Individual and combined stressor effects on freshwater invertebrate communities and an associated ecosystem function: an ExStream mesocosm study
The combined effects of multiple stressors on freshwater biota are still poorly understood. Recent studies on benthic invertebrates have shown that complex interactions frequently occur, leading to often unexpected responses. Recent developments in DNA metabarcoding have revealed that responses typically inferred for benthic invertebrates are often underestimated due to a lack of taxonomic resolution. Furthermore, stressor impacts on important biological traits such as biomass and size have rarely been addressed.
The two latter aspects can now be potentially overcome by coupling DNA-based identification with optical methods and machine learning. The innovative combination of approaches has the potential to significantly advance multiple stressor research by matching changes in species and OTU (operational taxonomic units) diversity with their biological traits. Project A08 will investigate multiple stressor impacts on benthic invertebrate communities and taxon-specific traits and link these to changes in organic matter (CPOM) decomposition as an essential ecosystem function. The project has three goals and workpackages (WPs): In WP1 we will assess individual and combined effects of increased temperature, salinity and reduced flow on the invertebrate community by building on results of the three replicated ExStream field experiments coordinated by project Z02.
WP1 will utilise automatic image-based taxon recognition and biomass estimation to also register changes in size and biomass per taxon. While we expect a reduction of species richness upon stressor application, responses are predicted to be largely taxon specific. Furthermore, we expect sensitive benthic invertebrate taxa (e.g. Ephemeroptera, Plecoptera, Trichoptera, ‘EPT’) to decrease in abundance but also overproportionately in biomass, as a higher proportion of the energy budget has to be allocated to physiologically coping with the stressor (homeostasis, higher basal metabolism), while a smaller proportion of the energy budget remains for somatic growth. In WP2 we will analyse all counted and measured benthic invertebrates obtained from WP1 using DNA metabarcoding to maximise response pattern detection at species/OTU level.
We expect a much greater diversity of responses due to improved resolution especially for smaller taxa such as Chironomidae. Based on the Asymmetric Response Concept (ARC), we predict community recovery after stress release to be largely stochastic, yet community composition to be significantly influenced by the endpoints of communities under stress, e.g. due to priority effects. WP3 will assess degradation of CPOM degradation under multiple stressor increase and release and link these important ecosystem function measures to the measured community change parameters obtained in WPs 1 and 2. Through this link we can identify stressor scenarios and taxa associated to ecosystem function decline.