The highly industrial metropolitan Ruhr area of Germany is one of the largest in Europe. For over a century, its streams have been exposed to massive pollution and degradation by industrial activities and domestic wastewater, with many of them transformed to concrete channels and used as open sewers. However, over the last three decades, the entire stream network has been gradually restored, spurred by investments exceeding €5 billion. The aim of the present study was to assess to what extent these improvements of stream hydromorphology and water quality have entailed changes in leaf-litter decomposition, a pivotal component of stream ecosystem functioning. We conducted a decomposition experiment at 20 stream sites differing in the time when restoration measures were completed. Freshly fallen leaves were collected in autumn, placed in mesh bags and submerged in the streams in late October 2022. Six pairs of fine-mesh and coarse-mesh bags per site were retrieved after 5 weeks to determine mass loss, microbial respiration, fungal sporulation, and the community composition and contribution to mass loss of leaf-shredding macroinvertebrates. In contrast to our hypothesis that decomposition is least efficient at recently restored sites, decomposition dynamics proved similar across all streams, with highly consistent microbial decomposition but important variability when macroinvertebrates could access leaves. All other response variables proved insensitive to restoration history. This suggests either extremely rapid recovery of ecosystem functioning, no effect of stream degradation prior to restoration, or, most likely, persisting functional impairment across the stream network, including at reference sites previously considered unimpacted.

Predicted future climatic change stressors and land-use drivers pose a significant threat to freshwater ecosystems, aquatic communities as well as associated ecosystem services. Improved understanding of the complex interactive effects between climate and land-use stressors are necessary for freshwater management in order to inform mitigation of potential negative impacts. The aim of this study was to investigate the individual and combined effects of two climate stressors (increased variability of flow velocity, carbon dioxide enrichment) and a key land-use stressors (deposited fine-sediment) as well as shading on stream benthic macroinvertebrate communities. To address this aim, a full-factorial experiment of these four factors was carried out using the ExStream System (an experimental stream mesocosm system comprising of 128 circular stream channels) in Annamoe, Co. Wicklow, on the east coast of Ireland between May and July 2022. Stressors were either applied in pulses or continuously. Stressors applied in pulses included increased flow velocity variability (pulsed fast- and slow-flow periods vs. constant velocity) and deposited fine-sediment (two pulses vs. absent). While shading and carbon dioxide were applied continuously and were either present/enriched or absent/ambient, respectively. The results focus on the macroinvertebrates remaining in the mesocosms at the end of the experiment. The findings are expected to inform catchment management to mitigate negative impacts of climate change stressors and provide further empirical support for mechanistic frameworks of stressor interactions.

National macroinvertebrate diversity has steadily improved in England over the past 30 years, but why? In this study we matched macroinvertebrate records over 30 years with location, chemical and physical records across England. The measurement locations and sample timings were different, so this required considerable effort to reconcile 62,000 biological records with about 500,000 chemical (and physical) records. We selected 1,519 macroinvertebrates sites across England from the Freshwater river macroinvertebrate surveys (Biosys) dataset, on the basis of their long and consistent records. The analysis focused on family richness, Ephemeroptera, Plecoptera, Trichoptera (EPT) family richness, BMWP_NTAXA and BMWP_ASPT. In this exercise, we matched macroinvertebrate sites with the nearest chemical measurement sites from the WIMS chemicals dataset. All environments showed improvements, although high upstream urban land use exerted a greater drag than high wastewater exposure or high cropland use in the catchment. Whilst improvements in family richness and BMWP_NTAXA have slowed in the last decade, EPT family richness and BMWP_ASPT have not. This implies that water quality is no longer the key limiting factor. We used GLMM statistical analysis to identify which stressor or stressors can best explain the variability in macroinvertebrate family richness over 30 years in English rivers. All possible combinations were compared by GLMM which requires up to 26,000 different model runs (no a priori assumptions are made). Zinc, copper, ammonia and BOD are currently coming out strongly as most closely associated with the selected indices for macroinvertebrate diversity. The latest results will be presented at the conference.

Effects of multiple stressors on stream ecosystems and their recovery are often analysed with a focus on structural community parameters such as species richness and community composition. However, not only biodiversity but also ecosystem functions may be affected by stressor increase and release. By assessing food webs, a community’s structure can be linked to its function, e.g. energy and matter pathways and fluxes. In order to analyse the effect of stressor recovery on food web structure, we measured stable isotopes (δ¹³C, δ¹⁵N) from benthic macroinvertebrate communities sampled annually over 11 years at different stream sites in the Boye River catchment (North Rhine-Westphalia, Germany). All sites had formerly been used as open sewers and were restored between 1993-2011. We analysed responses of food web parameters, e.g. food chain length, resource diversity, and trophic niche width, and linked them to time since restoration. Our results show that time since restoration affects food chain length positively. Additionally, resource diversity showed a negative response to time since restoration; probably the increasing amounts of leaf litter input with re-growth of the riparian vegetation became the dominating food source with time. Overall, our study demonstrates the value of food web analysis for multiple stressor and restoration research by combining complex structural and functional responses.

Macroinvertebrate dispersal plays an important role for recovery of streams from multiple stressors. Predictions of the dispersal capabilities of macroinvertebrates often rely on indirect proxies at species level (e.g., dispersal traits) rather than on actual dispersal measurements at the individual specimen level. This renders predictions of dispersal highly uncertain. Using stable isotope enrichment (¹⁵N), we non-invasively labelled large quantities of macroinvertebrates at eight sites in sand-bottom lowland streams of Germany. At each site, between 15 and 40 g ¹⁵NH₄Cl were dissolved in 40 l pure water and slowly released to the water column over a period of 42 days (enrichment). During enrichment, flow measurements were frequently taken at all sites. After enrichment, benthic invertebrates, phytobenthos, and coarse particulate organic matter (CPOM) were sampled repeatedly to evaluate the enrichment with and depletion of ¹⁵N in the biomass over a distance of up to 2 km downstream of the release point. In order to estimate the total number of labelled individuals, larval density and adult emergence of selected species were determined using hand nets and emergence traps, respectively. Overall, between approx. 45.000 (Potamophylax rotundipennis) and 4.000.000 (Gammarus sp.) individuals could be labelled with ¹⁵N at the eight sites. Species-specific results show that enrichment was highest in grazers (and phytobenthos) as compared to shredders (and CPOM) and predators. Our findings show that isotope enrichment is suited to label large quantities of macroinvertebrates across species and feeding types, thus providing a sound basis for the measurement of their dispersal distances and rates.

The exposure of biological systems to various stress factors is widespread under natural conditions. So is adaptation of individuals, populations, and communities to natural and anthropogenic stressors. The basic processes of effect and adaptation are known; however, the quantification of the stress-effect relationships is only possible to a very limited extent because the biological effect is determined by the combination of stressors and the adaptation by the duration of exposure to the stressors and the dynamics of the environmental conditions. In the case of incomplete adaptation, a trade-off is observed that leads to a reduction in performance, especially under multiple stress conditions. The lecture will provide examples for the processes listed here and furthermore shows possibilities of quantifying these processes.