Hyporheic ecology of alluvial rivers in Canterbury, New Zealand

Aspects of the ecology of hyporheic river communities in Canterbury, New Zealand were examined using field surveys in association with field and laboratory experiments. Seasonal pump-sampling of Ashley River tributaries revealed an invertebrate fauna dominated numerically by harpacticoid copepods, although insects (particularly Chironomidae and Polycentropodidae) dominated biomass. Dissolved oxygen (minimum concentration = 2.1 mg 1-1) was negatively related to invertebrate abundance in reaches receiving upwelling groundwater in summer, but not winter. Thus, seasonal limitation of dissolved oxygen may occur in river reaches where upwelling is prevalent. Colonisation pots embedded in the Waipara River collected a high proportion of epigean taxa, notably the snail Potamopyrgus antipodarum, whereas pump-samples were biased towards collecting non-insect taxa, including harpacticoids and mites. In colonisation pots, the hyporheic biota (15-45 cm depth) represented about 50% of total (0-45 cm) invertebrate abundance and community respiration. Willow leaves added to colonisation pot gravels increased invertebrate abundance and community respiration, but their effect declined with depth. Low concentrations of silt (<2.5 g per litre of sediment) appeared to enhance the food resource for some collector-filtering taxa (particularly oligochaetes and ostracods), whilst lessening its value to the grazers P. antipodarum and Hydora sp. (Elmidae). The epilithic microbial community found in the hyporheic zone was similar to that of heavily-shaded surface epilithon, and both had lower biomass and a less diverse microbiota (algae and fungi) than epilithon grown in full light. While the epigean caddisfly 0. Feredayi ingested hyporheic foods, it did not grow in the absence of either higher quality light-grown epilithon, or particulate organic matter. Fine sediment (