Restoration of riparian vegetation is widely recognized as a tool in stream rehabilitation, but information on whether local riparian characteristics can mitigate the effects of catchment-level stressors on in-stream processes is limited. We measured community metabolism in 21 streams in the Canterbury region of New Zealand along 2 independent gradients of agricultural intensity and riparian cover (from closed canopied to open canopied) to assess relative effects of landscape and local factors on stream trophic state. We measured stream metabolism with the single-station open-channel diel O2 method. We found a correlation between gross primary production (GPP) and ecosystem respiration (ER), indicating a gradient of trophic states across sites. Streams were strongly heterotrophic with P:R values varying from 0.01 to 0.25. GPP and ER increased with % agriculture and % macrophyte cover, but decreased with % shade from riparian vegetation. Hierarchical partitioning analysis indicated that % agriculture was the only landuse variable to have a significant independent effect on GPP and ER. Among local variables, % shade and % macrophyte cover had significant independent effects on GPP. Percent shade was the only local variable to have a significant independent effect on ER. Percent shade had a stronger effect on both GPP and ER than did % agriculture, and a trade-off exists between the importance of agricultural and forest cover on stream metabolism at different spatial scales. Our results highlight the role of local riparian conditions in controlling trophic state and the importance of riparian buffers as a tool to mitigate eutrophication in streams and rivers.
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Vol. 33 • No. 1