In‐Lake GHG Production Constrained by Lake DOM Mass Balance in a Boreal Catchment

Abstract The processing of organic carbon in surface waters represents a critical component of the carbon (C) cycle in boreal landscapes and is closely linked to greenhouse gas (GHG) emissions, primarily carbon dioxide (CO 2 ) and methane (CH 4 ). Utilizing a unique data set with dense monitoring of dissolved organic carbon (DOC), dissolved CO 2 , and CH 4 at the inlets and outlet of the boreal humic headwater lake Langtjern (Norway), we constructed an organic and inorganic C budget for the wet year 2023. Catchment inputs of DOC (14.7 g C m −2 yr −1 ) exceeded lake export (12.4 g C m −2 yr −1 ), whereas CO 2 (in: 0.9, out: 1.2 g C m −2 yr −1 ) and CH 4 (in: 0.001, out: 0.008 g C m −2 yr −1 ) exhibited the opposite pattern. The atmospheric evasion of CO 2 and CH 4 were 1.50 and 0.012 g C m −2 _catchment yr −1 , respectively, demonstrating that the lake was a hotspot of GHGs. Estimated mineralization rates derived from inorganic (1.80 ± 0.83 g C m −2 _catchment yr −1 ) and organic C (1.30 ± 0.43 g C m −2 _catchment yr −1 ) mass balances were in reasonable agreement, and their main uncertainties came from catchment inputs and lake evasion. Lake dissolved organic matter mineralization and CH 4 production rates were positively correlated with water temperature. Catchment C inputs were strongly dependent on precipitation while DOC removal was limited by water residence time. We expect that a wetter climate will exert a stronger future control on boreal lake GHG production than warming by reducing aquatic processing time.