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

Two northern Californian streams, an intermittent and a perennial with simi lar climate, geology vegetation and land use were compared to examine the e ffects of seasonal drying on surface and hyporheic invertebrate assemblages . Aquatic insects composed 95% and 94% of the surface fauna in the intermit tent 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 stream s, respectively, and were dominated by archiannelids and harpacticoid and c yclopoid copepods. Faunal overlap between the intermittent and perennial st reams was high (Jaccard coefficient 0.88 for surface fauna and 0.82 for hyp orheic fauna). The intermittent stream surface fauna had lower total densit ies, taxon richness, and species diversity compared to that of the perennia l stream; the hyporheic fauna in the intermittent stream had lower densitie s, 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 s urface flow ceases. If the hyporheic zone serves as a refuge, then hyporhei c densities during the dry period should increase in the intermittent strea m relative to the perennial stream. However, during the drying process, pop ulations of temporary hyporheic resident invertebrates remained unchanged ( Sweltsa) or decreased (Baetis, Parthina, Ceratopogonidae), but densities of the permanent hyporheic resident (Archiannelida) increased. This result su ggests that the hyporheic zone was not a refuge from drying for surface inv ertebrates inhabiting this intermittent stream.