The Emscher is a river, that flows through the Ruhr area in North Rhine-Westphalia in western Germany. Its overall length is 83 kilometres. Emscher and its tributaries had the reputation until the middle of 20th century of being the catchment with the greatest pollution in Germany. Since the 1860s and the beginning of the industrialisation, the streams were canalized and used to discharge the untreated wastewater of industries, agriculture and coal mining. When coal mining came to an end, Emscher and its tributaries started being restored with the aim to reach a good ecological status, as part of the effort directed towards the European Water Framework Directive (2000/60/CE). Underground sewers were built to transport the wastewater, the stream beds and the riparian areas were restored.
In order to assess the effect of restoration based on benthic diatom communities, we sampled 9 sites in five restored rivers of the Boye, a tributary catchment of the Emscher (Fig 1). The sites differ in time since restoration and were sampled between 9 and 27 years after restoration. The main aim of this study was also to assess the ecological status and to show the seasonal differences between samplings in summer and spring.
We also observed the functional groups and the behaviour of diatoms in restored rivers at the Boye catchment. The diversity of diatoms makes them an excellent bioindicator especially of rivers. The biological metrics of biomonitoring are life-forms, size classes and ecological guilds. The diversity of life-forms can be regrouped in larger groups, so called ecological guilds. An ecological guild is a group of taxa belonging to the same functional group that live in the same environment but may have adapted differently to abiotic factors. Three kinds of ecological guilds were taken into account in this study: low-profile guild, high-profile guild and motile guild.
The diatom communities at the Boye catchment in summer comprised 119 species. Downstream more species richness were found comparing to upstream and middle stream, which could be due to a higher concentration of nutrients permitting a higher number of species to survive. Moreover, there was a high diversity and dissimilarity between the sites according to the species.
To see the seasonal differences between the samplings, we observed the most common species in summer and in spring (Fig 2). In summer the pioneer species dominated, which occurred with a lower number in spring. But according to the permutation test, the season did not have a significant effect on diatom community composition and on abundance at the Emscher/Boye catchment, because the p value was greater than 0.05.
Both in summer and in spring very tolerant species occurred, which can resist different abiotic factors, so this could be the reason, why we didn’t have seasonal differences of the diatom community composition and abundance at the Boye catchment. As we saw, the ecological status was in summer as good as in spring, so this could be an effect of restoration.
The pioneer species are R-strategists, this means that they are very small species and have a very short time of reproduction. They also can easily adapt to new habitats and are more competitive there, so this explains their high number in summer comparing to spring.
Several trophic and saprobic diatom indices according to the trophic level and organic pollution showed, that the samples at the Boye catchment in summer and in spring were in a good ecological status, except of one sample, which was in a very good ecological status in summer and one other sample, which was in a mediocre ecological status in spring.
In nutrient poor sites, the low-profile guild dominated, because they do not tolerate nutrient enrichment. In nutrient rich sites, the motile and the high-profile guild should dominate, because they favour environments with high nutrients, but this was not the case.
According to the trophic level and organic pollution in rivers, the low-profile guild dominated in almost all of the sites at the Boye catchment. In nutrient-rich sites dominated the variable guild instead of high-profile and motile guilds. The variable guild can build the taxa, which can change their life-form during their life, to survive interspecies competition or to resist different abiotic factors.
Time since restoration, had a significant effect on abundance of diatom communities at the Boye catchment. The analysis of variance (ANOVA) showed a p-value lower than 0.05.
The species richness of diatom communities showed variation, it went up and down between the sites, which were restored in different years. Most of the samples improved during the time of restoration, except of one sample in spring, which was restored in year 2005, because the sample was in a mediocre ecological status, comparing to the others (Fig 3).
We didn’t have seasonal differences between the diatom communities and the abundance at the Boye catchment by this observation. The next step will be to observe more different years of restoration in order to have better results, including recently restored and reference rivers (never canalized), so we can see more precisely the differences between the time of restoration.
Figure 1. The location of the study area. V_43 (Vorthbach_unten), V_57 (Vorthbach mitte), V_42 (Vorthbach_oben), B_45 (Boye_unten), B_46 (Boye_oben), H_41(Haarbach unten), H_60 (Haarbach oben), W_48 (Wittringer-Mühlenbach_unten), K_39 (Kirchschemmsbach_unten)
Figure 2. Low-profile: Achnanthidium minutissimum (a), Achnanthidium saprophilum (b), Meridion circulare (f), Encyonema silesiacum (d), Planothidium lanceolatum (g), Planothidium frequentissimum (i). High profile: Gomphonema saprophilum (e). Motile: Caloneis lancettula (c), Surirella brebissonii var kuetzingii (h)
Figure 3. Species richness in spring and in summer since time of restoration
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