Stream oxygen flux and metabolism determined with the open water and aquatic eddy covariance techniques

Abstract

We quantified oxygen flux in a coastal stream in Virginia using a novel combination of the conventional open water technique and the aquatic eddy covariance technique. The latter has a smaller footprint (sediment surface area that contributes to the flux; 10 m 2 ), allowing measurements to be made at multiple sites within the footprint of the open water technique ( 1000 m 2 ). Sites included an unvegetated stream pool with cohesive sediment, a macrophyte bed with sandy sediment, and an unvegetated sand bed with rippled bedforms. Nighttime eddy covariance oxygen uptake was always smaller than uptake produced by the open water technique. At the pool and unvegetated sand bed sites, nighttime eddy covariance uptake was 20-fold smaller than open water uptake. At the macrophyte bed site, gross primary production quantified with the two techniques was similar but eddy covariance uptake was 2.4-fold smaller. The difference in oxygen uptake between eddy covariance and open water techniques could not be accounted for by uncertainties in the gas transfer velocity but could be accounted for by anoxic groundwater inflow through stream banks outside of the eddy covariance footprint. Nighttime oxygen uptake was also measured with eddy covariance in a tidal freshwater part of the stream, where pore space in the sandy sediment near the sediment-water interface was flushed with stream water at peak water velocities. As a result of this advective hyporheic exchange, nighttime oxygen flux increased fourfold with a doubling of water velocity.

Probing whole‐stream metabolism: influence of spatial heterogeneity on rate estimates

“…Algae and substrate can display spatial heterogeneity at the millimetre‐scale (Dodds, ; Wilson & Dodds, ), which can translate into differences in metabolism and nutrient uptake at the micro‐scale (Hoellein et al ., ; Koopmans & Berg, ; Lupon et al ., ). Eddy‐covariance methods are currently being developed for use in streams (Koopmans & Berg, ) that could be used to investigate the centimetre to metre scale of heterogeneity more thoroughly.…”

“…With finer grain sampling, future research could examine cross-section variability in O 2 by deploying several O 2 probes and coupling them with fine-scale flow measurement. Algae and substrate can display spatial heterogeneity at the millimetre-scale (Dodds, 1991;Wilson & Dodds, 2009), which can translate into differences in metabolism and nutrient uptake at the micro-scale (Hoellein et al, 2009;Koopmans & Berg, 2015;Lupon et al, 2016). Eddy-covariance methods are currently being developed for use in streams (Koopmans & Berg, 2015) that could be used to investigate the centimetre to metre scale of heterogeneity more thoroughly.…”

Continuous measurement of air–water gas exchange by underwater eddy covariance

“…A number of approaches have been used to study and determine values for k. For smaller rivers and streams, they include targeted parallel up-and across-stream additions of volatile tracers (e.g., propane) and hydrologic tracers (e.g., dissolved chloride), where the latter is added to correct for dilution of propane due to hyporheic mixing (Genereux and Hemond, 1992;Koopmans and Berg, 2015). A common approach for smaller reservoirs and lakes relies on additions of inert tracers, e.g., SF 6 (Wanninkhof, 1985;Cole et al, 2010), whereas floating chambers are often deployed in larger rivers, reservoirs, lakes, and estuaries (Marino and Howarth, 1993).…”

“…For most gases, C water and C air are straight forward to measure with modern sensors (Koopmans and Berg, 2015;Fritzsche et al, 2017), or calculate from known functions, but the complexity of gas exchange and its many controlling variables is contained in k ( MacIntyre et al, 1995;McKenna and McGillis, 2004;Cole et al, 2010). For sparingly soluble gases such as O 2 , CO 2 , and CH 4 , the ratio between the molecular diffusivity in air and water is on the order of 10 4 .…”

A Structure Function Model Recovers the Many Formulations for Air‐Water Gas Transfer Velocity

“…This interfacial transport phenomenon is featured prominently in global balances of carbon dioxide, methane, nitrous oxides, dimethyl sulfide (DMS), and other gases (Bastviken et al, 2011;Bolin, 1960;Butman & Raymond, 2011;Cole et al, 2010;Garbe et al, 2014;Heiskanen et al, 2014;Huotari et al, 2011;Jähne & Haußecker, 1998;Liu et al, 2016;Mammarella et al, 2015;Rantakari et al, 2015;Raymond et al, 2013;Raymond & Cole, 2001;Upstill-Goddard, 2006;Wanninkhof et al, 2009;Wüest & Lorke, 2003). Regionally and locally, gas transport across the air-water interface is used as a water quality index (e.g., dissolved oxygen and aeration rates) and is often needed when determining evasion rates of volatile pollutants from lakes, estuaries, or even large water treatment plants (Chu & Jirka, 2003;Frost & Upstill-Goddard, 1999;Koopmans & Berg, 2015;Liss et al, 2014;Prata et al, 2017). Given their RESEARCH ARTICLE 10.1029/2018WR022731 significance to ecosystem metabolism and uncertainty associated with model formulations (Genereux & Hemond, 1992;Marx et al, 2017;Raymond & Cole, 2001), studies on air-water gas transport in streams and rivers are now experiencing a renaissance partly driven by the rapid advancements in miniature eddy-covariance sensors (Berg & Pace, 2017).…”