Carbonate weathering as a driver of CO2 supersaturation in lakes

Abstract

Most lakes and reservoirs have surface CO 2 concentrations that are supersaturated relative to the atmosphere 1 . The resulting CO 2 emissions from lakes represent a substantial contribution to the continental carbon balance 2-4 . Thus, the drivers of CO 2 supersaturation in lakes need to be understood to constrain the sensitivity of the land carbon cycle to external perturbations 4-6 . Carbon dioxide supersaturation has generally been attributed to the accumulation of inorganic carbon in lakes where respiration exceeds photosynthesis 7,8 , but this interpretation has faced challenges 9-11 . Here we report analyses of water chemistry data from a survey of Spanish reservoirs that represent a range of lithologies, using simple metabolic models. We find that, above an alkalinity threshold of 1 mequiv. l −1 , CO 2 supersaturation in lakes is directly related to carbonate weathering in the watershed. We then evaluate the global distribution of alkalinity in lakes and find that 57% of the surface area occupied by lakes and reservoirs-particularly in tropical and temperate latitudes-has alkalinity exceeding 1 mequiv. l −1 . We conclude that lake inputs of dissolved inorganic carbon from carbonate weathering should be considered for the CO 2 supersaturation of lakes at both regional and global scales.There are two main mechanisms leading to CO 2 supersaturation in lakes and reservoirs: in situ net ecosystem production (NEP) imbalanced towards net heterotrophy 7 (that is, respiration exceeding photosynthesis); and inputs of groundwater or surface water with high dissolved inorganic carbon (DIC) content coming from both weathering of minerals and soil respiration in the watershed 10-14 . Among these, NEP is usually considered to be the main factor driving CO 2 supersaturation in lakes 7,8 . However, most studies relating CO 2 supersaturation to NEP have focused on relatively dilute, low-alkalinity lakes, despite the fact that CO 2 concentration in water is strongly modulated by the carbonate equilibrium (that is, the chemical reactions relating the different forms of DIC). Therefore, we lack conclusive evidence of the impact of NEP on CO 2 supersaturation in a range of systems showing contrasting DIC content.To test how DIC content can drive CO 2 supersaturation and modulate the relationship between NEP and CO 2 concentration, we use data from a nationwide study 15 including 202 measurements of dissolved oxygen (DO), DIC, alkalinity and CO 2 concentration in the surface layer of a set of Spanish reservoirs covering a wide range of DIC content and trophic states (Supplementary Table 1 and Data). We use the observed DO disequilibrium relative to the atmosphere as a surrogate for surface lake NEP, and investigate its impact on observed DIC and CO 2 concentrations considering three metabolic models. The models combine assumptions concerning the DIC loading from the watershed and the effect of lake NEP (Fig. 1a). All three models assume that the DIC generated during weathering of minerals in the watershed (DIC W ) dominates DIC...

CO<sub>2</sub> emissions from German drinking water reservoirs estimated from routine monitoring data

“…Since the surface CO 2 7 concentration in the reservoirs is probably predominantly determined by inflow water quality 8 rather than reservoir internal processes, CO 2 emissions are probably largely regulated by 9 catchment processes (Stets et al, 2009). This confirms studies showing that the CO 2 emission 10 from lakes may be controlled by catchment productivity (Maberly et al, 2013) or carbonate 11 weathering (Marcé et al, 2015). Catchment processes and inflow water quality are obviously 12 more important than hydrodynamics in regulating the annual CO 2 emission from German 13 drinking water reservoirs.…”

“…In other regions, CO 2 in lakes seems to 13 be driven by DIC input from the catchment . In high alkalinity lakes 14 in calcareous regions, CO 2 oversaturation is primarily caused by carbonate weathering (López 15 et al, 2011; Marcé et al, 2015). We used our dataset to get some information about the 16 principle drivers of the CO 2 flux from low DOC, low alkalinity waters, which are typical for 17 temperate drinking water reservoirs.…”

“…The pH is a result of alkalinity (mainly influenced by catchment geochemistry) and TIC 18 (influenced both by catchment processes and aquatic metabolism) (Marcé et al, 2015;Müller 19 et al, 2015). Especially between pH 5 and 7 even small changes in CO 2 significantly alter the 20 pH.…”

Biogeochemistry of a large and deep tropical lake (Lake Kivu, East Africa: insights from a stable isotope study covering an annual cycle

“…This concept seems to hold especially true for oligotrophic unproductive ecosystems (Del Giorgio et al, 1997), that are subsidised by substantial inputs of allochthonous OM of terrestrial origin, which support the production of heterotrophic organisms. Net heterotrophy has been recognised as one of the main cause for the net emission of carbon dioxide (CO 2 ) from freshwater ecosystems to the atmosphere (Prairie et al, 2002), although there is growing evidence of the contribution from external hydrological CO 2 inputs from the catchment (Stets et al, 2009;Finlay et al, 2010;Borges et al, 2014;Marcé et al, 2015). However, the current understanding of the role of inland waters on CO 2 emissions could be biased because most observations were obtained in temperate and boreal systems, and mostly in medium-to-small lakes, during open-water (ice-free) periods, but tropical and temperate lakes differed in some fundamental characteristics.…”

Do invasive quagga mussels alter CO2 dynamics in the Laurentian Great Lakes?

“…Recently, carbon cycling and magnitude of CO 2 fluxes across the air-water interface in lakes and inland waters have received increasing attention15161718, although studies on carbon dynamics in the Great Lakes remain scarce1719.…”