Polymictic, adj. Polymictic lakes are shallow and located in warmer regions, exposed to the wind. This combination of exposure and shallow water results in repeated and varying thermal stratification - The water is warmest on top when the weather is hot, approximately the same temperature throughout when the weather cools down, and is warmest at the bottom when the weather is coldest. This results in varying seasonal cycles of rapid growth and die-off for the less hardy organisms in the lake, such as algae. This also results in a complete or near-complete turnover of the water, due to temperature changes, mixing and aerating the lake.
Whale Songs, Lesson Plan #2 (http://www.whalesongs.org/dominica/lesson/lesson2.html)
Water on the Web - Understanding, Glossary (http://wow.nrri.umn.edu/wow/under/glossary.html)