Seasonal Changes in a Temperate Zone Lake

A Winter. lce covers the thin layer of slightly warmer water just below it. Densest (4°C) water is at bottom. Winds do not affect water under the ice, so there is little circulation. B Spring. Ice thaws. Upper water warms to 4°C and sinks. Winds blowing across water create vertical currents that help overturn water. bringing nutrients up from bottom. C Summer. Sun warms the upper water, which floats on a thermocline. a layer across which temperature changes abruptly. Upper and lower water do not mix because of this thermal boundaw. D Fall. Upper water cools and sinks downward. eliminating the thermo cline. Vertical currents mix water that was sepa rated during the summer. water between 0°C and 4°C water at 4"C overturn thermocline cvertu rn