Stream metabolism sources a large fraction of carbon dioxide to the atmosphere in two hydrologically contrasting headwater streams

Abstract

Headwater streams are control points for carbon dioxide (CO 2 ) emissions to the atmosphere, with relative contributions to CO 2 emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in-stream metabolic processes. We analyzed continuous measurements of stream dissolved CO 2 and oxygen (O 2 ) concentrations during spring and early summer in two Mediterranean headwater streams from which we evaluated the contribution of in-stream net ecosystem production (NEP) to CO 2 emission. The two streams exhibited contrasting hydrological regimes: one was non-perennial with relatively small groundwater inflows, while the other was perennial and received significant lateral groundwater inputs. The non-perennial stream exhibited strong inverse coupling between instantaneous and daily CO 2 and O 2 concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO 2 -O 2 relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. Mean NEP contribution to CO 2 emission was 51% and 57% at the non-perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO 2 concentration, in-stream CO 2 production consistently and substantially contributed to total atmospheric CO 2 flux in both streams. We conclude that in-stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed.Streams and rivers are control points in global carbon (C) cycling because of high carbon dioxide (CO 2 ) emission rates (Caraco and Cole 2003;Cole et al. 2007;Drake et al. 2018). High emissions from small streams are attributed principally to groundwater inputs of dissolved CO 2 from soil organic matter decomposition (

Patterns and Drivers of Dissolved Gas Concentrations and Fluxes Along a Low Gradient Stream

“…A synthesis of US streams reports an average CO 2 concentration of 5.3 mg L −1 [this study: 2.3; 0.2-7.1] (mean; range shown in brackets throughout section) and flux of 18.9 g m −2 d −1 [this study: 2.9; −0.4 to 10.5] in third order streams (Butman & Raymond, 2011). While the total flux from New Hope Creek is lower, the fraction of CO 2 flux due to in stream metabolism is comparable to that in some streams (Bernal et al, 2022;Rocher-Ros et al, 2020) but higher than is reported in most streams of similar size (Gómez-Gener et al, 2016;Lupon et al, 2019;Rasilo et al, 2017). The fraction we measured is likely a conservative estimate, as we would expect the lowest contribution of instream metabolism during the winter, which is when we collected most of our measurements.…”

“…It has been widely assumed that aquatic production of greenhouse gases (GHGs) is likely to be important only in large rivers (J. J. Cole & Caraco, 2001; Vannote et al., 1980) due to the higher connectivity of headwaters to terrestrial gas inputs (Hotchkiss et al., 2015) and shorter processing time available for organic carbon in these systems (Catalán et al., 2016). However, recent studies have shown that mineralization of allochthonous organic matter can be a primary source of river GHG fluxes in some headwater streams as well (Bernal et al., 2022; Rocher‐Ros et al., 2020), contributing to the high GHG fluxes observed in headwater streams (Li et al., 2021; Stanley et al., 2016).…”

Basin‐Scale CO2 Emissions From the East River in South China: Importance of Small Rivers, Human Impacts and Monsoons

“…The absence of high p CO 2 could result from the lack of C supply from the surrounding landscape. High p CO 2 in small rivers are typically derived from two sources, including groundwater carrying inorganic C originated from soils and in‐stream decomposition of terrestrial organic C (Bernal et al., 2022; Duvert et al., 2018; Marx et al., 2017). In the ERB, groundwater CO 2 input could be substantially diluted due to extensive precipitation in this watershed (B. Liu et al., 2021), implying that in‐stream metabolism could be the key mechanic controlling the p CO 2 dynamics in small rivers.…”

River Corridor Sources Dominate CO2 Emissions From a Lowland River Network

“…Since we only measured CO 2 concentrations-not those of other DIC species (but see the efforts of Duvert et al, 2019;Wang et al, 2021)-carbonate buffering changes when lateral inflows mix with river water are unknown. The "other" flux reported in this study, arrived at by mass balance, encompasses all those potential carbonate equilibria, but also fluxes due to anaerobic respiration (Bernal et al, 2022) and methane oxidation (Rasilo et al, 2017). Further work will be needed to fully constrain all the overlapping elements of riverine C cycle.…”

“…There are two main pathways—the “reactor” and “chimney”—for terrestrial C to subsidize riverine CO 2 emission, and many recent studies have focused on partitioning the contribution of each (Bernal et al., 2022; Hotchkiss et al., 2015; Lupon et al., 2019; Rasilo et al., 2017; Saccardi & Winnick, 2021). The “reactor” pathway arises from lateral transport of terrestrially derived organic matter, which fuels in‐stream ecosystem respiration (ER), producing net CO 2 in the stream that then off‐gases (Cole & Caraco, 2001; Mayorga et al., 2005).…”