When native grassland catchments are converted to pasture, the main effects on stream physicochemistry are usually related to increased nutrient concentrations and fine-sediment input. We predicted that increasing nutrient concentrations would produce a subsidy-stress response (where several ecological metrics first increase and then decrease at higher concentrations) and that increasing sediment cover of the streambed would produce a linear decline in stream health. We predicted that the net effect of agricultural development, estimated as percentage pastoral land cover, would have a nonlinear subsidy-stress or threshold pattern. In our suite of 21 New Zealand streams, epilithic algal biomass and invertebrate density and biomass were higher in catchments with a higher proportion of pastoral land cover, responding mainly to increased nutrient concentration. Invertebrate species richness had a linear, negative relationship with fine-sediment cover but was unrelated to nutrients or pastoral land cover. In accord with our predictions, several invertebrate stream health metrics (Ephemeroptera-Plecoptera-Trichoptera density and richness, New Zealand Macroinvertebrate Community Index, and percent abundance of noninsect taxa) had nonlinear relationships with pastoral land cover and nutrients. Most invertebrate health metrics usually had linear negative relationships with fine-sediment cover. In this region, stream health, as indicated by macroinvertebrates, primarily followed a subsidy-stress pattern with increasing pastoral development; management of these streams should focus on limiting development beyond the point where negative effects are seen.

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.