Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter.

Andreas Bruder ,Nathan J Waltham ,Sophie Cauvy‐Fraunié ,Damien Banas ,Oleksandra Shumilova ,Lluís Gómez‐Gener ,Roland Corti ,Núria Cid ,Stephanie M Carlson ,Émile Faye ,Björn Gücker ,Jason L Hwan ,Angus R Mcintosh ,Pierre Gnohossou ,Dev K Niyogi ,Alisha L Steward ,Arturo Elosegi ,Thibault Datry ,Simone D Langhans ,Simone Guareschi ,Athina Papatheodoulou ,Stefan Lorenz ,Clara Mendoza‐Lera ,Nick Bond ,María Isabel Arce ,Annamaria Zoppini ,Catherine M Febria ,Kate Brintrup ,Ross Vander Vorste ,Fiona Dyer ,Klement Tockner ,Brian Four ,Kate S Boersma ,S Kubheka ,Daniel Von Schiller ,Richardo Figueroa ,Ryan M Burrows ,Núria Bonada ,Petr Pařil ,Rosa Gómez Cerezo ,Musa C Mlambo ,Marko Miliša ,Manuel A S Graça ,Biel Obrador ,Florian Altermatt ,Robert J Rolls ,Marcos Moleón ,Chelsea J Little ,Andy Banegas-Medina ,Juan F Blanco-Libreros ,Erin E Beller ,Daniel C Allan ,Michaël Danger ,A Taleb ,Michael T Bogan ,Shai Arnon ,Vladimir Pešić ,Rachel Stubbington ,Isabel Pardo ,Christiane Zarfl ,Elisabeth I Meyer ,María M Sánchez‐Montoya ,Peter M Negus ,Melanie L Blanchette ,Pablo Rodríguez‐Lozano ,Jonathan Marshall ,Tommaso Cancellario ,Joanna Blessing ,Rubén Del Campo ,Iola G Boëchat ,Ana Savić ,Catherine Leigh ,Mark O Gessner ,Dominik Zak ,Anna Maria De Girolamo ,Arnaud Foulquier ,Bianca De Freitas Terra

doi:10.1111/gcb.14537

Abstract

Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events.

Highlights

Human activities and climate change cause global‐scale alterations in the flow regimes of rivers, which in turn are tightly linked to bio‐ geochemical processes such as carbon processing (Arnell & Gosling, 2013; Bernhardt et al, 2018; Tonkin, Merritt, Olden, Reynolds, & Lytle, 2018)
More than half of the global river network length is represented by intermittent rivers and ephemeral streams (IRES) – systems that cease to flow at some point in time and space (Acuña et al, 2014; Datry, Larned, & Tockner, 2014)
The same was found for the amounts of leached substances, where the explained variance for biofilms was due to the effect of environmental variables (PET and fraction of different land use types), and for leaves due to the effect of sub‐ strate characteristics (%C, %N)

Summary

| INTRODUCTION

Human activities and climate change cause global‐scale alterations in the flow regimes of rivers, which in turn are tightly linked to bio‐ geochemical processes such as carbon processing (Arnell & Gosling, 2013; Bernhardt et al, 2018; Tonkin, Merritt, Olden, Reynolds, & Lytle, 2018). Eutrophication and hypoxia can be a conse‐ quence of excess nutrient transport to downstream lakes, reservoirs, and coastal areas, where the mortality of fish and other aquatic organisms can increase (Bunn, Thoms, Hamilton, & Capon, 2006; Datry, Corti, Foulquier, Schiller, & Tockner, 2016; Hladyz, Watkins, Whitworth, & Baldwin, 2011; Whitworth, Baldwin, & Kerr, 2012) Despite their widespread distribution and distinct role in bio‐ geochemical cycling, IRES are notably missing in current analy‐ ses of global carbon budgets and other biogeochemical processes such as cycling of nutrients and DOM (Datry et al, 2018). We hypothe‐ sized that quantitative and qualitative composition of leachates will depend on substrate characteristics, which in turn are expected to correlate with environmental variables sampled at the study sites

| MATERIALS AND METHODS

Findings

| DISCUSSION