Underice Circulation and Greenhouse Gas Evasion in Arctic Lakes
Monday, 15 December 2014
Arctic lakes are ice-covered nine months of the year during which time greenhouse gases increase in near bottom waters and increased discharge associated with snowmelt supplies additional greenhouse gases and dissolved organic carbon into the lakes. We tested the hypothesis that evasion of greenhouse gases depends on the duration of the mixing period after ice off by quantifying the greenhouse gases produced over the winter, those introduced at snowmelt, and those remaining after stratification set up in five lakes in the Alaskan Arctic whose area ranged from 1 ha to 150 ha. Horizontal CTD and oxygen transects taken in one lake while the lake was ice covered and afterwards illustrated the path of incoming snowmelt water, numerous locations with hypoxia, and gravity current formation due to warming of bottom waters under the ice which also moderated stratification dynamics. Temperature-gradient microstructure profiling prior to and after ice off illustrated limited mixing despite heating when ice was present and winds after ice off. None of the lakes had fully mixed at ice off. The fraction of carbon dioxide emitted in spring depended on intensity of wind, density stratification, and lake morphometry, as described by the Lake number, and time for stratification to set up. We hypothesize that the fraction of methane which evades can be quantified by dimensionless indices which incorporate residence time and reaction times. Use of dimensionless numbers which incorporate hydrodynamics and rates of biogeochemical reactions enable scaling up across the landscape.