According to triathlonworld.gr the lake helds each summer the Sprint Distance Triathlon:
Lake Doxa, in the middle of a marvelous pine forest at 900m altitude, will held another Sprint Distance Triathlon, in its crystal waters and asphalt roads in its perimeter.
750m of Swimming, 21km of Biking and 4.2km of running, with the view of mount Ziria and mount Helmos tops. An exprerience that everybody should live, from begginers to experienced triathletes, in individual and team categories, only 2 hours away for Athens! – The Location
The start of the race will be at 11 in the morning, on Sunday, 15th of July and the road wll be closed for vehicles. The race includes 750m of swimming in a triangle route in the lake, 5 laps of the lake perimeter by bicycle and 1 lap running in the opposite direction of biking.