Abstract

Rivers are severely affected by human activities and many are simultaneously impacted by multiple stressors. Water diversion for hydropower generation affects ecosystem functioning of the bypassed reaches, which can alternate between periods with natural discharge and others with reduced flow that increase the surface of dry riverbeds. In parallel, urban pollution contributes a complex mixture of nutrients, organic matter, heavy metals, pesticides, and drugs, thus becoming an important stressor in rivers. However, there is little information on the interaction between both stressors on ecosystem functioning and, particularly, on organic matter processing, a key process linked to the input of energy to food webs. To assess the impact of water diversion and urban pollution on organic matter processing, we selected four rivers in a pollution gradient with a similar diversion scheme and compared reaches upstream and downstream from the diversion weirs. We measured leaf-litter decomposition and carbon dioxide (CO2) fluxes in both the wet channel and the dry riverbed. Water diversion and pollution in the wet channel did not affect CO2 fluxes but reduced microbial decomposition, whereas in the dry riverbed, their interaction reduced total and microbial decomposition and CO2 fluxes. Thus, both stressors affected organic matter processing stronger in dry riverbeds than in the wet channel. These results show that dry riverbeds must be taken into account to assess and manage the impacts of human activities on river ecosystems.

Highlights

  • The increase of human population and its demands for water and energy impact biodiversity and ecosystems worldwide (Crist et al, 2017), streams and rivers being among the most affected ecosystems (Vörösmarty et al, 2010)

  • Our results show that water diversion and pollution have interactive effects on river organic matter (OM) processing, dry channels being more reactive to the interaction than wet channels

  • We expected pollution to reduce the stock of OM in the wet channel and no effects in the dry channel, we found no effect in the wet channel and reduced coarse benthic OM (CBOM) stocks in the dry channel

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Summary

Introduction

The increase of human population and its demands for water and energy impact biodiversity and ecosystems worldwide (Crist et al, 2017), streams and rivers being among the most affected ecosystems (Vörösmarty et al, 2010). European rivers are severely affected by human activities (Tockner et al, 2009), and almost half of them are simultaneously impacted by multiple stressors, such as hydromorphological alterations and pollution (Schinegger et al, 2012). Dry Riverbeds, Diversion, and Pollution depend on river water (Albert et al, 2021), and the dams and weirs built for these purposes fragment the world river network (Grill et al, 2019) and impact their biodiversity (Vörösmarty et al, 2010), as well as their contribution to global biogeochemical cycles (Syvitski et al, 2005). The environmental impact of individual weirs and small dams is likely smaller, their extremely high numbers probably result in a very significant cumulative impact, as they account for over 91% of the barriers in streams and rivers worldwide (Belletti et al, 2020)

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