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

Abstract

We examined the effects of summer lowflow 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 M01093; Published 20 March 2003 Received 7 December 2001; accepted 10 September 2002from 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.

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

“…Similar shifts in community composition during periods of stable flow have been reported from several other field and experimental studies (e.g., Fisher et al 1982;Peterson & Stevenson 1992;Dent & Grimm 1999). The high algal biomass at this site, and the dominance of the community by filamentous green algae, suggests that periphyton production was far greater than potential invertebrate grazing pressure (Suren et al 2003). Although invertebrate densities at the Waipara River were much higher than in the Okuku River, ranging from c. 3700 individuals m-2 at the beginning of the study, to c. 10 000 at the end (Suren et al 2003), the community there was not dominated by grazers.…”

“…As invertebrate density increased, periphyton biomass decreased as a result of grazing. In the present study, however, at least 2.5 months had passed since a flood intensive enough to disturb invertebrates had occurred, and so invertebrate densities were moderately high (mean total invertebrate density = 2500 m-2 ; Suren et al 2003). Grazers such as the mayfly Deleatidium, elmid beetles, and the caddisfly Pycnocentrodes also were common at this site.…”

“…The high algal biomass at this site, and the dominance of the community by filamentous green algae, suggests that periphyton production was far greater than potential invertebrate grazing pressure (Suren et al 2003). Although invertebrate densities at the Waipara River were much higher than in the Okuku River, ranging from c. 3700 individuals m-2 at the beginning of the study, to c. 10 000 at the end (Suren et al 2003), the community there was not dominated by grazers. Instead, taxa such as ostracods and elmid beetles were dominant, whereas grazing animals such as the caddisfly Pycnocentrodes, and the snails Physa and Potamopyrgus were less common, making <4% of total density.…”

“…In the present study we monitored the hydraulic habitat and periphyton communities during the summer low-flow period to compare the extent and composition of periphyton growth in the two rivers, and to indicate the potential extent of habitat degradation. Then, in the second study (Suren et al 2003), changes in benthic macroinvertebrate communities were monitored to compare their composition and degree of change over the summer under the two enrichment regimes.…”

Extreme Climatic Events Alter Aquatic Food Webs

“…Drought also has the potential to alter ecosystem functioning and the supply of ecological goods and ecosystem services, such as carbon and nutrient cycling and fish production. Organic matter processing by microbes and detritivores may decline during severe drought (Schlief and Mutz, 2011), whereas primary production can at first increase greatly because of elevated temperature and nutrients, but then decline dramatically and even cease as drying intensifies (Ledger and Hildrew, 2001;Ledger et al, 2008;Suren et al, 2003;Wade et al, 2002). Direct impacts on the secondary production of invertebrates and fish depend on drought intensity, with greatest effects occurring where flow cessation occurs (Lake, 2011;Ledger et al, 2011;Matthews and Marsh-Matthews, 2003).…”