Littoral and benthic macroinvertebrate community responses to contrasting stressors in a large New Zealand river

Large rivers present major challenges for biomonitoring and the development of biocriteria because of difficulties obtaining samples representative of the range of habitats present and a paucity of information on taxa responses to environmental conditions. We sampled littoral and deepwater benthic habitats to investigate macroinvertebrate community composition associated with organic, sediment and heated water discharges at 22 transects along a 73 km section of the lower Waikato River, North Island, New Zealand. Significant community differences in near-field exposure versus no exposure sites were found only in littoral habitats, partly reflecting changes in relative abundances of Crustacea and Oligochaeta. Littoral and benthic habitats supported different macroinvertebrate communities, particularly downstream of the sediment input and following the onset of a river-floodplain morphology which may have enhanced deposition of suspended sediment. Distance down river was a significant covariate for most metrics derived from benthic samples, but did not influence the relative abundance of most macroinvertebrate groups in littoral samples. Collectively, these results suggest that littoral habitats are likely to be more suitable than benthic habitats for biomonitoring since macroinvertebrate faunas were responsive to near-field stressor exposure and were less likely to be influenced by longitudinal changes in river conditions.