The Gelså River Restoration Revisited: Community persistence of the macroinvertebrate community over an 11-year period

The study was undertaken on the River Gelså, Denmark, where a 1.8 km meandering course was estab-lished in 1989 to replace a channelized river reach. This restoration project was the first of its kind inDenmark and has the longest time-series of post-intervention data of any restoration project conductedworld-wide. Additionally, a 0.5 km upstream (control) reach that remained channelized has been sampledsince 1989. In this paper, we examined macroinvertebrate assemblages in distinct habitats in 2008, 19years after the restoration, and community persistence between two years, 1997 and 2008, to investigatethe longer-term effects of restoration on the biota. We found that habitat type influenced macroinverte-brate community composition to some degree, while there were no clear effects on - and -diversity ofhabitat or reach type. Stony substrate habitats introduced as part of the restoration could, however, stillbe separated from other habitat types and were much more frequent in the restored reach. Furthermore,very little change had occurred over the 11-year period from 1997 to 2008, suggesting a high degree ofcommunity persistence. Our results suggest that the local species pool was already close to saturationin 1997 and that only limited immigration of new species occurred in the intervening period until 2008.The lack of long term benefits could be attributed to the simultaneous cessation of weed cutting (whichhad almost as big a positive impact as restoration), other types of stress on the river (eutrophication)and dispersal limitations. However, it might also reflect that River Gelså is still functionally channelizedand is far from exhibiting a dynamic river morphology governed by natural processes that create a rangeof habitats for the biota and this might explain why there has not been a more pronounced increase inmacroinvertebrate diversity in River Gelså.