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.

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

“…This river has a high conductivity suggesting higher levels of enrichment. Extensive blooms of filamentous algae were recorded in the companion study (Suren et al 2003). Gauging stations have operated in both rivers for over 10 years, and examination of the historical hydrological data showed that discharge during the study was the second lowest on record.…”

“…These sites are described in Suren et al (2003), so only a brief account is given here. The Okuku River flows through a catchment dominated by unmodified land cover with a geology dominated by nutrient-poor metamorphic greywacke rocks.…”

“…In a companion paper, Suren et al (2003) quantified changes in periphyton communities during summer low-flows in two rivers of contrasting enrichment status. It was found that the periphyton community of each river was different, and successional changes in community composition varied.…”

Responses of epibenthic algal assemblages to water abstraction in Hong Kong streams

“…Such responses have been widely documented to inform management decisions about the implementation of environmental flows; i.e., the flows left in streams to sustain biodiversity, ecological integrity and ecosystems service (Acreman & Dunbar, 2004;Brisbane Declaration, 2007;Arthington et al, 2010). Nonetheless studies on the relationships between flow reduction and responses of stream algal assemblages are scarce compared with those dealing with macroinvertebrates and fishes, and most such studies are focused mainly on single streams in temperate or subtropical latitudes (Clausen & Biggs, 1997;Downes et al, 2003;Soininen, 2003;Suren et al, 2003;Wang et al, 2009;Wu et al, 2009). Comparable studies on tropical or monsoonal streams-with markedly seasonal flow regimes-are scarce (Niu & Dudgeon, 2011a).…”

“…Physical shear stress imposed by current strongly influences the relative abundance of algae with different growth forms or morphologies (e.g., erect, prostrate, or filamentous) and also affects colonization dynamics, thereby influencing species composition (Stevenson, 1996;Hart & Finelli, 1999). With reduced current speed and flow volume, algal community development is less constrained by shear stress, and compositional shifts can take place from, for instance, low-stature diatoms to nuisance growths of filamentous green algae, although the precise outcome depends on nutrients and temperature regimes (Biggs & Price, 1987;Suren et al, 2003;Berthon et al, 2011). The strength of grazing pressure upon epibenthic algae can vary according to flow conditions (Poff & Ward, 1995;Chester & Norris, 2006), potentially influencing the intensity of topdown control of assemblage composition and biomass (Opsahl et al, 2003).…”