The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influences the estimate of the global terrestrial carbon dioxide (CO2) sink. At present, no reliable number of this export is available, and the few studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function along a continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lake CO2 production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes in carbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using global data on in-lake CO2 production by mineralization as well as CO2 loss by emission, primary production, and carbonate precip- itation in lakes, we estimated that the global lateral DIC export can lie within the range of 0:70þ0:27 −0:31 to 1:52þ1:09 −0:90 Pg C yr−1 depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DIC export from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO2 sinks and sources will require the classification of lakes into their predominant function. This functional lake classification concept becomes particularly important for the estimation of future CO2 sinks and sources, since in-lake carbon transformation is predicted to be altered with climate change. | Funding information. Financial support was received from the Swedish Research Council (Grant No. 2016-04153), the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 643052 (C-CASCADES project), and from the Knut and Alice Wallenberg Foundation (KAW project). This work profited from the Global Lake Ecological Observatory Network (GLEON). The Estonian partners were supported by institutional research funding IUT 21-02 of the Estonian Ministry of Education and Research, and American partners were supported, in part, through NSF EF- 1137327.

The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influences the estimate of the global terrestrial carbon dioxide (CO2) sink. At present, no reliable number of this export is available, and the few studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function along a continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lake CO2 production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes in carbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using global data on in-lake CO2 production by mineralization as well as CO2 loss by emission, primary production, and carbonate precipitation in lakes, we estimated that the global lateral DIC export can lie within the range of [Formula: see text] to [Formula: see text] Pg C yr-1 depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DIC export from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO2 sinks and sources will require the classification of lakes into their predominant function. This functional lake classification concept becomes particularly important for the estimation of future CO2 sinks and sources, since in-lake carbon transformation is predicted to be altered with climate change. | Fil: Engel, Fabian. Uppsala University; Suecia | Fil: Farrell, Kaitlin J.. University of Georgia; Estados Unidos. Virginia Tech University; Estados Unidos | Fil: McCullough, Ian M.. University of California; Estados Unidos | Fil: Scordo, Facundo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina | Fil: Denfeld, Blaize A.. Universidad de Umea; Suecia | Fil: Dugan, Hilary A.. University of Wisconsin; Estados Unidos | Fil: de Eyto, Elvira. Marine Institute; Irlanda | Fil: Hanson, Paul C.. University of Wisconsin; Estados Unidos | Fil: McClure, Ryan P.. Virginia Tech University; Estados Unidos | Fil: Nõges, Peter. Estonian University of Life Sciences. Centre for Limnology; Estonia | Fil: Nõges, Tiina. Estonian University of Life Sciences. Centre for Limnology; Estonia | Fil: Ryder, Elizabeth. Dundalk Institute of Technology; Irlanda | Fil: Weathers, Kathleen C.. Cary Institute Of Ecosystem Studies; Estados Unidos | Fil: Weyhenmeyer, Gesa A.. Uppsala University; Suecia

Funding information. Financial support was received from the SwedishResearch Council (Grant No. 2016-04153), the European Union’sHorizon 2020 research and innovation program under the MarieSklodowska-Curie grant agreement No 643052 (C-CASCADES project),and from the Knut and Alice Wallenberg Foundation (KAW project). Thiswork profited from the Global Lake Ecological Observatory Network(GLEON). The Estonian partners were supported by institutional researchfunding IUT 21-02 of the Estonian Ministry of Education and Research,and American partners were supported, in part, through NSF EF-1137327. | The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influencesthe estimate of the global terrestrial carbon dioxide (CO2) sink. At present, no reliable number of this export is available, and thefew studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function alonga continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lakeCO2 production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes incarbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using globaldata on in-lake CO2 production by mineralization as well as CO2 loss by emission, primary production, and carbonate precip-itation in lakes, we estimated that the global lateral DIC export can lie within the range of 0:70þ0:27−0:31 to 1:52þ1:09−0:90 Pg C yr−1depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DICexport from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO2 sinks and sources will requirethe classification of lakes into their predominant function. This functional lake classification concept becomes particularlyimportant for the estimation of future CO2 sinks and sources, since in-lake carbon transformation is predicted to be altered withclimate change.

The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influences the estimate of the global terrestrial carbon dioxide (CO₂) sink. At present, no reliable number of this export is available, and the few studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function along a continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lake CO₂ production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes in carbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using global data on in-lake CO₂ production by mineralization as well as CO₂ loss by emission, primary production, and carbonate precipitation in lakes, we estimated that the global lateral DIC export can lie within the range of [Formula: see text] to [Formula: see text] Pg C yr⁻¹ depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DIC export from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO₂ sinks and sources will require the classification of lakes into their predominant function. This functional lake classification concept becomes particularly important for the estimation of future CO₂ sinks and sources, since in-lake carbon transformation is predicted to be altered with climate change.