Ecological evaluation of restored former sewage channels in the urbanised Emscher catchment
Background My thesis aimed at the ecological evaluation of restored former sewage channels located in the highly urbanised Emscher catchment in western Germany. Prior to restoration the study streams had been used as open sewers for decades and benthic invertebrate life was not possible except for some sewage tolerant Oligochaeta. Restoration measures included the construction of underground sewers for the wastewater, the near-natural remodeling of riparian areas and of the stream bed. The unique situation in the streams of the Emscher catchment allowed to investigate the recolonisation of restored urban streams by benthic invertebrates and follow-up, the restoration success and additionally, the primary factors influencing the recolonisation. According to literature the following factors predominantly influence recolonisation: the recolonisation potential (e.g. Sundermann et al. 2011a; Tonkin et al. 2014), the species dispersal capability (Cañedo-Argüelles et al. 2015), the environmental conditions and landscape context (Hughes et al. 2008; Reynolds et al. 2013) and the succession processes (McCook 1994). These influencing factors were mainly investigated in streams of the open landscape. Therefore, their influence on the urban streams of the Emscher catchment was analysed in this thesis. For this purpose, new indicators were developed and indicators of the Water Framework Directive (WFD) were used. The WFD aims at improving the chemical and biological quality of European waters. For heavily modified water bodies, like the streams of the Emscher system, the goal is to reach the “Good Ecological Potential” until 2015 (or under certain circumstances until 2021 or 2027) (European Commission 2000). In summary, this thesis focussed on the ecological evaluation of the three main topics: First, the ecological assessment of the restored streams after restoration according to the WFD and environmental parameters influencing the Good Ecological Potential; second, the primary colonisation by benthic invertebrates after restoration and their recolonisation patterns; and third, the succession of benthic invertebrates communities and environmental parameters steering the succession process. Ecological assessment The thesis starts with an overview of the ecological status of restored sites in the whole Emscher catchment. Based on 248 taxa lists of benthic invertebrates sampled in restored sites by different sampling methods, the analysis focused on the Ecological Potential according to the requirements of the WFD. As possible explanatory parameters for the Ecological Potential, amongst others, riparian land use, stream habitats, and time since restoration were included into a PCA analysis. Almost 40 % of the sites already achieve the Good Ecological Potential at the recent sampling. Environmental parameters enhancing the probability of meeting the Good Ecological Potential include: connection of restored sites to an unmodified stream section upstream, dead wood in the stream bed, good hydro-morphological structure, deciduous riparian vegetation and unsealed surface in the stream´s surrounding, while the occurrence of iron ochre and sewage overflows located upstream of the sampling sites hinder the achievement of the Good Ecological Potential. This study also reveals that the Ecological Potential of the restored streams in the Emscher system is only indirectly determined by the factor time. Although the statistical analysis presented this factor as most influencing, in the minority of cases a parallel development of the Ecological Potential and time was found. Instead, the above mentioned environmental conditions of a restored site are of greater importance for the assessment and the achievement of the Good Ecological Potential. In conclusion, the factor “time since restoration” must be interpreted as a proxy for the overall development of the (aquatic and terrestrial) habitats in and at the restored sites. Recolonisation The recolonisation success and a good ecological assessment of the streams are amongst others dependent on recolonisation sources and the dispersal capabilities of taxa. Therefore, in a second step I analysed the recolonisation processes by benthic invertebrates. For this analysis, the case study catchment of the Boye, a 77 km² sub-catchment of the Emscher was chosen. This catchment has a high number of restored streams with almost the same ecological conditions as the whole Emscher catchment. In the spring 2012, seven restored sites connected to near-natural upstream sections were sampled, which were never used as sewage channels and are in good status morphologically. Furthermore, six unconnected restored sites were sampled. Restoration measures had been conducted between one and 19 years before sampling. Additionally, 21 near-natural sites within the catchment and eleven near-natural sites in neighbouring catchments were sampled. Near-natural sites were considered to be potential source sites from which benthic invertebrates might colonise the restored sites. 