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

We hypothesized that interstitial invertebrate assemblages should have fair ly distinct taxonomic composition in stream reaches characterized by differ ent subsurface flowpaths. We examined invertebrate assemblages in a perenni al and an intermittent stream during summer baseflow conditions to determin e if the direction of subsurface flow within each stream influenced taxonom ic composition. The subsurface regions are described as 1) infiltration (wh ere stream surface-water recharges the aquifer), 2) exfiltration (where int erstitial water discharges into the stream surface), and 3) dry-channel sub surface region (i.e. the saturated interstices beneath a dry intermittent s tream channel). In both perennial and intermittent streams, the infiltratio n 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 hyp ogean taxa (Archiannelida, the amphipod Stygobromus, and the isopod Calasel lus) in either stream. In these groundwater-dominated streams where water i nfiltrates and exfiltrates through large interstices within coarse substrat es, homogeneous water quality resulted in similar invertebrate assemblages among subsurface regions within a small-spatial scale in both the perennial and intermittent stream.