Stream Communities Along a Catchment Land-Use Gradient: Subsidy-Stress Responses to Pastoral Development |
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Authors: | Dev K Niyogi Mark Koren Chris J Arbuckle Colin R Townsend |
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Institution: | (1) Department of Zoology, University of Otago, Dunedin, New Zealand;(2) Present address: Department of Biological Sciences, University of Missouri-Rolla, Rolla, MO 65409-1120, USA |
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Abstract: | 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. |
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Keywords: | Streams Land cover Land use Macroinvertebrates Stream health Nutrients Sedimentation |
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