Start at zero: succession of benthic invertebrate assemblages in restored former sewage channels

We analysed the development of benthic invertebrate assemblages in restored urban streams in Germany. Prior to restoration these streams were open sewers and not inhabited by benthic invertebrates except Oligochaeta. Therefore, almost all taxa recolonised the streams after restoration. Restoration included the improvement of the water quality and the hydro-morphology. Following stream restoration, benthic invertebrate assemblages are supposed to undergo a distinct succession, as new habitats have been generated. These processes are poorly understood in streams. Our study revealed succession patterns and processes of benthic invertebrate assemblages in restored streams formerly transporting sewage, with pioneer taxa being dominant in the first years and a gradual assemblage maturation. We sampled 13 sites in seven restored streams in the springs of 2012 and 2013. Seven of these sites are connected to near-natural sections, whereas six sites lack this connection. The 13 sites differ in time since restoration and were sampled between one and 20 years after restoration. Additionally, we sampled 21 near-natural sites within the catchment and 11 near-natural sites in neighbouring catchments as potential recolonisation sources. Within 1 year, the restored sites underwent succession, which led to a higher resemblance of their assemblages to those of source sites. Derived from change values and non-metric multidimensional scaling, assemblages of young restored sites changed more than assemblages of old ones. In the first years after restoration we found assemblages with high abundances of pioneer taxa, while 5 years after restoration assemblages were increasingly similar to those of the source sites and mature assemblages were observed a decade after restoration. The succession towards near-natural assemblages is influenced by the instream habitats, catchment conditions and the recolonisation sources in the surroundings. Our findings reveal that monitoring results obtained fewer than 10 years after restoration will still be influenced by ongoing succession.