21 - Stable nitrogen isotope tracing of trophic relations in food webs of river and hyporheic habitats

Summary

ABSTRACT Natural isotopic abundances of N (d¹⁵N) in organic matter are used to examine the trophic relations in river and hyporheic habitats of the Flathead River, Montana. The d¹⁵N of biofilms and Plecopteran-dominated food webs are compared for river and hyporheic waters. We examine the concept that the isotope ratio of an animal undergoes both the food source effect and in vivo metabolic effects. Metabolic effects for nitrogen isotopes in animal tissues relates to isotopic fractionation, which exhibits progressive enrichment of the d¹⁵N in body tissues at higher trophic levels. These ∇ d¹⁵N enrichments are in excess of isotopic abundances of food sources. Comparisons show greater enrichment for the consumer trophic levels of the hyporheic than river channel habitat. The d¹⁵N contents of the hyporheic food webs suggest influences of different concentrations and d¹⁵N contents of NO₃ and NH_4. These inorganic conditions may be controlled by physical mixing of various water masses, Ntransformations associated with biofilm microbiota, and feedbacks of NH₄ through excretion by animals. Our observations indicate that stable isotopes may be powerful tools for determining nitrogen source and process information in hyporheic habitats.

INTRODUCTION

Little is known about the trophic ecology of food webs within the hyporheic ecotones of river ecosystems (Hendricks, 1993). In our study, stable nitrogen isotopes measurements (d¹⁵N) are used to examine the trophic relations in river and hyporheic habitats of the Flathead River, Montana. The d¹⁵N of biofilms and Plecopteran-dominated food webs are compared for river and hyporheic waters. The almost total coverage on most rock surfaces by biofilms suggests that attached microbiota comprise a dominant food resource in both the river channel and hyporheic habitats.