Changes in agricultural intensity and river health along a river continuum

1. The impact of agricultural activities on waterways is a global issue, bu t the magnitude of the problem is often not clearly recognized by landowner s, 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 we re sampled along a 119-km stretch of the river. Headwater sites were surrou nded by low-intensity sheep farming, with high-intensity pasture and dairyi ng occurring in the mid-reach and lower reaches. 3. Water clarity decreased significantly from about 6 m in the headwaters t o less than 2 m in the lower reaches. Benthic sediment levels increased sig nificantly downriver, peaking at 35 mg m(-2) below several tributaries with high-intensity agriculture in their catchments. Periphyton levels were als o significantly greater in the lower reaches than the headwaters, and coinc ided with increased nitrogen (DIN) and phosphorus (SRP) concentrations. 4. Macro-invertebrate species richness did not change significantly through out the river, but species composition did with Ephemeroptera, and to a les ser extent, Plecoptera and Trichoptera dominating the headwater sites (wher e there was high water clarity, and low nutrient and periphyton levels). Do wnriver these assemblages were replaced by molluscs, oligochaetes and chiro nomids. 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 R iver. 6. The present results indicate that quantifying agricultural intensity wit hin a catchment, particularly relative Livestock densities, may provide a u seful tool for identifying threshold levels above which river health declin es.