128 taxa were recorded and were categorised into five dispersal classes reflecting dispersal capabilities and degree of ecological specialisation, according to a literature review. Assemblages at restored sites were characterised by lower numbers of taxa and/or high abundances of hololimnic taxa and poorly dispersing winged species and by higher species numbers and abundance of strongly dispersing generalists. A recolonisation sequence was derived from the observed patterns, in which winged, strongly dispersing generalists colonised most rapidly and were followed by hololimnic species, weakly dispersing generalists and habitat specialists. Restored sites connected to near-natural upstream sections were colonised more rapidly than unconnected restored sites, particularly by habitat specialists. Almost 90 % of the recolonisation events originated from sources within a distance of 5 km. A succession from pioneer assemblages to more mature communities, which resembles that of the surrounding near-natural sites, was observed. In summary, assemblages in connected, restored sites needed 9 to 19 years to reach maturation, while the settlement of assemblages in unconnected sites are expected to require more time. Succession While successional changes of assemblages in lakes or wetlands are well documented, these processes are poorly understood in streams. Following stream restoration and primary recolonisation the benthic invertebrate assemblage is also supposed to undergo a succession, as new habitats have been generated. These successional changes are important to predict the taxonomical development, thus indirectly the development of the ecological assessment, of restored sites. Therefore, the same 13 sites in the seven restored streams as of the Boye sub-catchment in chapter 3 were investigated again in the spring 2013. For each site environmental parameters expected to steer the succession process were collected. Their influence on the inter-annual taxonomical change was tested with correlation analyses (Spearman’s rho). The 21 near-natural sites within the Boye catchment and 11 near-natural sites in neighbouring catchments sampled in 2012 served again as source sites for the analysis. Within 1 year time, the restored sites have undergone further succession, which lead to a higher resemblance of their assemblages to those of the source sites. These results were derived from similarity analyses, non-metric multidimensional scaling and therefrom developed change values, which show the taxonomical change of a site after 1 year dependent of the recolonisation sources. The assemblages of young restored sites changed more markedly than assemblages of old restored sites within the time span of 1 year. In the first years after restoration instable assemblages with high abundances of eurytopic pioneer taxa were found, while 5 years after restoration assemblages were increasingly similar to those of the source sites and mature assemblages were observed 9 to 10 years after restoration. Differences between young restored sites connected and unconnected to near-natural upstream sections were observed, suggesting a strong colonisation with organisms from upstream sections particularly in the first years. The succession towards near-natural assemblages is further supported by recolonisation sources in the surroundings, the presence of gravel/stones at the stream bottom and a low share of urban land use in the surrounding. Especially urban land use is associated with several stressors (e.g. diffuse inputs, morphological degradation, hydraulic stress) leading to unstable, less predictable and less favourable conditions. Conclusion and future prospects From the results of the first study, suggestions, as the creation/enhancement of growth of deciduous woody riparian vegetation along buffer strips of the streams, the reduction and improvement of sewage overflows, the provisioning of a connection to the streams tributaries, and active addition of dead wood to the streams, for further optimisation of the restoration of urban streams were derived. They serve as recommendations to improve restoration measures in the future. Furthermore, the second study showed that the establishment of mature habitat conditions, in particular woody riparian vegetation, is a prerequisite for the recolonisation of habitat specialists, which indicate the maturation progress of a restored site. In the planning phase of a restoration this knowledge can be used to especially create habitats in order to promote the recolonisation of sensitive species. Several streams of the Emscher catchment are still isolated after restoration and not directly connected to colonisation sources. For these streams a possible approach might be an assisted migration of invertebrates, which would not reach these streams on their own. This can probably help to reach the target assemblage and the Good Ecological Potential in due course. The results of the second and third study suggest that the invertebrate assemblages will reflect the restoration effects in sense of maturation at earliest 5 years, but more likely a decade, after restoration. Applying the “change value” used in the third study, timeframes for the monitoring of restored sites can be aligned for each river network, dependent of the recolonisation sources of the surrounding and the environmental conditions of the study streams´ catchment. My findings about succession help to set realistic goals in stream restoration. It can be detected at an early stage, whether the restoration target or the achievement of the Good Ecological Potential is realistic for a stream or not. Often, the success of a restoration measure initially does not result in the Good Ecological Potential. Nevertheless, it is important to know, whether a site has reached a “small success”, in terms of a taxonomic improvement, after a restoration measure. For this purpose, the dispersal classes of the second study can be used. The dispersal classification is transferable to other catchments and additional taxa could be classified following the described rules. As the number of taxa representing different dispersal classes is more constant than species richness or similarity patterns, dispersal classes could be used as a generic trait to analyse assemblage maturation. Small successes like the recolonisation of demanding taxa can get apparent. To better understand the processes of recolonisation and succession, long term studies, ideally also addressing population genetics, are advisable in the future.