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

Abstract

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 3× lower on a per‐meter‐squared basis in the restored reach than in the upstream unrestored reference reaches (9764 vs. 29,636 mg DM/m2/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 3.4× 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.

Reconnecting a stream channel to its floodplain: implications for benthic diatoms and macroinvertebrate trophic structure

“…Approximately, 45% of mountain stream length in the western United States is categorized as mostly (20%) or intermediately (25%) disturbed (Stoddard et al 2005). Anthropogenically disturbed and degraded streams are increasingly the focus for restoring hydro-geomorphic processes and ecosystem functions to recover lost ecosystem services and provide climate change resilience (Wohl et al 2021;Jennings et al 2023). This is exemplified in the Pacific Northwest (PNW), which is one of the epicenters of innovative stream restoration in North America (Bernhardt et al 2005).…”

“…However, restoration best practice subsequently evolved and now takes a more process-based approach. The goal of many restoration projects is recovery of the morphological complexity and habitat diversity necessary to support robust aquatic and riparian ecosystems (Beechie et al 2010;Wohl et al 2021;Jennings et al 2023). Although the evidence base demonstrates that the benefits of process-based stream restoration is growing rapidly (Wohl et al 2021;Flitcroft et al 2022), quantifying the direct effects of process-based restoration on hydro-geomorphic processes and ecosystem functions remains a challenge (Jähnig et al 2011).…”