Recycling and fluxes of carbon gases in a stratified boreal lake following experimental carbon addition

Abstract. Partly anoxic stratified humic lakes are important sources of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. We followed the fate of CH₄ and CO₂ in a small boreal stratified lake, Alinen Mustajärvi, during 2007–2009. In 2008 and 2009 the lake received additions of dissolved organic carbon (DOC) with stable carbon isotope ratio (δ¹³C) around 16‰ higher than that of local allochthonous DOC. Carbon transformations in the water column were studied by measurements of δ¹³C of CH₄ and of the dissolved inorganic carbon (DIC). Furthermore, CH₄ and CO₂ production, consumption and emissions were estimated. Methane oxidation was estimated by a diffusion gradient method. The amount, location and δ¹³C of CH₄-derived biomass and CO₂ in the water column were estimated from the CH₄ oxidation pattern and from measured δ¹³C of CH₄. Release of CH₄ and CO₂ to the atmosphere increased during the study. Methane production and almost total consumption of CH₄ mostly in the anoxic water layers, was equivalent to the input from primary production (PP). δ¹³C of CH₄ and DIC showed that hydrogenotrophic methanogenesis was the main source of CH₄ to the water column, and methanogenic processes in general were the reasons for the ¹³C-enriched DIC at the lake bottom. CH₄ and DIC became further ¹³C-enriched in the anoxic layer of the water column during the years of DOC addition. Even gradient diffusion measurements showed active CH₄ oxidation in the anoxic portion of the water column; there was no clear ¹³C-enrichment of CH₄ as generally used to estimate CH₄ oxidation strength. Increase in δ¹³C-CH₄ was clear between the metalimnion and epilimnion where the concentration of dissolved CH₄ and the oxidation of CH₄ were small. Thus, ¹³C-enrichment of CH₄ does not reveal the main location of methanotrophy in a lake having simultaneous anaerobic and aerobic oxidation of CH₄. Overall the results show that organic carbon is processed efficiently to CH₄ and CO₂ and recycled in the anoxic layer of stratified boreal lakes by CH₄ oxidation. In spite of this, increased DOC input led to increased greenhouse gas release, mainly as CO₂ but also as CH₄. Due to the predominantly anaerobic CH₄ oxidation, a relatively small amount of CH₄-derived biomass was produced, while a large amount of CH₄-derived CO₂ was produced in the anoxic bottom zone of the lake.