INVERTEBRATE RECOLONIZATION OF SMALL PATCHES OF DEFAUNATED HYPORHEIC SEDIMENTS IN A SONORAN DESERT STREAM

1. Fine mesh (63-mu-m) traps filled with defaunated sediment and open to recolonization from upstream, downstream or below were buried 30 cm deep in the hyporheic zone of a gravel run in a desert stream and recovered after 1, 3, 7, 14 and 27 days. Temporal changes in organic matter and faunal composition and abundance were compared among treatments and with those in completely closed 'control' traps. 2. Closed traps consistently contained little detritus (< 30 mg AFDM per 5000 cm3). Traps facing upstream accumulated significantly more organic matter than those facing below or downstream, indicating downstream hyporheic transport of fine particulate detritus. 3. Upstream-facing traps also captured significantly greater numbers of taxa and individuals. Low numbers of most taxa recolonized from all three directions within 1 day; total densities of individuals in the traps increased before levelling off after 2 weeks. 4. The prediction that taxa with planktonic epigean relatives (e.g. cyclopoid copepods) would predominate in upstream facing traps was unsupported, nor did unpigmented, blind phreatic forms preferentially colonize traps from below. Two closely related taxa exhibited major differences in recolonization dynamics, illustrating the risk of masking such trends when pooling data for more than one taxon. 5. Experimental manipulations such as this supplement the predominantly descriptive studies of the hyporheos, and demonstrate the complex dynamics of organic matter and biota in the hyporheic zone that must be considered in analyses of most stream ecosystems.