New Zealand Journal of Marine and Freshwater Research abstracts
Benthic community dynamics during summer low-flows in two rivers of contrasting
enrichment
1. Periphyton
Alastair M. Suren
Barry J. F. Biggs
Cathy Kilroy
Liz Bergey*
National Institute of Water and Atmospheric
Research Limited
P.O. Box 8602, Riccarton
Christchurch, New Zealand
email: a.suren@niwa.co.nz
*Present address: Oklahoma Biological Survey, 11 E. Chesapeake St, University
of Oklahoma, Norman OK 73019, United States.
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
M01092 Received 7 December 2001; accepted 10 September 2002; Published
20 March 2003
New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37:
53-70
0028-8330/03/3701-0053 $7.00 © The Royal Society of New Zealand
2003
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