Carbonate buffering and metabolic controls on carbon dioxide in rivers

Abstract

Multiple processes support the significant efflux of carbon dioxide (CO2) from rivers and streams. Attribution of CO2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO2 production is closely related to O2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO2 and O2 concentrations and other water quality parameters were analyzed in two data sets: a synoptic field study and nationwide water quality monitoring data. CO2 and O2 concentrations were strongly negatively correlated in both data sets (ρ = −0.67 and ρ = −0.63, respectively), although the correlations were weaker in high‐alkalinity environments. In nearly all samples, the molar oversaturation of CO2 was a larger magnitude than molar O2 undersaturation. We used a dynamically coupled O2CO2 model to show that lags in CO2 air‐water equilibration are a likely cause of this phenomenon. Lags in CO2 equilibration also impart landscape‐scale differences in the behavior of CO2 between high‐ and low‐alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO2 dynamics are primary steps in understanding the sources and magnitude of CO2 oversaturation in rivers and streams.

No long‐term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams, and rivers

“…Stets et al . [] highlighted the importance of carbonate buffering for understanding CO 2 dynamics in freshwaters as CO 2 concentrations can be buffered despite large changes in the DIC pool. However, this effect is greatest in waters with high alkalinity and high pH.…”

Carbon dioxide and methane dynamics in a human-dominated lowland coastal river network (Shanghai, China)

“…Riverine CO 2 oversaturation and subsequent CO 2 emissions are primarily driven by internal metabolic CO 2 production (hereafter “internal production”) as well as external hydrological inputs of DIC through upstream runoff (hereafter “longitudinal input”) and lateral groundwater fluxes (hereafter “lateral input”) at segment scales (Figure S1) [ Abril et al , ; Gómez‐Gener et al , ]. In this study, a dynamically coupled DIC‐CO 2 ‐O 2 model [ Gómez‐Gener et al , ; Stets et al , ] was used to quantify and differentiate the relative contribution of these sources to the measured p CO 2 and CO 2 evasion along the longitudinal transect of the Huangpu river (Figure ). Lateral surface water inputs to this transect (e.g., tributaries or sewage discharges) were small (<2% of the net annual mean discharge) (Shanghai Water Bureau, 2011, http://www.shanghaiwater.gov.cn/swEng/index.jsp, accessed 10 May 2016).…”

“…Changes in DIC concentration between position x and x + Δ x are simulated by accounting for gas exchange, internal production, and reequilibration of the DIC pool: where IP is the internal production of CO 2 (g C m −2 d −1 ) and D is the mean river depth (m). We assumed that alkalinity is a conservative quantity during the transport, since the loss or addition of dissolved CO 2 does not change the charge balance of the system [ Stets et al , ]. The p CO 2( x + Δ x ) can therefore be calculated from DIC ( x + Δ x ) and initial alkalinity at each time step (details and equations are provided in the supporting information).…”

“…Additionally, longitudinal transport of DIC originating from terrestrial ecosystems via upstream runoff can also be an external contributor to CO 2 evasion in downstream river segments [ Abril et al , ; Gómez‐Gener et al , ] (Figure S1). This longitudinal flux may be particularly important in high‐alkalinity watersheds because of the carbonate buffering effect that diminishes CO 2 evasion rates, resulting in more consistent CO 2 concentration moving from small upstream waters to downstream rivers [ Stets et al , ] (Figure S1). Therefore, the relative importance of these major CO 2 pathways is controlled by the interactions among climatological, hydrogeological, and biogeochemical processes [ Butman and Raymond , ; Striegl et al , ], as well as physical gas transfer at the air‐water interface [ Alin et al , ; Beaulieu et al , ; Raymond et al , ], which all can vary substantially along a stream‐river continuum [ Dubois et al , ; Hotchkiss et al , ].…”

Groundwater data improve modelling of headwater stream CO&lt;sub&gt;2&lt;/sub&gt; outgassing with a stable DIC isotope approach

“…The main reasons are uncertainties in groundwater input as well as poorly defined surface areas and gas trans-fer velocities (Marx et al, 2017a;Schelker et al, 2016). In addition, pCO 2 and subsequent CO 2 outgassing fluxes typically decline rapidly from stream source areas to river sections further downstream (van Geldern et al, 2015;Stets et al, 2017). Poor definition of these gradients adds another uncertainty to the global carbon budget.…”