Evaluating habitat suitability curves for predicting variation in macroinvertebrate biomass with weighted usable area in braided rivers in New Zealand

We investigated changes in seasonal biomass of benthic macroinvertebrates over whole floodplain transects upstream and downstream of existing flow diversions in the Wilberforce and South Ashburton Rivers, Canterbury, New Zealand. We tested the predictive performance of a 1-dimensional hydraulic-habitat model informed with existing general habitat suitability curves (HSC), as well as river- and channel-specific HSC developed from the studied rivers. Diversions resulted in significant declines in wetted width, habitat complexity, and resulting declines in benthic biomass, but community composition was unchanged. Weighted usable area (WUA) predictions based on general HSC were often poorly correlated with cross-sectional macroinvertebrate biomass. River- and channel-type-specific HSC greatly improved the accuracy of predictions; however, all HSC generally under-predicted the total proportional change in cross-sectional macroinvertebrate biomass associated with flow change. We conclude that WUA was an insensitive indicator of flow-related macroinvertebrate biomass change. We encourage development of HSC categorised by river hydrology, size and channel form.