Potential methane emission from north-temperate lakes following ice melt
Methane, a radiatively active “greenhouse gas, is emitted from lakes to the atmosphere throughout the open-water season. However, annual lake CH4 emissions calculated solely from open-water measurements that exclude the time of spring ice melt may substantially underestimate the lake CH4 source strength. We estimated potential spring CH4 emission at the time of ice melt for 19 lakes in northern Minnesota and Wisconsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littoral zone sediment type. Regression analyses indicated that lake area explained 38\% of the variance in potential CH4 emission for relatively undisturbed lakes; as lake area increases potential CH4 emission per unit area decreases. Inclusion of a second term accounting for the presence or absence of soft organic-rich littoral-zone sediments explained 83\% of the variance in potential spring CH4 emission. Total estimated spring CH4 emission for 1993 for all Minnesota lakes north of 45° with areas \textgreater=4 ha was 1.5 × 108 mol CH4 assuming a 1 : 1 ratio of soft littoral sediment to hard littoral sediment lakes. Emission estimates ranged from 5.3 × 107 mol assuming no lakes have soft organic-rich littoral sediments to 4.5 × 108 mol assuming all lakes have soft organic-rich littoral sediments. This spring CH4 pulse may make up as much as 40\% of the CH4 annually emitted to the atmosphere by small lakes.