RESPONSE OF A HYPORHEIC INVERTEBRATE ASSEMBLAGE TO DRYING DISTURBANCEIN A DESERT STREAM

The effects of flow variation on surface invertebrates have been well documented; however, studies involving the subsurface (hyporheic) envi ronment are rare. We assessed the response of hyporheic invertebrates to drying by sampling 5 sets of wells (30, 50, and 100 cm deep) from 8 May 1995 to 4 August 1995 in an intermittent desert stream undergoing drying. Drying began with discontinuity of surface water, and continu ed through loss of surface now to recession of the water table (at a r ate of 11 cm/wk) to >1 m depth by August. Twenty taxa were collected d uring the study. Shallow (30- and 50-cm) assemblages consisted mainly of insect larvae and cyclopoid copepods whereas bathynellaceans, isopo ds, and harpacticoid copepods were the major taxa in deeper (100-cm) s ediments. Total abundance of all taxa combined and abundance of many i ndividual taxa differed between depths, but also changed significantly over time. The interaction between date and depth was also significan t, indicating that the direction of temporal change was not consistent among well depths. Once surface-water dried, total invertebrate abund ance began to increase in wells that remained inundated. Thereafter, a s the water table continued to recede, abundance in the 100-cm wells i ncreased while numbers in shallow wells decreased. This pattern suppor ts the hypothesis that the hyporheos migrates into deeper sediments to avoid conditions associated with drying.