Thermal stratification, mixing, and heat budgets of Florida lakes

We measured temperature and oxygen profiles at approximately monthly intervals over a period of 15 months in relatively deep (z(max) = 17.5-24 m) Florida (USA) lakes. Johnson Pond and Lake Sheelar lie in north Florida. Lakes Tulane and Verona are in south-central Florida. All four lakes were strongly stratified thermally in summer and weakly stratified or isothermal during winter. Sheelar displayed winter isothermy at similar to 17 degrees C and Johnson Pond at similar to 13 degrees C. The southern lakes were isothermal at similar to 19 degrees C. Surface water temperatures exceeded 30 degrees C in all lakes in summer. Annual heat budgets for each lake were calculated twice, using heat loss from summer to winter and heat gain from winter to summer in 2006 and 2007. Three lakes showed similar annual heat budgets despite differences in latitude and size: Sheelar (9069 and 9955 cal cm(-2)), Tulane (8459 and 9319 cal cm(-2)), and Verona (7948 and 8444 cal cm(-2)). Johnson Pond displayed lower values of 5105 and 6049 cal cm(-2). Johnson Pond had lower isothermic winter temperatures. It also had smaller annual heat budgets because heat is not transferred effectively to great depth in the take during the period when thermal stratification is established. This is a consequence of wind protection by surrounding vegetation, short fetch, and high amounts of dissolved and suspended material in the water. All four study basins have annual heat budgets that are low compared with temperate lakes of similar size and depth, but similar to values for many tropical basins. During winter circulation, oxygen penetrates to great depths in Lakes Sheelar, Tulane and Verona. The entire water column was oxygenated in Lake Sheelar during winter isothermy. Lakes Tulane and Verona, however, failed to oxygenate completely during winter mixing. Furthermore, oxygen was quickly depleted in deep waters of these relatively unproductive lakes after the onset of thermal stratification in spring. Johnson Pond was nearly completely deoxygenated during winter mixing. This was probably a consequence of dilution of oxygen in surface waters by deeper anoxic water and consumption of O(2) by reduced ions that accumulate in the hypolimnion.