Effects of process-based floodplain restoration on aquatic macroinvertebrate production and community structure

Recent stream restoration approaches, such as Stage 0 restoration, aim to restore natural processes to regain lost ecosystem functions, but project implementation can also represent a reach-scale disturbance. Assumed outcomes of these restoration actions, like greater biological productivity, are rarely evaluated. In this study, we examined the short-term effects of Stage 0 floodplain restoration on the secondary production of aquatic macroinvertebrate communities in the South Fork McKenzie River, Oregon, 1-2 years following project implementation. We seasonally sampled macroinvertebrates from benthic and submerged wood surfaces in the restored reach, and two unrestored reference reaches located upstream, to estimate annual secondary production. Macroinvertebrate production estimates were 3x lower on a per-meter-squared basis in the restored reach than in the upstream unrestored reference reaches (9764 vs. 29,636 mg DM/m(2)/yr). However, because there was 4.5-times greater wetted area available in the restored reach, overall macroinvertebrate production per unit of valley length was 3.4x higher in the restored reach than in unrestored reference reaches (2744 vs. 802 kg DM/km). Additionally, the mosaic of aquatic habitats created via restoration (main-channel, side-channel, and wetted forest habitats) supported a diversity of macroinvertebrate assemblages both within and among reaches. Our findings suggest Stage 0 project implementation, which can include dewatering and filling incised channels, may reduce aquatic macroinvertebrate production on a per-unit-area basis for at least 1 or 2 years following restoration. However, this short-term disturbance effect may be offset by channel aggradation and widening, which can provide a more wetted area for macroinvertebrate production and may support greater macroinvertebrate community diversity. Future studies are needed to examine the longer term (2-10 years) aquatic macroinvertebrate response to Stage 0 restoration, and the impacts of shifting resource availability on stream fishes.