The dominant role of sunlight in degrading winter dissolved organic matter from a thermokarst lake in a subarctic peatland

[摘要] Dissolved organic matter (DOM) leaching from thawing permafrostmay promote a positive feedback on the climate if it is efficientlymineralized into greenhouse gases. However, many uncertainties remain on theextent of this mineralization, which depends on DOM lability that isseemingly quite variable across landscapes. Thermokarst peatlands areorganic-rich systems where some of the largest greenhouse gas (GHG) emissionrates have been measured. At spring turnover, anoxic waters releasethe GHG accumulated in winter, and the DOM pool is exposed tosunlight. Here, we present an experiment where DOM photoreactivity and bioreactivitywere investigated in water collected from a thermokarst lake in a subarcticpeatland during late winter (after 6 months of darkness). We appliedtreatment with or without light exposure, and manipulated the bacterialabundance with the aim to quantify the unique and combined effects of lightand bacteria on DOM reactivity at ice-off in spring. We demonstrate thatsunlight was clearly driving the transformation of the DOM pool, part ofwhich went through a complete mineralization into CO 2 . Up to 18 % of the initial dissolved organic carbon (DOC, a loss of 3.9 mgC L −1 ) was lostover 18 d of sunlight exposure in a treatment where bacterial abundancewas initially reduced by 95 %. However, sunlight considerably stimulatedbacterial growth when grazers were eliminated, leading to the recovery ofthe original bacterial abundance in about 8 d, which may have contributedto the DOC loss. Indeed, the highest DOC loss was observed for the treatmentwith the full bacterial community exposed to sunlight (5.0 mgC L −1 ),indicating an indirect effect of light through the bacterial consumption ofphotoproducts. Dark incubations led to very limited changes in DOC,regardless of the bacterial abundance and activity. The results also showthat only half of the light-associated DOC losses were converted intoCO 2 , and we suggest that the rest potentially turned into particlesthrough photoflocculation. Sunlight should therefore play a major role inDOM processing, CO 2 production and carbon burial in peatland lakesduring spring, likely lasting for the rest of the open season in mixingsurface layers.

[学科分类] 大气科学