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.
Rivers and streams have been identified as important links in the global
carbon cycle. They receive and transport terrestrial carbon from the land to
the ocean and are also shown to be a net source of greenhouse gases (GHG),
i.e., carbon dioxide (CO
Ecosystem-scale fluxes of CO
As eddy-covariance (Baldocchi, 2014) measurements are not suitable for small streams, gas flux chambers that float on the water surface are a straightforward and inexpensive method for direct measurements of gas fluxes, and can easily be replicated over time and space (Bastviken et al., 2015). The gas flux is determined from the change of the gas concentration in the chamber headspace over time. Floating chambers have been frequently applied for measuring gas fluxes in large rivers, reservoirs and lakes (e.g., Beaulieu et al., 2014; DelSontro et al., 2011; Eugster et al., 2011).
Chamber measurements have been criticized because submerged chamber edges are thought to disrupt the aquatic boundary layer, thereby affecting the gas exchange (Kremer et al., 2003). Comparisons of floating chambers with other flux measurement techniques were performed in lakes, rivers, and estuaries. While some studies have reported a tendency of floating chambers to yield higher fluxes than other methods (Raymond and Cole, 2001; Teodoru et al., 2015), others found reasonable agreement (Gålfalk et al., 2013; Cole et al., 2010).
In streams and rivers, floating chambers have been deployed anchored at one spot (anchored chambers; Sand-Jensen and Staehr, 2012; Crawford et al., 2013), or freely drifting with the water (drifting chambers; Alin et al., 2011; Beaulieu et al., 2012). Although based on the same principle, the two deployment modes have fundamental differences. Because of the higher velocity difference between the chamber and the surface water, anchored chambers in running waters may create additional turbulence around the chamber edges (Kremer et al., 2003). If the effect of this turbulence on fluxes is minor, anchored chambers would be advantageous as the area covered by the chamber can be controlled and because practical work with anchored chambers is relatively simple. Drifting chambers will likely induce less turbulence in the surface water; however it is difficult to control their coverage, potentially resulting in spatially biased measurements. Drifting chambers are also complicated for several reasons, e.g., the presence of obstacles in the streams or in terms of logistics, as the chambers may travel far during measurement periods.
While the establishment of efficient methods for running water gas emissions is needed to improve the global GHG budget, progress in chamber-based methods is prevented by the lack of comparative assessments of anchored versus drifting chambers. In this study, we compared measurements of GHG fluxes and the gas exchange velocity using drifting and anchored chambers in various streams and rivers. Because chamber performance is expected to depend strongly on chamber design, the field experiments were conducted using three different chamber types. In laboratory experiments, we analyzed the flow field and the turbulence under both anchored and drifting chambers at different flow velocities. The primary objective of this study was to answer the following question: do anchored chambers produce reliable measurements of localized GHG fluxes in running waters?
Summary of the three data sets obtained in field measurements. Pictures show the three different chambers used for the anchored and drifting approach. Additional information about the sampling procedures is provided in the Supplement.
Field measurements were conducted in nine different rivers and streams in
Germany and Poland using three different chambers (Table 1).
All three data sets included anchored measurements, where the chambers were tethered to stay at a
fixed position as well as drifting measurements, where the chambers freely moved with the
current. In two of the data sets (A and B), the temporal change of CO
The chamber flux measurements were supplemented by measurements of dissolved
gas concentrations (CO
The flux
The gas exchange velocity of the respective gas at in situ temperature
The flow fields under freely drifting and anchored chambers were measured
using particle image velocimetry (PIV) in a 3 m long laboratory flume. The
chamber type and geometry was identical to the chamber in data set C
(Table 1). The flow field under the drifting chamber was
measured for 50 repeated chamber runs (58 s cumulative velocity observations
under the chamber) at a mean flow velocity of 0.10 m s
The flow fields were analyzed by illuminating neutrally buoyant seeding
particles (diameter of 20
The two-dimensional (longitudinal and vertical) flow velocities within the
field of view were estimated using an adaptive correlation algorithm
(Dynamic Studio, DantecDynamics) with a final spatial resolution of
The turbulent kinetic energy (TKE) was estimated by assuming isotropy in the
unresolved velocity component to be
The mean fluxes measured with anchored and drifting chambers in the
respective field data sets were compared using paired
In all measurements, the CO
Gas exchange velocities
Discharge rate, flow velocities, gas fluxes (
When both gases were measured, the gas exchange velocities estimated from
CO
When combining all data sets, there was no correlation between gas exchange
velocities and the measured current velocity for drifting chambers for
either CO
The laboratory measurements revealed pronounced differences in the flow fields and turbulence under the anchored and drifting chambers. The mean longitudinal flow velocity was strongly reduced within the submerged part of the anchored chamber and increased below the submerged chamber edge. Recirculating eddies were formed under the leading (upstream) edge of the chamber (vector graphs of the mean velocity distributions are provided in Appendix B). These eddies detached and injected turbulence below the chamber (Fig. 3). The turbulent kinetic energy which was produced by the submerged edge of the anchored chambers increased with increasing current speed (Appendix B). Under the drifting chambers, the flow velocities were slightly enhanced below the submerged chamber edge, but no recirculating eddies were formed.
