Invertebrates in intermittent and perennial streams: is the hyporheic zone a refuge from drying?

Two northern Californian streams, an intermittent and a perennial with similar climate, geology vegetation and land use were compared to examine the effects of seasonal drying on surface and hyporheic invertebrate assemblages. Aquatic insects composed 95% and 94% of the surface fauna in the intermittent and perennial streams, respectively, and were dominated by chironomids and caddisflies (eg, Apatania, Neothremma, Parthina). Noninsects composed 73% and 59% of the hyporheic fauna in the intermittent and perennial streams, respectively, and were dominated by archiannelids and harpacticoid and cyclopoid copepods. Faunal overlap between the intermittent and perennial streams was high (Jaccard coefficient 0.88 for surface fauna and 0.82 for hyporheic fauna). The intermittent stream surface fauna had lower total densities, taxon richness, and species diversity compared to that of the perennial stream; the hyporheic fauna in the intermittent stream had lower densities, similar richness, but higher species diversity. We used a Before-After-Control-Impact (BACI) design to test the hypothesis that the hyporheic zone serves as a refuge for surface invertebrates when surface flow ceases. If the hyporheic zone serves as a refuge, then hyporheic densities during the dry period should increase in the intermittent stream relative to the perennial stream. However, during the drying process, populations of temporary hyporheic resident invertebrates remained unchanged (Sweltsa) or decreased (Baetis, Parthina, Ceratopogonidae), but densities of the permanent hyporheic resident (Archiannelida) increased. This result suggests that the hyporheic zone was not a refuge from drying for surface invertebrates inhabiting this intermittent stream.