Carbon dioxide partial pressure and 13C content of north temperate and boreal lakes at spring ice melt

Carbon dioxide (CO 2 ) accumulates under lake ice in winter and degasses to the atmosphere after ice melt. This large springtime CO 2 pulse is not typically considered in surface‐atmosphere flux estimates, because most field studies have not sampled through ice during late winter. Measured CO 2 partial pressure (pCO 2 ) of lake surface water ranged from 8.6 to 4,290 Pa (85–4,230 µatm) in 234 north temperate and boreal lakes prior to ice melt during 1998 and 1999. Only four lakes had surface pCO 2 less than or equal to atmospheric pCO 2 , whereas 75% had pCO 2 >5 times atmospheric. The δ 13 C DIC (DIC = ΣCO 2 ) of 142 of the lakes ranged from –26.28‰ to +0.95‰. Lakes with the greatest pCO 2 also had the lightest δ 13 C DIC , which indicates respiration as their primary CO 2 source. Finnish lakes that received large amounts of dissolved organic carbon from surrounding peatlands had the greatest pCO 2 . Lakes set in noncarbonate till and bedrock in Minnesota and Wisconsin had the smallest pCO 2 and the heaviest d 13 C DIC , which indicates atmospheric and/or mineral sources of C for those lakes. Potential emissions for the period after ice melt were 2.36 ± 1.44 mol CO 2 m −2 for lakes with average pCO 2 values and were as large as 13.7 ± 8.4 mol CO 2 m −2 for lakes with high pCO 2 values.