Vertical and seasonal distribution of hyporheic invertebrates in streams with different glacial histories

Abstract

We investigated the vertical, lateral, and seasonal distribution of hyporheic invertebrates at five stream sites in Idaho (unglaciated areas) and seven sites in Montana (glaciated areas), U.S.A. Conservative tracer releases and transient storage zone analyses were conducted to help characterize study sites. Hyporheic invertebrates and substrate were sampled by freeze-coring with electro-shocking, and mini-piezometers were used to measure other hyporheic conditions, including vertical hydraulic gradient (VHG). Wells were installed at some Montana sites (range: 5–175 m lateral to the channel) and shallow pits were excavated at some Idaho sites (5 m lateral to the channel) to sample hyporheic invertebrates in these saturated sediments regions. Environmental conditions varied widely among sites and between surface and hyporheic zones at each site, and most hyporheic habitat measures differed significantly between glaciated and unglaciated streams. In-channel invertebrate density and taxa richness generally decreased with increasing depth into the streambed, regardless of geographic location, history of glaciation, or season. Although hyporheic invertebrate mean density and (to a lesser extent) richness values at individual depth layers (e.g., 10–20 cm, 20–30 cm) mostly were greater in the glaciated region than the unglaciated region, density and richness between 10–50 cm did not differ significantly between the two regions. Hydrologic variables (VHG; AS/A [ratio of transientstorage- zone- to channel- cross-sectional areas]; TTS [turnover time of storage]; and Rh [hydraulic retention factor]) provided the best explanation for differences in sub-channel invertebrate density and richness patterns. The observation that benthic invertebrates occurred in freeze-core, well, and open-pit samples contributes to existing evidence that aquatic invertebrates occupy habitats that extend considerable distances from the active channel.