The effect of instream rehabilitation structures on macroinvertebrates in lowland rivers.
Many lowland rivers in Western Europe have been substantially modified to aid land drainage and support the intensification of agriculture. Although there have been many attempts at rehabilitation, few have been systematically evaluated on ecological criteria. Macroinvertebrates were assessed in 13 UK lowland rivers containing instream rehabilitation structures, seven with artificial riffles (intended to mimic natural gravel riffles) and six with flow deflectors (intended to increase flow, depth and substrate heterogeneity within the channel). In each river, invertebrates were compared between stretches of river with and without rehabilitation structures. Rehabilitated and reference stretches were subdivided into different benthic and macrophyte habitats. Three macroinvertebrate samples were taken once in July/August 1999 from each habitat across all schemes and rivers. Current velocity, depth and substratum particle size were recorded at the same time from each habitat. Artificial riffle benthos had faster current, a coarser substratum and was shallower than reference benthos. Depth and substratum particle size differed little between flow deflector and reference benthos, although velocity downstream of the deflector tip was greater, and velocity in the lee of the deflector lower, than reference benthos. At a habitat scale, the benthos of artificial riffles, but not flow deflectors, had higher abundance, taxon richness and diversity than reference benthos. The impact of artificial riffles was most marked for benthic rheophilic taxa. In all rivers, macroinvertebrate diversity was highest in marginal macrophytes and abundance highest in instream macrophytes. Although invertebrate communities were distinct between artificial riffle (but not flow deflector) and reference benthos, these differences were negligible in comparison to those between benthic and macrophyte habitats. Neither artificial riffles nor flow deflectors had any significant impact on the taxon richness of the benthos or of the rehabilitated stretch of the river as a whole. Invertebrate diversity of rehabilitated stretches related closely to that of reference stretches, indicating that larger scale factors constrained any impact of rehabilitation. Synthesis and applications. Local rehabilitation structures appeared to have minor biological effects in lowland rivers. We suggest that post-project appraisal should be more rigorously applied to rehabilitation schemes, measuring success against more clearly defined goals. We also advocate a greater emphasis on large-scale riparian, floodplain and catchment rehabilitation, rather than small-scale channel rehabilitation. Such a change in approach needs more effective cooperation and collaboration between all catchment users.