Future directions

Future directions

In Phase I of RESIST, we have conducted a series of experiments and field studies to address the project’s three main hypotheses. These hypotheses were addressed to varying degrees; several were supported, but in some cases, inconclusive results were obtained. As a consequence, and also based on results published by other authors since 2020, we have decided to introduce moderate modifications to our research concept for the proposed RESIST Phase II.

Introducing new projects

Five additional projects will be included for Phase II that complement RESIST by strengthening the organismic focus on viruses, parasitic protists and aquatic fungi, and by complementing the hydrological modelling with a large-scale approach. The formerly separate projects of Phase I will be combined into a single project with a main focus on studying the links between food webs and stable isotopes.

Stronger focus on recovery from multiple stress

While Phase I mainly addressed stressor interactions in phases of degradation, the focus of Phase II will shift to recovery trajectories and mechanisms. Here, we will distinguish two main types: (1) Recovery from severe degradation, i.e. the eradication of major parts of the community; and (2) recovery from moderate degradation. While light or moderate degradation can be a consequence of all types of stressor impacts, severe degradation requires exceptionally strong stressor impacts, e.g. strong pollution or desiccation. Consequently, in Phase II of RESIST we will include two examples of recovery from severe degradation:

  • Recovery from experimentally generated and naturally occurring droughts
  • Recovery of the Emscher main stem, which was exclusively transporting raw sewage until recently. Since early 2022, it is wastewater-free and we can investigate the recovery process in a fascinating natural experiment.

These recovery processes will be compared to the recovery from moderate degradation, e.g. following temporary low flow, salinization or temperature increase. The intensity of degradation will be measured by environmental parameters (stressors); only if these are not available, they will be inferred from the biota (e.g. through Ecological Quality Ratios).

Adding drought as an additional stressor

Droughts / low flow is another consequence of the climate change. While low flow is affecting aquatic organisms indirectly, e.g. through increased salt and nutrient concentration, increased water temperatures or reduced oxygen contents, the drying of streams eradicates most organisms directly. Unlike the gradual degradation imposed by most other stressors, drought requires a complete recolonization e.g. from dormant stages or dispersing organisms. As a focus of Phase II will be on recovery, drought is the most appropriate stressor to rapidly initiate a complete recovery process.

Introducing new experimental systems

To address more specific hypotheses resulting from Phase I, several projects will perform individual lab experiments in well-established beaker systems or using lab flumes. A prototype of these indoor flumes has already been established and first trials have been successfully conducted.

Moreover, we plan to refurbish the ExStream system by introducing the SIGMA (stressor interaction gauging mesocosm approach) system. The general principle of SIGMA is the same as in ExStream. Water is redirected from an adjacent stream to header tanks on a scaffold, from which the water is gravity-fed into the flume-like mesocosms. This creates environmentally realistic physico-chemical conditions of the supplied water and allows for natural colonisation dynamics of the experimental units by organisms being transported with the water. Compared to the formerly used circular ExStream mesocosms (3.5 L, 25 cm diameter), the SIGMA flumes have a unidirectional linear flow, they are larger in volume, allowing for a deeper sediment layer, which is crucial to create sub-surface flow, and have more stable flow as they allow for higher discharge.

Finally, while indoor experimental systems and ExStream/SIGMA allow for a high level of control and replicated experiments, and the field studies for a high level of realism, Phase I lacked a system in between that combines these advantages. We will therefore complement the suite of experimental and observational systems with an additional component: replicated field flumes of 10 m length that are fed with discharged stream water and allow for a modification of discharge and habitat composition.

Extension of field work to the recolonization of the Emscher main stem

The restoration of the Emscher system is one of the largest river rehabilitation projects worldwide with an overall budget of 5.3 billion €. Originally a slowly flowing lowland river with surrounding wetlands, it was heavily polluted in the late 19th and early 20th century. More than 100 years ago, the Emscher and its tributaries were transformed into an open sewer system, because coal mining and subsequent subsidence of the terrain rendered sub-surface sewers impractical. Restoration included the removal of untreated sewage and hydromorphological restoration. The restored sections differ in their exposure to various stressors. In particular, many sections are still hydromorphologically modified and impacted by elevated water temperature and salinity. Since the beginning of 2022 the Emscher main stem, which transported raw sewage for a century, is wastewater-free.

While the sites in the Emscher catchment were initially restricted to the sub-catchment of the Boye, 14 additional sites in the Emscher main stem were included into the overall sampling design with the beginning of 2022, to explore the recolonization of the formerly wastewater transporting river. Here, sampling intervals were shorter with seven sampling events to date. These sampling programmes will be continued throughout Phase II, thus extending the existing time series and providing data for a multitude of purposes, including investigations of recolonization effects, of spatial and temporal variability in multiple stressor effects and of interactions between different taxonomic groups.

Improved coherence and scope of modelling approaches by using data beyond study catchments

In Phase II, the links will be strengthened and implemented in a top-down approach, securing more formal interfaces between models and the coverage of all scales and components relevant for testing the project’s main hypotheses. At the same time, we will maintain a variety of modelling techniques, including statistical and mechanistic modelling, tailor-made to address the individual project’s research questions. A main novelty of Phase II will be the extension of models from the scale of the Boye/Emscher and Kinzig catchments to larger scales, i.e. entire Germany including transboundary catchments. Much of the catchment-specific modelling activities will now be mirrored by respective models addressing catchments of the size of Rhine or Weser, allowing for a generalisation of results obtained in the case study catchments.

In parallel to the hydrological modelling of the Boye/Emscher and Kinzig catchments, we will build a hydrological model for the entire Germany that will provide hydrological and physico-chemical stressor variables on a fine resolution and for the period from 1950 on. These data will be used for relating both temporally and spatially distributed biotic data to stressor intensities and legacies. It will also be possible to compare the spatial and temporal response of different organism groups to increasing and decreasing stress. Moreover, the data will be used to mechanistically model metacommunities for a large Central European catchment since 1950, integrating temporal and spatial scales.

Introducing a public relation project

The results of RESIST are relevant for a wide range of stakeholders and for the general public. Newly established project will closely cooperate with regional water boards to communicate with representatives of the civil society and to transfer RESIST’s results to the relevant stakeholders. We will use Citizen Science and “Blue classrooms” approaches among other to better communicate and disseminate our science to the general public, pupils & students, teachers and water managers.