INFLUENCE OF SUBSTRATE TYPE AND PHYSICOCHEMICAL CONDITIONS ON MACROINVERTEBRATE FAUNAS AND BIOTIC INDEXES OF SOME LOWLAND WAIKATO, NEW-ZEALAND, STREAMS

Lowland streams in agricultural areas are often considered degraded be cause of high turbidity, high biomass of adventive macrophytes, and a predominance of fine bed substrates. Invertebrate faunas in these stre ams are being increasingly used in biomonitoring, and an understanding of factors affecting their distribution and abundance is important fo r interpreting the results of these studies. We measured physicochemic al parameters and sampled macroinvertebrate faunas on inorganic substr ates, wood, and macrophytes to investigate factors influencing inverte brate distribution and abundance in Waikato, New Zealand, lowland stre ams, and to evaluate six biotic indices and a rapid bioassessment prot ocol for lowland stream environments. At most sites, invertebrate faun as were dominated numerically on all substrate types by the hydrobiid snail Potamopyrgus antipodarum (Gray), the eusirid amphipod Paracallio pe, and Diptera (mostly the chironomid Neonella forsythii Boothroyd an d the blackfly Austrosimulium spp.). Stepwise regression analyses indi cated that densities of some common taxa on inorganic substrates (0.1 m(-2)) and macrophytes (gl wet weight) were correlated with combinatio ns of reach water depth, percentage of bed covered by macrophytes or s and/silt, and water quality factors including conductivity, water temp erature, and measures of dissolved oxygen (DO) concentration and magni tude of diel fluctuation. These factors and also the amount of wood pr esent explained 36-87% of the variation in several of the biotic indic es examined on different substrate types. Some taxonomic (no. of taxa) and compositional (% dominant taxon) indices appeared sensitive to me asured habitat conditions, and this may affect their utility for detec ting 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 tho ught to be more associated with direct and indirect effects of riparia n and catchment land use on water quality (DO, conductivity, temperatu re, macrophyte cover). Comparison of indices derived from intensive an d rapid bioassessment sampling yielded significant regressions with sl opes that were not significantly different to 1 for % EPT and MCI. The se indices appear to be particularly useful for rapid bioassessment of lowland stream environments.