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.

Mimoza Dani (University of Duisburg-Essen)

Microphytobenthos community recovery: time scales of asymmetric recovery and the modulation of priority effects by stressor severity

Microphytobenthic or periphyton communities, often dominated by diatoms, are main contributors to autochthonous primary production in stream ecosystems. We investigate the responses of microphytobenthic communities to warming and drought, as well as their recovery after exposure to stressors of different intensities. In relation to the central theoretical construct of the CRC, the Asymmetric Response Concept, we focus on abiotic sorting via differential tolerances, and on biotic interactions, primarily community closure and priority effects during the recovery process. Having seen indications of priority effects hindering early-phase recolonization after stressor release in phase I, we now address the duration of such priority effects by working across different geographic and temporal scales and levels of ecological realism in laboratory and outdoor stream mesocosm experiments as well as in field studies. Besides, we investigate the suitability of diatom guilds, a widely used trait-based framework attempting to capture functional aspects of periphytic diatom diversity, for characterising aspects relevant to community re-assembly after stressor release.

Contact: mimoza.dani@uni-due.de

First Supervisor: Prof. Dr. Bánk Beszteri (University of Duisburg-Essen, Phycology)
Second Supervisor: Prof. Dr. Ralf Schäfer (University of Duisburg-Essen, Ecotoxicology)
Mentor: Dr. Julia Förster (LANUV NRW)