Diurnal cycles in thermokarst lakes of a permafrost peatland
Despite the importance of surface waters of permafrost landscapes in carbon (C) emission, dissolved C, metal storage and export, the diel pattern of dissolved components and gases in thermokarst lakes remain virtually unknown. Here we discovered a factor of 2 to 3 higher CO2 concentrations and fluxes during the night compared to day-time in the high-DOC lake. The emission fluxes in the low-DOC lake increased from zero to negative values during the day to highly positive values during the end of the night and early morning. The bulk of dissolved (< 0.45 m) hydrochemical parameters remained highly stable with 10% variation in concentration over 2 days of observation. Overall, the impact of diel cycle on dissolved CH4, DOC, nutrient and metal concentration was below 10%. However, neglecting night-time period may underestimate net CO2 emission by ca. 30 to 50% in small organic-rich thaw ponds and switch the CO2 exchange from uptake/zero to net emission in larger thermokarst lakes. Given the dominance of large lakes in permafrost regions, the global underestimation of the emission flux may be quite high. As such, monitoring CO2 concentrations and fluxes in thermokarst lakes during months of extended night time (August to October) is mandatory for assessing the net emissions from lentic waters of frozen peatlands.
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