Longitudinal changes in biota along four New Zealand streams: Declines and improvements in stream health related to land use

Abstract

We studied four streams in southern New Zealand in 2002 to document downstream changes in water quality, habitat, and stream biota in relation to land use. Two streams were in catchments that had increasing intensity of agricultural development downstream from relatively pristine headwaters. A third stream had the most intense land use in the headwaters and a riparian corridor of regenerating native forest along its middle reaches. A fourth stream had low intensity pasturing in its lower reaches, but also downstream increases in sedimentation from natural and historic mining sources. Four to six sites were sampled along each stream. Pastoral land cover in catchments was positively related to nutrient concentrations and fine sediment cover in the streams. At the most agricultural sites, dissolved inorganic nitrogen reached concentrations of 2 mg/litre, and fine sediment covered 33% of the stream bottom. Several biotic indices for invertebrates, including the Macroinvertebrate Community Index (MCI), were lower at the agricultural sites, which had MCI scores around 100. The indices were negatively related to fine sediment cover. The site with the intact riparian zone had declines in sedimentation downstream, which were paralleled by increases in invertebrate richness and biotic indices. Our findings support the notion that the restoration of riparian zones can improve stream habitat and invertebrate health.

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