1. Riparian management has been embraced by water and land managers globally to offset the deleterious effects of intensive agricultural land use on aquatic ecosystems. However, the documented responses of stream communities to riparian management have been variable, particularly in highly degraded systems.
2. We used boosted regression trees and structural equation models to assess the effects of riparian condition and stream size on the invertebrate communities of 64 agricultural waterways on the Canterbury Plains, New Zealand. We hypothesized that small streams would be more degraded than larger waterways but would show a greater increase in the abundance of pollution-sensitive aquatic invertebrates in response to riparian management. We also predicted that land-use legacies of poor in-stream habitat would reduce the effectiveness of current riparian management. The two strongest determinants of community structure were primarily in-stream habitat, where sedimentation and low water velocity had negative impacts on stream communities, and stream size, with smaller waterways generally more impacted than large waterways. Not surprisingly, with >150 years of agriculture and patchy riparian management on the plains, current management has not greatly improved in-stream habitat and thus had little effect on the abundance of sensitive aquatic insect (EPT) taxa.
3. Managed streams did, however, have more pollution-sensitive communities in general. This was largely mediated by decreased stream temperature, narrower/deeper channels and greater organic matter resources in streams with riparian planting and restricted stock access. Thus, if water velocity and sedimentation issues can be mitigated, then riparian management should become more effective.
4. Synthesis and applications. Within the context of a degraded agricultural landscape, we identified factors limiting the effectiveness of riparian management for stream invertebrate communities. Riparian management should primarily target and protect small streams and those without degraded in-stream habitat. Intensive management, such as in-stream habitat or channel morphology modification, may be needed to address historical factors (e.g. low velocity and sedimentation), which otherwise may continue to limit community recovery.