Abstract
Investigating the influence of biodiversity on ecosystem functioning over environmental gradients is needed to anticipate ecosystem responses to global change. However, our understanding of the functional role of freshwater biodiversity, especially for microbes, is mainly based on manipulative experiments, where biodiversity and environmental variability are minimized. Here, we combined observational and manipulative experiments to analyse how fungal biodiversity responds to and mediates the impacts of drying on two key ecosystem processes: organic matter decomposition and fungal biomass accrual. Our observational data set consists of fungal biodiversity and ecosystem processes from 15 streams spanning a natural gradient of flow intermittence. Our manipulative design evaluates the responses of ecosystem processes to two fungal richness levels crossed with three levels of drying. For the observational experiment, we found that increasing the duration of drying reduced fungal species richness and caused compositional changes. Changes in species composition were driven by species turnover, suggesting resistance mechanisms to cope with drying. We also found that fungal richness had a positive effect on organic matter decomposition and fungal biomass accrual. Positive effects of fungal biodiversity were consistent when controlling for the effects of drying duration on richness by means of structural equation modelling. In addition, our results for the manipulative experiment showed that the positive effects of higher richness on both ecosystem processes were evident even when exposed to short or long simulated drying. Overall, our study suggests that maintaining high levels of biodiversity is crucial for maintaining functional freshwater ecosystems in response to ongoing and future environmental changes.
Highlights
Microbial communities sustain biogeochemical cycles in ecosystems by playing a key role in carbon processing, nutrient cycling and energy transfer to higher tropic levels (Gessner and others 2010; Besemer 2015; Manning and others 2018)
There is unequivocal evidence that biodiversity enhances the stability of ecosystem processes through time and that biodiversity buffers ecosystems against environmental variations (Pascoal and others 2010; van Rooijen and others 2015; Tredennick and others 2017)
It remains unclear whether the underlying mechanisms explaining changes in organic matter decomposition and microbial biomass production are related to stress-induced biodiversity changes, indirect abiotic effects on species performance or their joint effects (Baert and others 2018)
Summary
Microbial communities sustain biogeochemical cycles in ecosystems by playing a key role in carbon processing, nutrient cycling and energy transfer to higher tropic levels (Gessner and others 2010; Besemer 2015; Manning and others 2018). Intermittent rivers and ephemeral streams (IRES) are widespread examples of freshwater systems exposed to recurrent abiotic stress, that is, drying (Datry and others 2014; Mora-Gomez and others 2020), and are expanding in scope as a result of climate change and water extraction (Doll and Schmied 2012; Koutroulis and others 2019) Due to their global extension and temporal dynamics, IRES have a strong influence on global carbon processing and greenhouse gas emissions (Datry and others 2018; von Schiller and others 2019). There is unequivocal evidence that biodiversity enhances the stability of ecosystem processes through time and that biodiversity buffers ecosystems against environmental variations (Pascoal and others 2010; van Rooijen and others 2015; Tredennick and others 2017) It remains unclear whether the underlying mechanisms explaining changes in organic matter decomposition and microbial biomass production are related to stress-induced biodiversity changes, indirect abiotic effects on species performance or their joint effects (Baert and others 2018). Understanding how microbial biodiversity mediates drying impacts on organic matter processing and microbial biomass production is essential for predicting future variations in global carbon dynamics and its transfer to higher trophic levels in aquatic food webs
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