Project A03 focuses on microphytobenthos, in particular diatoms. In Phase I, the project tested hypotheses related to stressor effects and interactions, speed of community recovery, and decoupling of community function and composition. In ExStream mesocosms, microphytobenthic communities showed priority effects two weeks after stressor removal, while field studies of recovering streams revealed no further community change beyond 1-2 years after restoration, specifying the Asymmetric Response Concept for microorganisms. One aspect of the Phase II A03 project will be to close this temporal gap and characterise the time scale of progression from the transitory priority effect phase of recovery to the later (equilibrium) phase, by working across different time spans and experimental scales (laboratory experiments, ExStream mesocosms, field observations). In line with the overall Phase II concept, drought as a potentially severe stressor will play a central role. To improve predictive modelling of microphytobenthic community re-assembly after stressor exposure, we will experimentally test a trait-based approach using diatom guilds as a framework to capture taxonomic variability in drought tolerances and in biotic interactions using simple artificial communities composed of selected clonal strains. We will further test the hypothesis that priority effects during community reassembly are weaker after more severe stressor exposure, both in laboratory and mesocosm approaches. A field time series from the Emscher main stem will be used to follow community recovery after removal of sewage load on a longer time scale. A03 will also continue field sampling in all RESIST model catchments and provide data to other modelling and analysis projects.

Serge Mayombo (University of Duisburg-Essen)

Functional and compositional responses of stream microphytobenthic communities to multiple stressors increase and decrease

Microphytobenthos form a key group of autochthonous primary producers in stream ecosystems. Microphytobenthic communities occur at the base of all aquatic food webs and play an important role in the transfer of energy to higher trophic levels. Diatoms are arguably the best‐studied organisms in these communities, e.g. changes in their community structures against environmental gradients, including a combination of anthropogenic stressors, in a range of systems. Diatom assemblages are widely used for biomonitoring of various environmental conditions and paleoecological reconstruction.

This is possible because the species composition of diatom communities reflects directly the influence of the specific combination of long-term environmental factors prevailing in their thriving ecosystems. Inside the framework of RESIST, our goal is to investigate the effect of multiple stressors on microphytobenthos, mainly diatoms, and photosynthesis-related traits. Very few studies reported the effects of anthropogenic stressors upon photosynthetic fitness and performance of stream microphytobenthic communities up to date. An integrative characterisation of community reorganisations and changes in photosynthesis‐related traits accompanying stressor load and release under highly controlled conditions has been very seldom attempted.

This study, on the one hand, aims to fill this gap by assessing compositional responses of benthic diatoms (using digital microscopy and DNA metabarcoding) in parallel with functional changes (in photosynthesis‐related traits) of microphytobenthic communities to stressor (flow, salinity and temperature) increase and release in the large-scale ExStream mesocosm experimental setting. On the other hand, field samplings taken on two anthropogenically impacted river catchments, aim to characterise field microphytobenthos assemblages in a similar manner, in order to assess to what extent differences in these communities reflect changes in adjacent land-use (along the Kinzig network) and differing time periods since urban stream restoration (in the Emscher/Boye localities).

Contact: ntambwe.mayombo@uni-due.de

First Supervisor: Prof. Dr. Bánk Beszteri (University of Duisburg-Essen, Phycology)
Second Supervisor: Prof. Dr. Daniel Hering (University of Duisburg-Essen, Aquatic Ecology)
Mentor: Dr. Julia Förster (LANUV NRW)