Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Astrid Harjung,Elisabet Ejarque,Jakob Schelker,Tom Battin,Andrea Butturini,Francesc Sabater,Masumi Stadler

doi:10.1002/lno.11018

Abstract

AbstractSubalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.

Highlights

Net heterotrophy is reported for most other fluvial ecosystems (Mulholland et al 2001; Hoellein et al 2013) where high ecosystem respiration (ER) is maintained by a steady supply of particulate and dissolved organic matter (POM and DOM) from the terrestrial ecosystem (Battin et al 2008)
Our experiment demonstrated an increase of DOC concentration with discharge reduction that originated from autochthonous sources with assumingly high availability for heterotrophic metabolism
The change in DOM bioavailability during drought was paralleled by an initial phase of increased gross primary production (GPP) that was superimposed with high ER

Summary

Introduction

Net heterotrophy is reported for most other fluvial ecosystems (Mulholland et al 2001; Hoellein et al 2013) where high ER is maintained by a steady supply of particulate and dissolved organic matter (POM and DOM) from the terrestrial ecosystem (Battin et al 2008). Heterotrophy largely depends on the hydrological connectivity of soils and streams. The effects of hydrological variation on DOM quantity have been well studied in the context of stormflow events. Several studies report that DOM quantity increases with discharge (Ågren et al 2008; Wiegner et al 2009; Bass et al 2011; Guarch-Ribot and Butturini 2016). Little is known about how extended periods of reduced flow may affect DOM quantity and quality, in humid regions (Larned et al 2010)

Objectives

Methods

Results

Conclusion