No 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.

1. Invertebrates were collected semi-quantitatively from four relatively undisturbed wetlands in the west coast of New Zealand's South Island: two acidic fens and two swamps. Samples were collected from up to four discrete habitats within each wetland: large open-water channels, small leads (small, ill-defined channels with emergent vegetation in them) and large (>10 m diameter) or small (<10 m diameter) ponds. Samples were also collected from different plant species within each wetland, each with different morphology, and from areas without vegetation. This was done to determine whether invertebrate communities varied more between-wetlands than within-wetlands, as the results had implications for future wetland monitoring programmes. 2. Principal components analysis of water chemistry data revealed striking differences in pH, conductivity and nutrients between the four wetlands. Not surprisingly, pH was lowest in one of the acidic fens, and highest in one of the swamps, where conductivity was also high. Midges (Tanytarsus, Tanypodinae, Orthocladiinae and Ceratopogonidae), nematodes, harpactacoid copepods and the damselfly Xanthocnemis dominated the invertebrate fauna. Orthoclad midges and mites were the most widespread taxa, found in 91 of 94 samples. Diptera were the most diverse invertebrate group, followed by Trichoptera and Crustacea. 3. Ordination analysis of the invertebrate data showed that the four wetlands supported different invertebrate communities. However, species composition did not change completely along the ordination axes, suggesting that a relatively species-poor invertebrate fauna was found in the wetlands. Taxa such as molluscs were restricted to wetlands with high pH. Multi-response permutation procedures (MRPP) was used to analyse resultant ordination scores to see how they differed according to five terms: 'Wetland', 'Habitat', 'Growth Form', 'Morphology' and 'Plant'. Most of the sample separation along ordination axes reflected differences between wetland, although the 'Habitat' and 'Plant' terms also explained some of the variation. The 'Growth Form' and 'Morphology' terms had only minor effects on community composition. 4. A multivariate regression tree modelled invertebrate assemblages according to the five predictor terms. The resultant model explained 54.8% of the species variance. The 'Wetland' term contributed most to the explanatory power, followed by 'Habitat'. 'Growth type' and 'Morphology' explained only a small amount of variance to the regression tree, while the different plant species explained none of the variation. 5. Variation in these New Zealand wetland invertebrate communities appears to be controlled most by large-scale factors operating at the level of individual wetlands, although different habitats within individual wetlands contributed slightly to this Correspondence: A. M. Suren, 727 variation. Based on these results, sampling programmes to describe wetland invertebrate communities do not need to sample specific habitats or plant types within a wetland. Instead,...

We characterised water chemistry, aquatic habitat, macrophytes, and invertebrate assemblages in eight lowland streams-five in Westland, South Island, and three in the Waikato, North Island, New Zealand. Factors influencing invertebrate community structure over large (between ecoregions) and small (within an ecoregion) spatial scales were investigated. The Westland sites had generally lower nutrient concentrations, conductivity, and water clarity, and coarser substrates than the Waikato sites, reflecting differences in geological history, hydrology, and land-use intensity. The macrophyte communities in each region were very different in species composition and structure, but seasonal abundance patterns in both regions were fairly typical of New Zealand streams in general. Alien obligate submerged macrophyte species were absent from the Westland lowland stream sites, and may partly reflect the isolated nature of this region where colonisation by asexual macrophytes would be difficult. Waikato stream invertebrate faunas were *Author for correspondence. M02006; Published 14 November Received 23 January 2002; accepted 19 July 2002dominated by molluscs (mainly Potamopyrgus) and crustaceans (mainly amphipods), whereas Ephemeroptera, Trichoptera, and Coleoptera taxa dominated the Westland stream faunas. The overall structure of invertebrate assemblages appeared to be influenced by a combination of regional differences in substrate type, nutrient concentrations, water clarity, and macrophyte cover. Westland streams with more upstream pastoral development had higher proportions of molluscs than those with predominantly forested or scrub catchments. Our study suggests that large-scale ecoregional differences may override smaller-scale land-use effects on lowland stream invertebrate communities, and that management strategies should be developed on an ecoregional basis for lowland stream ecosystems.