Benthic community dynamics during summer low-flows in two rivers of contrasting enrichment 2. Invertebrates

We examined the effects of summer low-flow on invertebrate communities in two New Zealand rivers of contrasting enrichment and associated periphyton development. Quantitative benthic samples were collected and hydraulic measurements made from three runs in each river over a 6-week period. Although hydraulic conditions and time since last flood disturbance were similar in both rivers, invertebrate communities were dissimilar. This dissimilarity reflected differences in periphyton communities within each river. The invertebrate community in the low enrichment river (Okuku River) was dominated by high numbers of "clean-water" insect taxa, and this community changed little over time. Periphyton biomass was low here, and the community was composed of diatoms. Major changes occurred to the invertebrate community in the high enrichment river (Waipara River) over the summer, where the relative abundance of dominant invertebrate taxa changed from insects and snails to ostracods. Periphyton biomass was high in this river, which was originally dominated by diatoms and cyanobacteria but which became dominated by filamentous green algae. Changes to invertebrate communities were linked to this increase in filamentous green algae. Multiple regression analysis showed that changes to the invertebrate community in the enriched river were strongly correlated with the number of days at low flow, suggesting that successional processes occurred during the low-flow period. Such changes may occur naturally during summer low-flows in enriched rivers or could be exacerbated if velocities are reduced by abstraction. Our results suggest that a river's degree of enrichment should be considered when assessing minimum flows for consents or for the development of flow management guidelines.