EFFECTS OF BURIED LEAF-LITTER AND VERTICAL HYDROLOGIC EXCHANGE ON HYPORHEIC WATER CHEMISTRY AND FAUNA IN A GRAVEL-BED RIVER IN NORTHERN NEW-SOUTH-WALES, AUSTRALIA

1. Large amounts of coarse particulate organic matter (CPOM) are burie d in the sand and gravel beds of many rivers during spates. The effect s of these patchily distributed resources on hyporheic invertebrates a nd water chemistry are poorly understood. Buried CPOM may provide loca l 'hot-spots' of food for hyporheic detritivores and their predators, alter nutrient supply to nearby sediment biofilms, and generate habita t for some invertebrates. 2. To examine potential short-term effects o n hyporheic water chemistry, nutrient concentrations and invertebrate assemblage composition, leaf packs were buried in downwelling (surface water infiltrating the hyporheic zone) and upwelling (hyporheic water emerging to the surface) zones at two sites along a gravel-bed river in northern New South Wales. At one site, pits were excavated to simul ate leaf burial (procedural control) and plastic 'leaves' were buried to test whether invertebrates might respond to leaves as refuges rathe r than food. Hyporheic CPOM, sediment size fractions, and interstitial silt content were also quantified at these sites. 3. Dry weights of n aturally buried CPOM (leaf litter and wood fragments) varied substanti ally (0.6-71.7 g L-1 sediment). Amounts of CPOM did not differ between up- vs. downwelling zones or between sites. Hyporheic dissolved oxyge n saturation was generally high (> 75%), and was lower in upwelling zo nes. The hyporheos was dominated taxonomically by water mites (approxi mate to 20 species), whereas small oligochaetes were most abundant (40 % of total abundance). Tiny instars of elmid beetle larvae and leptoph lebiid mayfly nymphs were also common. Before experimental manipulatio n, faunal composition differed between up- and downwelling zones. In u pwelling zones, bathynellaceans and blind peracarids were found, where as small individuals of the surface benthos were common in samples fro m downwelling zones. This validated stratification of the experiment a cross zones of hydrologic exchange. 4. Twenty days after leaf burial, there was no effect of the treatments at either site on changes in mos t variables, including mean numbers of taxa and individuals per sample . Similarly, changes in faunal composition of the hyporheos in the tre atments paralleled those in the controls except for a weak response in the buried leaves treatment in the upwelling zone at site 1. Artifici ally buried leaf litter does not seem to influence hyporheic water che mistry or fauna at these two sites. It is probable that naturally buri ed leaf Litter is swiftly processed soon after entrainment and that re peating this experiment immediately after a flood may yield different results.