5 - Interstitial fauna along an epigean-hypogean gradient in a Rocky Mountain river

Summary

ABSTRACT Interstitial animals were collected during spring, summer, and autumn from eight sampling sites along a Rocky Mountain river to examine small scale patterns of diversity, abundance, and faunal composition across the groundwater/surface water ecotone. At each site samples were taken from benthic habitats (superficial bed sediments), hyporheic habitats (underflow 30 cm below the bed surface), and phreatic habitats (30 cm below the water table of adjacent alluvial bars). Total taxa decreased markedly along the epigeanhypogean gradient. Crustaceans and insects (the majority of animals in all three habitats) progressively increased (Crustacea) or progressively declined (Insecta) in relative abundance along the epigean-hypogean gradient. Most of the 142 common taxa exhibited one of four types of distribution patterns along the gradient (hypogean, epigean, transitional, eurytopic). This study demonstrated a marked faunal gradient on a scale of meters across the groundwater/surface water ecotone. Faunal similarity coefficients (e.g., Jaccard) and detrended correspondence analysis of faunal distributions may provide comparative measures of ecotone permeability between physically contiguous interstitial habitats.

INTRODUCTION

Riverine sediments form habitat patches of various sizes and complexity, reflecting the interactions of fluvial dynamics and biogeochemical features (Schumm, 1977; Richards, 1982; Rust, 1982). The distribution patterns of animals inhabiting porous alluvia reflect gradients in environmental conditions that occur at a variety of spatial scales (reviewed by Gibert et al., 1994; Ward & Palmer, 1994). Interstitial animals residing in the water-filled spaces between substrate particles are distributed along a gradient of habitat conditions from superficial bed sediments (benthic biotope), to deeper sediments beneath the channel (hyporheic biotope=underflow), to alluvial deposits situated some distance laterally from the active channel (phreatic biotope).