Reply on CC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Dissolved organic matter (DOM) leaching from thawing permafrost may promote a positive feedback on the climate if it is efficiently mineralized into greenhouse gases. However, many uncertainties remain on the extent of this mineralization, which depends on DOM lability that is seemingly quite variable across landscapes. Thermokarst peatlands are organic-rich systems where some of the largest greenhouse gas (GHG) emission rates have been measured. At spring turnover, anoxic waters release the GHG accumulated in winter, and the DOM pool is exposed to sunlight. Here, we present an experiment where DOM photoreactivity and bioreactivity were investigated in water collected from a thermokarst lake in a subarctic peatland during late winter (after 6 months of darkness). We applied treatment with or without light exposure, and manipulated the bacterial abundance with the aim to quantify the unique and combined effects of light and bacteria on DOM reactivity at ice-off in spring. We demonstrate that sunlight was clearly driving the transformation of the DOM pool, part of which went through a complete mineralization into CO<span class="inline-formula">₂</span>. Up to 18 % of the initial dissolved organic carbon (DOC, a loss of 3.9 mgC L<span class="inline-formula">^−1</span>) was lost over 18 d of sunlight exposure in a treatment where bacterial abundance was initially reduced by 95 %. However, sunlight considerably stimulated bacterial growth when grazers were eliminated, leading to the recovery of the original bacterial abundance in about 8 d, which may have contributed to the DOC loss. Indeed, the highest DOC loss was observed for the treatment with the full bacterial community exposed to sunlight (5.0 mgC L<span class="inline-formula">^−1</span>), indicating an indirect effect of light through the bacterial consumption of photoproducts. Dark incubations led to very limited changes in DOC, regardless of the bacterial abundance and activity. The results also show that only half of the light-associated DOC losses were converted into CO<span class="inline-formula">₂</span>, and we suggest that the rest potentially turned into particles through photoflocculation. Sunlight should therefore play a major role in DOM processing, CO<span class="inline-formula">₂</span> production and carbon burial in peatland lakes during spring, likely lasting for the rest of the open season in mixing surface layers.