Interstitial invertebrate assemblages associated with small-scale subsurface flowpaths in perennial and intermittent California streams

We hypothesized that interstitial invertebrate assemblages should have fairly distinct taxonomic composition in stream reaches characterized by different subsurface flowpaths. We examined invertebrate assemblages in a perennial and an intermittent stream during summer baseflow conditions to determine if the direction of subsurface flow within each stream influenced taxonomic composition. The subsurface regions are described as 1) infiltration (where stream surface-water recharges the aquifer), 2) exfiltration (where interstitial water discharges into the stream surface), and 3) dry-channel subsurface region (i.e. the saturated interstices beneath a dry intermittent stream channel). In both perennial and intermittent streams, the infiltration regions had higher invertebrate densities, taxa richness, and chironomid densities, but a lower percentage of hypogean individuals. The direction of subsurface flow did not generally affect the distribution of the three hypogean taxa (Archiannelida, the amphipod Stygobromus, and the isopod Calasellus) in either stream. In these groundwater-dominated streams where water infiltrates and exfiltrates through large interstices within coarse substrates, homogeneous water quality resulted in similar invertebrate assemblages among subsurface regions within a small-spatial scale in both the perennial and intermittent stream.