The effects of stream habitat restoration on benthic communities in boreal headwater streams

Abstract

The changes in stream benthic communities after restoration of channelized rivers was studied in northern headwater streams. More specifically, this thesis concentrates on the effects of restoration on short and long term changes in macroinvertebrate communities, habitat characteristics and coarse particulate organic matter (CPOM) retention in headwater streams. Restoration clearly increased the stream bed roughness and heterogeneity of the flow regime but not enough to mimic natural rivers effectively. Rivers were still, after restoration, characterized by too fast flows and deep waters. The CPOM retention capacity increases but not enough to reach the level of natural rivers. The restoration process detached mosses from large areas of the stream bed, and the full recovery of mosses was observed to last over a decade. As an immediate response to restoration disturbance, all abundant taxa were evenly reduced, thus leaving the structure of the benthic community essentially unchanged. The short-term recovery of stream macroinvertebrates after restoration was rapid, showing the great resilience of the stream biota. The long-term effects on macroinvertebrate communities were minor, even so small that they were hard to detect. The macroinvertebrate richness of natural rivers was not reached. The increased retention capacity of CPOM did not remove the resource limitation of detritus-feeding invertebrates. The results of the restoration were not a great success ecologically. The restoration may have been the first step to the right direction by increasing the heterogeneity of the stream bed and the flow regime, but the macroinvertebrate richness and CPOM retention capacity of natural rivers were still not reached. I suggest that the following three key factors, (i) the loss of mosses, (ii) the absence of debris dams and (iii) insufficient heterogeneity of the stream bottom, which slow down the recovery process considerably, be given adequate consideration in order to restore our channelized rivers more effectively.