The penetration depth of the chamber edges varied with time as the chamber moved vertically on the rough water surface (see Appendix B for snapshots of instantaneous velocity distributions and chamber penetration). However, at the same flow velocity the average penetration depth of the anchored chamber was higher than that of the drifting chamber (Fig. 3).
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
Laboratory measurements of the mean longitudinal flow
velocities (
The laboratory observation agrees with our field measurements, where the
ratio of the fluxes measured with anchored and with drifting chambers was
comparably small at flow velocities < 0.2 m s
The large (several-fold) potential overestimation of fluxes measured with anchored chambers calls into question its suitability for application in running waters, particularly at high flow rates. This agrees with the observations of Teodoru et al. (2015) who reported a linear dependency of the gas exchange velocity under anchored chambers on the water velocity relative to the chamber in a large river.
The correlation of the anchored chamber gas exchange velocity with flow velocity observed in our study could provide a potential means for correcting the artificial chamber flux, if the corresponding drifting chamber gas exchange velocity were also a function of flow velocity. However, no such correlation was present in our field observations, indicating that near-surface flow velocity is a poor predictor for the gas exchange velocities in streams. Therefore, it can be expected that river depth and bed roughness affect the near-surface turbulence more than flow velocity (Moog and Jirka, 1999; Raymond et al., 2012).
As the correction of the effects of chamber-induced turbulence on measured fluxes seems unlikely, it would be more reasonable to optimize the chamber design to completely avoid or to at least reduce this effect. The rectangular chamber B produced the largest error, although it remained unclear from our measurements whether this was caused by the geometry of the chamber or by the high flow velocity in data set B. On this basis, we recommend the use of more streamlined circular chambers to minimize the error under drifting conditions. Crawford et al. (2013) and McMahon and Dennehy (1999) used streamlined (canoe-shaped) instead of cylindrical or rectangular chambers to minimize the generation of chamber-induced turbulence at the upstream chamber edge during anchored chamber deployments. However, they did not provide evidence that this goal was reached.
Another approach to minimize the bias of anchored chambers would be to design chambers without submerged rigid walls. Submergence of the chamber edges can be avoided completely by using a piece of thin plastic foil which adheres to the water surface to seal the chamber headspace (Fig. 4a). Laboratory (PIV) measurements of the flow field were performed under a piece of foil, mimicking a chamber deployed in anchored mode. The measurements revealed a strong reduction of flow disturbances and chamber-induced turbulence (Fig. 4) in comparison to both anchored and drifting chambers. Such “flying” chambers require a frame to keep the chamber above the water surface, which can be supported by floats at a larger lateral distance to the chamber or, in small streams, also by a fixation at the river bank.
Our study clearly shows that anchored chambers strongly overestimate the gas flux in running water and are not suited to quantify greenhouse gas fluxes in streams and rivers. One possible way forward to reduce this bias while still maintaining the practical advantages of the anchored chambers could be the use of “flying” (anchored) chambers with flexible foil sealing at the water surface. Drifting chambers provide a practical and reliable solution, although they are not free of potential spatial bias. Because their measurement locations are difficult to control, their trajectories may not be representative of the areal mean flux from the study reach. Regions with locally enhanced turbulence, e.g., stream reaches with large emerging roughness of the river bed, cannot be surveyed with drifting chambers; however the gas exchange velocity is highest at these sites (Moog and Jirka, 1999). Similarly, mean flow trajectories may bypass backwaters and regions of reduced flow velocity along the stream banks. Observations in reservoirs and river impoundments revealed that the enhanced sedimentation of particulate organic matter can make these zones emission hot spots (Maeck et al., 2013; DelSontro et al., 2011). Anchored chamber deployments may provide a useful extension of drifting chamber measurements at such sites, if the flow velocity is sufficiently small. To truly validate a reliable chamber method for small streams, a multi-method comparison study, including tracer additions, should be performed.
This study shows that flux chamber approaches to measure GHG fluxes from running waters have a high potential, given sufficient knowledge about appropriate chamber design and deployment approaches. Thus, flux chambers are emerging as an important method to constrain greenhouse gas fluxes from stream networks.
Field measurements of five streams in the north-central European Plain in
Germany and Poland were conducted during October 2014. Gaseous CO
Measurements were performed on the Bode River between Egeln-Nord and
Staßfurt on 7 April 2014 (summer base flow 7.7 m
The flux of CO
Water temperature was continuously measured by temperature loggers (Tidbit, Onset, USA). The barometric pressure was recorded by the FTIR analyzer.
Under drifting conditions the CH
Chambers with a cross-sectional area of 0.066 m
Chambers were deployed fixed at a certain position (anchored) and freely
drifting. Triplicate measurements were conducted during each drifting run,
and three runs were conducted at each site. The anchored chambers were then
used for measuring the flux of CO
Laboratory measurements of flow velocity and turbulence under
anchored chambers at different mean current speeds (left: 0.06 m s
Parts of this study were financially supported by the German Research
Foundation (grant no. LO 1150/9-1) and conducted within the LandScales project
(“Connecting processes and structures driving the landscape carbon dynamics over scales”)
financed by the Leibniz Association within the Joint Initiative for
Research and Innovation (BMBF) and (partially) carried out within the SMART
Joint Doctorate (Science for the MAnagement of Rivers and their Tidal
systems) funded with the support of the Erasmus Mundus program of the
European Union and the Swiss National Science Foundation (grant no. PA00P2_142041). The development and production of the
chambers with built-in CO