The effect of landuse and geology on macroinverterbate communities in East Coast streams, Gisborne, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, New Zealand

Abstract

Chapter 1. Land use and geology both potentially influence streams and rivers by altering hydrology, water chemistry, light levels, sedimentation, channel form, food sources and habitat structure for the macroinvertebrate community assemblages. This study investigated the effect of geology and land use on stream macroinvertebrate communities in the East Coast region of the North Island, New Zealand. Macroinvertebrate communities were monitored every year between 2016-2018 at 79 sites during the austral summer in; exotic forest, indigenous forest and pasture streams that either had a hard or soft geology. Both land use and geology independently influenced biotic indices (MCI and %EPT abundance) and macroinvertebrate communities, but there seemed to be limited interactive effects. Exotic forest and pastoral land use were dominated by Molluscs and Ephemeroptera. Pastoral land use supported the highest number of macroinvertebrate individuals and indigenous forest had the highest densities of sensitive species. Sixty five percent of sites had deposited sediment levels greater than 20% cover. Sedimentation reduced the abundance of sensitive taxa, decreased biotic indices and was elevated in exotic forest, pastoral land use and soft geology. Calcium and conductivity levels were the parameters most strongly linked with macroinvertebrate community structure, with higher conductivity levels in exotic forest, pastoral land uses and soft geology. These results demonstrate the complex relationship between geology and land use. This is important for future regional decisions as the combination of land use in a certain geology type could have extrapolated negative effects on macroinvertebrate assemblages indirectly through changes in habitat and water chemistry. Chapter 2. The influence of forest harvest on macroinvertebrate communities in soft geology was investigated at fourteen stream sites in the upper Waipaoa River catchment, Gisborne, New Zealand. Stream reaches were surveyed under summer base flow conditions in March 2019 at sites that varied in time following forest harvest from recently harvested (0 years) to mature exotic forest (27 years). Macroinvertebrate communities were most influenced by turbidity, mean particle size, sediment size and bank erosion up to ten years following harvest. Macroinvertebrate communities following forest harvest were dominated by species sensitive to degraded stream conditions including Coleoptera and Diptera, with Diptera being one of the dominant taxa at all sites throughout the forest rotation. Fine sediment particle size did not change over the harvest cycle with all sites having silt present, and benthic sediment density decreasing as the forest established. The density of sensitive EPT taxa increased from ten years following forest harvest and communities became dominated by Hydrobiosis, Deleatidium, Austroclima and Coloburiscus. The QMCI increased with time since harvest with sites having high values after ten years. Recovery of macroinvertebrate communities was dependent on riparian shade, vegetation, hydraulic heterogeneity and habitat quality, with the recovery time being at least ten years following harvest. The results highlight the importance of retaining or establishing continuous riparian buffers along streams within harvested areas of forest to reduce negative impacts on freshwater communities.