Influence of substrate type and physico-chemical conditions on macroinvertebrate faunas and biotic indices of some lowland Waikato, New Zealand, streams

Lowland streams in agricultural areas are often considered degraded because of high turbidity, high biomass of adventive macrophytes, and a predominance of fine bed substrates. Invertebrate faunas in these streams are being increasingly used in biomonitoring, and an understanding of factors affecting their distribution and abundance is important for interpreting the results of these studies. We measured physicochemical parameters and sampled macroinvertebrate faunas on inorganic substrates, wood, and macrophytes to investigate factors influencing invertebrate distribution and abundance in Waikato, New Zealand, lowland streams, and to evaluate six biotic indices and a rapid bioassessment protocol for lowland stream environments. At most sites, invertebrate faunas were dominated numerically on all substrate types by the hydrobiid snail Potamopyrgus antipodarum (Gray), the eusirid amphipod Paracalliope, and Diptera (mostly the chironomid Neonella forsythii Boothroyd and the blackfly Austrosimulium spp.). Stepwise regression analyses indicated that densities of some common taxa on inorganic substrates (0.1 m(-2)) and macrophytes (gl wet weight) were correlated with combinations of reach water depth, percentage of bed covered by macrophytes or sand/silt, and water quality factors including conductivity, water temperature, and measures of dissolved oxygen (DO) concentration and magnitude of diel fluctuation. These factors and also the amount of wood present explained 36-87% of the variation in several of the biotic indices examined on different substrate types. Some taxonomic (no. of taxa) and compositional (% dominant taxon) indices appeared sensitive to measured habitat conditions, and this may affect their utility for detecting water quality impacts in lowland streams. Percent Ephemeroptera, Plecoptera, and Tricoptera (EPT), and the Macroinvertebrate Community Index (MCI) and its quantitative analogue (QMCI) reflected factors thought to be more associated with direct and indirect effects of riparian and catchment land use on water quality (DO, conductivity, temperature, macrophyte cover). Comparison of indices derived from intensive and rapid bioassessment sampling yielded significant regressions with slopes that were not significantly different to 1 for % EPT and MCI. These indices appear to be particularly useful for rapid bioassessment of lowland stream environments.