Technical note: drifting versus anchored flux chambers for measuring greenhouse gas emissions from running waters

A Lorke,C Noss,C Somlai-Haase,P Bodmer,K Premke,D Müller,M Koschorreck,S Flury,A Maeck,Z Alshboul,D F Mcginnis,D Bastviken

doi:10.5194/bg-12-7013-2015

Abstract

Abstract. Stream networks have recently been discovered to be major but poorly constrained natural greenhouse gas (GHG) sources. A fundamental problem is that several measurement approaches have been used without cross-comparisons. Flux chambers represent a potentially powerful methodological approach if robust and reliable ways to use chambers on running water can be defined. Here we compare the use of anchored and freely drifting chambers on various streams with different flow velocities. The study clearly shows that (1) anchored chambers enhance turbulence under the chambers and thus elevate fluxes, (2) drifting chambers have a very small impact on the water turbulence under the chamber and thus generate more reliable fluxes, (3) the bias of the anchored chambers greatly depends on chamber design and sampling conditions, and (4) there is a promising method to reduce the bias from anchored chambers by using a flexible plastic foil collar to seal the chambers to the water surface, rather than having rigid chamber walls penetrating into the water. Altogether, these results provide novel guidance on how to apply flux chambers in running water, which will have important consequences for measurements to constrain the global GHG balances.

Highlights

Rivers and streams have been identified as important links in the global carbon cycle
Our field observations showed consistently higher gas exchange velocities and gas fluxes measured with anchored in comparison to freely drifting chambers in a variety of small streams with flow velocities between 0.08 and 0.8 m s−1
Detailed observations of the flow field and turbulence under both types of chambers in the laboratory revealed a reduction of mean flow velocity and the generation of chamberinduced turbulence due to the shedding of eddies at the upstream part of the submerged edge of the anchored chamber

Summary

Introduction

Rivers and streams have been identified as important links in the global carbon cycle. Due to the lack of knowledge of surface area and gas exchange velocity, the smallest streams are considered to be a major unknown component of regional- to global-scale GHG emission estimates (Bastviken et al, 2011; Cole et al, 2007). A continental-scale analysis of CO2 efflux from streams and rivers revealed a continuous decline of the fluxes with increasing size and discharge of the aquatic systems (Hotchkiss et al, 2015)

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