Changes in agricultural intensity and river health along a river continuum

• Published in: Freshwater biology Vol. 42; no. 2; pp. 345 - 357
• Main Authors: Harding, JoN. S., Young, Roger G., Hayes, JohN. W., Shearer, Karen A., Stark, JohN. D.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01-09-1999; Blackwell Science; Wiley Subscription Services, Inc
• Subjects: Animal, plant and microbial ecology; Applied ecology; benthic invertebrates; Biological and medical sciences; biotic indices; Chironomidae; Conservation, protection and management of environment and wildlife; Environmental degradation: ecosystems survey and restoration; Ephemeroptera; Freshwater; Fundamental and applied biological sciences. Psychology; Invertebrata; land use; New Zealand; New Zealand, Pomahaka R; Plecoptera; river continuum; species richness; stream health; Trichoptera; water quality; New Zealand; Oceania; New Zealand, South I; Environmental factor; Zoobenthos; Multivariate analysis; Land use; Freshwater environment; Species diversity; Biotic integrity index; Animal community; Environment impact; Watershed; Water quality; Agriculture; Stream; Invertebrata; Otago New Zealand; Species richness; Anthropogenic factor; Environment quality
• ISSN: 0046-5070; 1365-2427
• DOI: 10.1046/j.1365-2427.1999.444470.x

1. The impact of agricultural activities on waterways is a global issue, but the magnitude of the problem is often not clearly recognized by landowners, and land and water management agencies.
2. The Pomahaka River in southern New Zealand represents a typical lowland catchment with a long history of agricultural development. Fifteen sites were sampled along a 119‐km stretch of the river. Headwater sites were surrounded by low‐intensity sheep farming, with high‐intensity pasture and dairying occurring in the mid‐reach and lower reaches.
3. Water clarity decreased significantly from about 6 m in the headwaters to less than 2 m in the lower reaches. Benthic sediment levels increased significantly downriver, peaking at 35 mg m−−2 below several tributaries with high‐intensity agriculture in their catchments. Periphyton levels were also significantly greater in the lower reaches than the headwaters, and coincided with increased nitrogen (DIN) and phosphorus (SRP) concentrations.
4. Macro‐invertebrate species richness did not change significantly throughout the river, but species composition did with Ephemeroptera, and to a lesser extent, Plecoptera and Trichoptera dominating the headwater sites (where there was high water clarity, and low nutrient and periphyton levels). Downriver these assemblages were replaced by molluscs, oligochaetes and chironomids.
5. Canonical correspondence analysis indicated that agricultural intensity and physical conditions associated with agriculture activity (e.g. impacted waters, high turbidity and temperature) were strongly associated with the composition of benthic assemblages at differing reaches down the Pomahaka River.
6. The present results indicate that quantifying agricultural intensity within a catchment, particularly relative livestock densities, may provide a useful tool for identifying threshold levels above which river health declines.