa for which the
15N tracer results were not definitive. The
15N 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
15N tracer approach proved to be very useful tools for identifying food resources in this stream ecosystem.
Keywords
Stream macroinvertebrates,
food resources,
N cycling,
tracer addition,
stable isotope,
15N,
13C
Citation
Mulholland, Patrick J.; Tank, Jennifer L.; Sanzone, Diane M.; Wollheim, Wilfrid M.; Peterson, Bruce J.; Webster, Jackson R.; Meyer, Judy L. 2000. Food resources of stream macroinvertebrates determined by natural-abundance stable C and N isotopes and a
15N tracer addition. Journal of the North American Benthol Society, 2000, 19(1): 145-157