Using in-stream biotopes to assess the effectiveness of stream rehabilitation projects

Hydromorphological rehabilitation is increasingly being used to reverse degradation and destruction of stream and river ecosystems. There have been many criticisms of river rehabilitation projects, because many have not met their goals, while many others have not been monitored sufficiently well to assess whether their goals were met. With increasing investment in rehabilitation, there is an urgent need to develop effective approaches to assessing treatment efficacy and effect. The lack of appropriate monitoring has meant that the effectiveness of stream rehabilitation has generally not been rigorously demonstrated. This research proposes a novel, structure- and function-based methodology for evaluating linkages between river and stream “hydromorphological rehabilitation”, “in-stream biotope heterogeneity”, and “macroinvertebrate community structure and function” in support of reach-scale river and stream hydromorphological rehabilitation ecology. In Before-After-Control-Impact (BACI) study designs, in-stream biotopes and their macroinvertebrate assemblages as structural and functional units were used to assess the ecological effectiveness of two different river rehabilitation projects – one using large woody material installation and the second one using entire channel hydromorphological rehabilitation. Both rehabilitation projects were successful in enhancing the rehabilitated reaches’ in-stream biotope number and diversity. Macroinvertebrate density, biomass, richness, diversity, and secondary production values were increased significantly. Macroinvertebrate community taxonomic composition and functional composition were enhanced to become more similar to those of the natural reaches. Changes in in-stream biotope number and their percentages of cover were significantly related to changes in the rehabilitated reaches’ macroinvertebrate community metrics. The results of both projects indicate that comparing in-stream biotopes between reaches can provide a rapid method for monitoring rehabilitation outcomes. Macroinvertebrate structural and functional metrics can provide a quantitative basis for assessing reach-level rehabilitation outcomes if samples are collected in a random sampling protocol stratified at the in-stream biotope-level using a BACI design.