Ch4 Variation in the Environment5.pdf

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University of Toronto St. George
University College Courses
Spencer Barrett

 coldest water at surface, warmer water (max. 4°C) at bottom  b/c density of water increases btn the freezing point and 4°C  in early spring, lake surface warmed gradually by the sun  warmed surface water sinks below until temp. exceeds 4°C  this vertical mixing distributes heat throughout the water  results in an uniform temperature profile  at the same time, winds drive surface currents that can cause deep water to rise (similarly to upwelling currents)  this spring overturn brings nutrients from the bottom sediments to the surface and oxygen to the depths  in late spring/early summer (sun rises higher and air above lake warms up), surface layers gain heat faster than the deeper layers  creates a tthermocline a zone of abrupt temperature change at intermediate depth  once established, water doesn’t move across it b/c of stratification  stratification is a condition where warmer, less dense surface water literally floats on the cooler, denser water below  depth of thermocline depends on winds and depth & turbidity of lake o around 5m-20m below surface o lakes less than 5m usu. lack stratification  epilimnion is the upper layer of warm water above the thermocline o where most of the production occurs (sunlight most intense here) o photosynthesis supplies oxygen here and supports animal life o but the abundance of plants and algae often deplete the mineral nutrients here, thus limiting their own production  hypolimnion is the deeper layer of cold water below the thermocline o
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