Effects of groundwater flux on open‐channel estimates of stream metabolism

Abstract

The open‐channel oxygen method can produce precise estimates of photosynthesis (P) and respiration (R) over a wide range of stream conditions. It is widely recognized that flux of groundwater contributes to the oxygen mass balance for a stream. However, groundwater flux is rarely considered in open‐channel estimates of stream metabolism, and no guidelines have been established regarding the conditions under which it can be ignored. The purpose of this paper is to describe a method for predicting the effect of groundwater flux on estimates of metabolism and thereby establish the conditions under which flux of groundwater can lead to large errors in estimates of metabolism. Estimation of P is not significantly affected by flux of groundwater. Ecosystem R, however, can be greatly overestimated where the oxygen concentration of groundwater is substantially lower than the concentration in the channel. Although the effects of groundwater flux on estimates of metabolism often are trivial, rates of flux can be sufficiently high in many streams, at least during some part of the year, to affect estimates of R where the oxygen concentration differs substantially between groundwater and surface water. Thus, the potential contribution of groundwater flux to oxygen mass balance should always be evaluated when the

A simple technique for continuous measurement of time‐variable gas transfer in surface waters

“…The ability to measure GTV continuously in real time could be useful in many types of open-channel gas calculations, particularly those in shallow and turbulent systems where atmospheric exchange is a large component of the gas budget. For example, determinations of whole-system denitrification rates rely on detecting small changes in N 2 concentration (Laursen andSeitzinger 2002, 2004;McCutchan et al 2003) and/or N 2 isotopic composition in the case of isotope tracer studies (Böhlke et al 2004, Mulholland et al 2004, against large ambient N 2 inventories in surface water and air. The use of gas ratios (e.g., N 2 /Ar) for this purpose can be susceptible to alteration from bubble injection and/or from bubble stripping.…”

“…Often, these reach-scale approaches require measurements of biogenically derived gases (or gas ratios) and a subsequent accounting of sources and sinks of these gases to calculate the process rates of interest. Numerous examples of this approach exist, including O 2 metabolism (Odum 1956;Young and Huryn 1999;Mulholland et al 2001), N 2 and N 2 O production via denitrification (McCutchan et al 2003;Laursen andSeitzinger 2002, 2004), and carbon respiration (Cole and Caraco 1998). More recently, these gas mass balances have been coupled with stable isotopic measurements to improve resolution of the rate estimate (Böhlke et al 2004;Mulholland et al 2004), simplify the solution of the gas mass balance (Quay et al 1995;Russ et al 2004), or provide additional insights into the underlying mechanisms controlling reaction rates (Bade and Cole 2006;Tobias et al 2007).…”

“…More recently, these gas mass balances have been coupled with stable isotopic measurements to improve resolution of the rate estimate (Böhlke et al 2004;Mulholland et al 2004), simplify the solution of the gas mass balance (Quay et al 1995;Russ et al 2004), or provide additional insights into the underlying mechanisms controlling reaction rates (Bade and Cole 2006;Tobias et al 2007). Several factors can confound interpretation of whole-system gas and/or isotopic mass balances, including groundwater fluxes, channel geometry, diel variability in reaction rates, and errors in estimating gas transfer velocity (GTV) (McCutchan et al 2002;Böhlke et al;Hall and Tank 2005;Laursen and Seitzinger 2005;McCutchan and Lewis 2006;Tobias et al 2007). Impacts of these different factors vary between systems and become important in the final calculations when they are large components of the mass balance equations and when they exhibit high degrees of variability and/or uncertainty.…”

“…GTV responds to changes in wind velocity (Wanninkhof et al 1985;Liss and Merlivat 1986;Wanninkhof 1992) as well as to changes in temperature and channel turbulence (Bennett and Rathbun 1972;Young and Huryn 1999). Given the linear form of the equations governing the dissolved gas mass balances such as O 2 and N 2 (Mulholland et al 2001;Laursen and Seitzinger 2002;McCutchan et al 2003;Böhlke et al 2004;Mulholland et al 2004;Tobias et al 2007), under-or overestimates of gas exchange translate proportionately into under-or overestimates of the biogenic gas production or consumption rate, respectively. Despite numerous sitespecific estimates of gas exchange using conservative gas tracer additions, this term is still often regarded as one of the most error-prone terms in the open-channel calculations.…”

Groundwater flux and open-channel estimation of stream metabolism: response to Hall and Tank

“…Rates of this magnitude can significantly affect the oxygen mass-balance of a stream and should be considered in the estimation of ecosystem metabolism by open-channel methods (McCutchan et al 2002;Hall and Tank 2005).…”