Mikko Tolkkinen

Are biological communities in naturally unproductive streams resistant to additional anthropogenic stressors

Studies on the interactive responses to multiple simultaneously acting stressors have focused on individual or population-level responses in laboratory microcosms, while field-based studies on community-level responses are rare. We examined the influence of a natural (non-anthropogenic acidity) vs. human-induced stress (land drainage) and their interaction on species richness and spatial turnover (β diversity) of stream diatom, bryophyte, and benthic invertebrate communities. Our four stream categories were: circumneutral reference, circumneutral impacted, naturally acidic, and naturally acidic impacted streams. We expected the most sensitive species to be present only in the circumneutral reference streams. Therefore, species richness should be highest in these streams and lowest in the naturally acidic streams additionally stressed by forest drainage. Alternatively, communities in acidic streams may consist of the most tolerant taxa that are unaffected by further stressors, species richness in these streams remaining unaffected by drainage. We also expected spatial turnover to be highest in the circumneutral near-pristine streams and lowest in the drainage-impacted acidic streams. In all three taxonomic groups, α diversity was lower in the naturally acidic than in circumneutral streams. The additional impact of the anthropogenic stress on species richness varied between groups, having no effect on diatoms, antagonistic effect on bryophytes, and additive effect on invertebrates. We also found differences in how each stressor modified β diversity of each taxonomic group. For diatoms, β diversity showed an overall tendency to decrease with increasing stress level, while bryophyte β diversity responded mainly to forest drainage. Benthic invertebrate β diversity did not differ between treatments. Our results suggest that non-additive effects among stressors need special attention to improve the understanding and management of multifactor responses in streams. Our results also argue for the primacy of a multi-taxon approach to environmental impact detection, and for the inclusion of a wide array of ecological responses, particularly community turnover, in bioassessment programs to detect responses that may go unnoticed by conventional richness-based measures.

Human disturbance increases functional but not structural variability of stream fungal communities

Summary Temporal stability of ecosystem functions is often regulated by the same environmental factors that also shape diversity. Therefore, species diversity, ecosystem functions and their environmental regulators should be considered together to understand and predict the consequences of anthropogenic disturbances on ecosystems. We studied the influence of land-use disturbance (agriculture) and a natural stressor (low pH due to specific geology) on the temporal variability (variability among successive years) of fungal decomposer communities and leaf decomposition rates in streams. We used next-generation sequencing techniques (pyrosequencing) to determine the composition of fungal assemblages. Temporal variability of leaf decomposition was higher in human-disturbed streams than in circumneutral reference or naturally acidic sites, whereas the latter two did not differ. Fungal operational taxonomic unit (OTU) richness and evenness were lower in human-impacted sites than in circumneutral reference sites. However, there were no significant differences between stream types in the temporal variability of fungal community composition. Fungal OTU evenness was negatively and among-year variability of water chemistry positively related to temporal variability in leaf decomposition. Partial regressions showed that these two factors had independent effects on decomposition rates. Although the dominant OTUs in the disturbed streams were temporally stable, they did not maintain stable ecosystem functions, suggesting that variability in decomposition was driven mainly by changes in the metabolic responses of dominant taxa to environmental fluctuations. Our results show that leaf decomposition rates in reference sites vary little through time, supporting the use of leaf decomposition assays in bioassessment. Our results also highlight the importance of measuring not only the mean rates, but also temporal variability of process rates when assessing the influence of human disturbance on ecosystem functioning.