28 Jan 2022
28 Jan 2022
Status: a revised version of this preprint is currently under review for the journal BG.

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

Flora Mazoyer1,2, Isabelle Laurion1,2, and Milla Rautio2,3 Flora Mazoyer et al.
  • 1Centre Eau Terre Environnement, Institut National de la Recherche Scientifique, Québec, QC, Canada
  • 2Centre for Northern Studies, Université Laval, Québec, QC, Canada
  • 3Département des sciences fondamentales, Université du Québec à Chicoutimi, QC, Canada

Abstract. Dissolved organic matter (DOM) leaching from thawing permafrost may promote a retroaction loop onto climate if it is efficiently mineralized into greenhouse gases. Yet, 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 are releasing the GHG accumulated in winter, and the DOM pool is being exposed to sunlight. Here, we present an experiment where DOM photo- and bioreactivity were investigated in water collected from a thermokarst lake in a subarctic peatland during late winter (after six months of darkness). We applied treatments 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, partly leading to its full mineralization into CO2. About 18 % of initial dissolved organic carbon (DOC, a loss of 3.9 mgC L−1) was directly lost over 18 days of sunlight exposure in a treatment where bacterial abundance was initially reduced by 95 %, while dark incubations lead to very limited changes in DOC, regardless of the bacterial abundance and activity. The highest DOC loss was observed for the treatment with the full bacterial community exposed to sunlight (5.0 mgC L−1), indicating an undirect effect of light through the bacterial consumption of photoproducts. Sunlight was an outstanding boosting factor on bacterial growth when grazers were eliminated, leading to the recovery of the original bacterial abundance in about 8 days. The results also show that only half of the light-associated DOC losses were converted into CO2, with the rest potentially turned into particles through photo-flocculation. Sunlight should therefore play a major role in DOM processing, CO2 production and carbon burial in peatland lakes during spring, likely lasting for the rest of the open-season in mixing surface layers.

Flora Mazoyer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-26', Anonymous Referee #1, 23 Feb 2022
    • AC1: 'Reply on RC1', Flora Mazoyer, 01 May 2022
  • RC2: 'Comment on bg-2022-26', Anonymous Referee #2, 17 Mar 2022
    • AC2: 'Reply on RC2', Flora Mazoyer, 01 May 2022
  • CC1: 'Comment on bg-2022-26', Liudmila Shirokova, 21 Mar 2022
    • AC3: 'Reply on CC1', Flora Mazoyer, 01 May 2022

Flora Mazoyer et al.

Flora Mazoyer et al.


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Short summary
Dissolved organic matter collected at the end of winter from a peatland thermokarst lake was highly transformed by sunlight, leading to bacterial stimulation and CO2 production, but also with a large fraction apparently lost by photo-flocculation. Over 18 days, 23 % of the incubated dissolved organic matter was lost under sunlight, while bacterial degradation was negligible in the dark. Sunlight thus has a marked effect on carbon cycling in organic-rich thermokarst lakes after ice-off.