Benthic community dynamics during summer low‐flows in two rivers of contrasting enrichment 1. Periphyton

Abstract

Many foothill rivers in the eastern regions of New Zealand undergo severe reductions in flow during summer. These reductions are often exacerbated by abstractions for out‐of‐river uses. It has been observed that at such times some river ecosystems are more detrimentally affected by reduced flows than others. We hypothesised that the degree of degradation in river ecosystems during summer low‐flows is dependent on a river's enrichment status. We examined the effect of low flows on periphyton community dynamics in two Canterbury rivers of contrasting enrichment. Three runs in each river were sampled on four occasions over a summer low‐flow period, during which discharge was amongst the lowest on record for these rivers. Percentage cover of major periphyton groups on the riverbed was estimated across five transects in each run, and periphyton was scraped from five replicate rocks per run to quantify taxonomic richness, community composition, and chlorophyll a biomass. Periphyton communities in each river were quite different. Diatoms with a low biomass dominated the communities in the unenriched Okuku River. These communities did not change significantly in composition or biomass over the 6‐week study period. Conversely, in the enriched Waipara River, the communities had a high biomass and there was a succession in dominant taxa from diatoms and cyanobacteria to filamentous green algae over summer. This resulted in the Waipara River exceeding guidelines for the maintenance of in‐river recreational and fisheries values, whereas the Okuku River did not. Thus, degree of enrichment should be taken into account when assessing in‐river flow requirements. We suggest that enriched rivers will be more sensitive to flow abstraction (i.e., they will become more degraded), and so may require higher minimum flows to maintain river health, than unenriched rivers.

Keywords:

• hydraulics
• flow recession
• low flows
• community composition
• nutrient status