Low contribution of internal metabolism to carbon dioxide emissions along lotic and lentic environments of a Mediterranean fluvial network

Abstract

Inland waters are significant sources of carbon dioxide (CO2) to the atmosphere. CO2 supersaturation and subsequent CO2 emissions from inland waters can be driven by internal metabolism, external inputs of dissolved inorganic carbon (DIC) derived from the catchment, and other processes (e.g., internal geochemical reactions of calcite precipitation or photochemical mineralization of organic solutes). However, the sensitivity of the magnitude and sources of CO2 emissions to fluvial network hydromorphological alterations is still poorly understood. Here we investigated both the magnitude and sources of CO2 emissions from lotic (i.e., running waters) and lentic (i.e., stagnant waters associated to small dams) waterbodies of a Mediterranean fluvial network by computing segment‐scale mass balances of CO2. Our results showed that sources other than internal metabolism sustained most (82%) of the CO2 emissions from the studied fluvial network. The magnitude and sources of CO2 emissions in lotic waterbodies were highly dependent on hydrology, with higher emissions dominated by DIC inputs derived from the catchment during high flows and lower emissions partially fueled by CO2 produced biologically within the river during low flows. In contrast, CO2 emissions in lentic waterbodies were low, relatively stable over the time and the space, and dominated by DIC inputs from the catchment regardless of the different hydrological situations. Overall, our results stress the sensitivity of fluvial networks to human activities and climate change and particularly highlight the role of hydromorphological conditions on modulating the magnitude and sources of CO2 emissions from fluvial networks.

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

“…Therefore, the emerged pattern of p CO 2 in the Shanghai river network both confirms and challenges the current conceptual understanding of river C cycling. On the one hand, our results corroborate the importance of placing river p CO 2 dynamics in a hydrological context for a complete examination of C fluxes along a river continuum [ Kaushal et al , ; Gómez‐Gener et al , ]. The Taihu Lake basin is an agricultural basin with high DIC concentrations (12–30 mg L −1 ) in the main body of the Taihu Lake and its discharging rivers [ Tao 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). In the model, IP is estimated using DO dynamics along the transect [ Hotchkiss et al , ; Gómez‐Gener et al , ]. The governing equation for DO dynamics is given by where k O2 is the specific gas transfer velocity for O 2 , DO (atm) is the saturated DO concentration in atmospheric equilibrium (mM), and NEP is the aquatic net ecosystem production (mmol O 2 m −2 d −1 ).…”

“…To date, lateral groundwater inputs of high CO 2 derived from soils and fringing wetlands [ Johnson et al , ; Crawford et al , ; Abril et al , ] and in‐stream oxidation of allochthonous organic C (OC) [ Richey et al , ; Mayorga et al , ] are considered the two major sources sustaining the widespread CO 2 oversaturation and evasion in streams and rivers [ Hotchkiss et al , ] (see Figure S1 in the supporting information for a schematic). 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).…”

Hydrologic controls on pCO2 and CO2 efflux in US streams and rivers

“…c) and its Q responsiveness (Fig. c) along stream order 1–6 were in line with decreasing landscape connectivity and terrestrial CO 2 input in these rivers (Hotchkiss et al ), internal river organic matter metabolism was still not a dominant process (Gomez‐Gener et al ; Winterdahl et al ). Short‐residence times and strong hydrologic responsiveness in these small rivers worked to shunt river organic matter quickly through the stream network (Raymond et al ), leaving little opportunity for channel metabolism and processing until the higher orders were reached.…”

Stream metabolism controls diel patterns and evasion of CO2 in Arctic streams

“…Importantly, GPP and ER also consume and produce CO 2 , respectively, and thus provide estimates of aquatic C processing rates that can be compared to independent measures of CO 2 . In this way, estimating metabolism modelled from O 2 data is a powerful tool to understand CO 2 sources to streams (Hotchkiss et al, 2015), yet few studies have coupled high frequency measurements of O 2 and CO 2 with the goal of resolving these different pathways (but see Gómez-Gener, von Schiller, et al, 2016;Stets et al, 2017).…”