OBSERVATIONS OF A DEEP-MIXING EVENT IN CRATER LAKE, OREGON

We present observations of the evolution of a deep-mixing event in a d eep, temperate: lake. The observations were obtained from thermistors mounted on a long-term mooring in the lake. The event seems to have or iginated near 150-m depth and resulted in a plume or layer of cold wat er from the upper half of the lake that descended to the lake bottom ( 590 m) over a 3-d period. Net mixing associated with this event result ed in an overall vertical heat exchange of nearly 10(15) J and a volum e exchange of 0.7-3.2 km(3) (4-18% of the lake volume) between the upp er and lower portions of the lake. The deep water displaced during the event is estimated to have carried 0.3-2.5 x 10(6) mol of nitrate to the upper lake, which accounts for a significant portion of the averag e annual nitrate flux (similar to 2-4 x 10(6) mol yr(-1)) thought to b e upwelled in this highly oligotrophic system.