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

Abstract

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.