Response of a hyporheic invertebrate assemblage to drying disturbance in a desert stream

The effects of flow variation on surface invertebrates have been well documented; however, studies involving the subsurface (hyporheic) environment 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 continued through loss of surface flow to recession of the water table (at a rate of 11 cm/wk) to >1 m depth by August. Twenty taxa were collected during the study Shallow (30- and 50-cm) assemblages consisted mainly of insect larvae and cyclopoid copepods whereas bathynellaceans, isopods, and harpacticoid copepods were the major taxa in deeper (100 cm) sediments. Total abundance of all taxa combined and abundance of many individual taxa differed between depths, but also changed significantly over time. The interaction between date and depth was also significant, indicating that the direction of temporal change was not consistent among well depths. Once surface water dried, total invertebrate abundance began to increase in wells that remained inundated. Thereafter, as the water table continued to recede, abundance in the 100-cm wells increased while numbers in shallow wells decreased. This pattern supports the hypothesis that the hyporheos migrates into deeper sediments to avoid conditions associated with drying.