POTENTIAL METHANE EMISSION FROM NORTH-TEMPERATE LAKES FOLLOWING ICE MELT

Methane, a radiatively active ''greenhouse'' gas, is emitted from lake s 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 t he lake CH4 source strength. We estimated potential spring CH4 emissio n at the time of ice melt for 19 lakes in northern Minnesota and Wisco nsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littora l zone sediment type. Regression analyses indicated that lake area exp lained 38% of the variance in potential CH4 emission for relatively un disturbed lakes; as lake area increases potential CH4 emission per uni t area decreases. Inclusion of a second term accounting for the presen ce or absence of soft organic-rich littoral-zone sediments explained 8 3% of the variance in potential spring CH4 emission. Total estimated s pring CH4 emission for 1993 for all Minnesota lakes north of 45 degree s with areas greater than or equal to 4 ha was 1.5 x 10(8) mol CH, ass uming a 1 : 1 ratio of soft littoral sediment to hard littoral sedimen t lakes. Emission estimates ranged from 5.3 x 10(7) mol assuming no la kes have soft organic-rich littoral sediments to 4.5 x 10(8) mol assum ing all lakes have soft organic-rich littoral sediments. This spring C H4 pulse may make up as much as 40% of the CH4 annually emitted to the atmosphere by small lakes.