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Most of Maine's lakes and ponds are classified "dimictic", which means that they mix, or "turn over" twice annually-once in the spring just after the ice melts, and again in the fall when the water temperature drops to the point where the temperature is uniform from the surface to the bottom.
Fall turnover can occur as early as late summer or early September in shallow lakes, and as late as October and November for some of Maine's larger, deeper lakes. In both cases, the weather has a strong bearing on the timing of the mixing event.
Air temperature, sunlight and wind are all factors that influence exactly when the fall turnover takes place. A period of strong, cold wind can produce enough energy to overcome the barriers to mixing that exist during the summer months when most lakes experience thermal stratification.
Fall turnover/mixing has a strong effect on lakes that have been stratified for up to five months, during which time the coldest water near the bottom of the lake is unable to replenish dissolved oxygen that is consumed by bacteria, in the process of decomposing accumulated organic matter.
When the mixing event occurs, water from the bottom is swept up to the surface, where exposure to the wind and the atmosphere quickly replenishes the oxygen. The turnover experience is quite literally like a "breath of fresh air" for our lakes!
Maine Volunteer Lake Monitoring Program
vlmp@mainevlmp.org
24 Maple Hill Road, Auburn, ME 04210
(207) 783-7733
www.MaineVolunteerLakeMonitors.org
copyright 2010 Maine Volunteer Lake Monitoring Program | website comments to: vlmp@mainevlmp.org
