Low flow and heatwaves alter ecosystem functioning in a stream mesocosm experiment

Raquel Arias Font; Kieran Khamis; Alexander M Milner; Gregory H Sambrook Smith; Mark E Ledger

doi:10.1016/j.scitotenv.2021.146067

Low flow and heatwaves alter ecosystem functioning in a stream mesocosm experiment

Sci Total Environ. 2021 Jul 10:777:146067. doi: 10.1016/j.scitotenv.2021.146067. Epub 2021 Feb 26.

Authors

Raquel Arias Font¹, Kieran Khamis², Alexander M Milner², Gregory H Sambrook Smith², Mark E Ledger²

Affiliations

¹ School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. Electronic address: R.AriasFont@bham.ac.uk.
² School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.

PMID: 33677285
DOI: 10.1016/j.scitotenv.2021.146067

Abstract

Climate change is expected to intensify the effect of environmental stressors on riverine ecosystems. Extreme events, such as low flow and heatwaves, could have profound consequences for stream ecosystem functioning, but research on the impact of these stressors and their interaction across multiple processes, remains scarce. Here, we report the results of a two-month stream mesocosm experiment testing the effect of low flow (66% water level reduction, without gravel exposure) and heatwaves (three 8-d episodes of +5 °C above ambient with 10-15 days recovery between each episode) on a suite of ecosystem processes (i.e. detrital decomposition, biofilm accrual, ecosystem metabolism and DOC quantity and quality). Low flow reduced whole system metabolism, suppressing the rates of gross primary production (GPP) and ecosystem respiration (ER), but elevated DOC concentration. Overall, habitat contraction was the main driver of reduced ecosystem functioning in the low flow treatment. By contrast, heatwaves increased decomposition, algal accrual, and humic-like DOC, but reduced leaf decomposition efficiency. Net ecosystem production (NEP) generally decreased across the experiment but was most pronounced for low flow and heatwaves when occurring independently. Assessment of NEP responses to the three successive heatwave events revealed that responses later in the sequence were more reduced (i.e. more similar to controls), suggesting biofilm communities may acclimate to autumn heatwaves. However, when heatwaves co-occurred with low flow, a strong reduction in both ER and GPP was observed, suggesting increased microbial mortality and reduced acclimation. Our study reveals autumn heatwaves potentially elongate the growth season for primary producers and stimulate decomposers. With climate change, river ecosystems may become more heterotrophic, with faster processing of recalcitrant carbon. Further research is required to identify the impacts on higher trophic levels, meta-community dynamics and the potential for legacy effects generated by successive low flows and heatwaves.

Keywords: Biofilm; Climate change; Dissolved organic matter; Extreme climatic events; Metabolism; Multiple stressors.