Dissolved organic matter collected at the end of winter from a peatland thermokarst lake was highly transformed and degraded by sunlight, leading to bacterial stimulation and CO₂ production, but a fraction was also potentially lost by photoflocculation. Over 18 days, 18 % of the incubated dissolved organic matter was lost under sunlight, while dark bacterial degradation was negligible. Sunlight could have a marked effect on carbon cycling in organic-rich thermokarst lakes after ice-off.