Mixing - Lecture 5 Vertical Structure and Mixing Regimes...

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EVPP/BIOL 350 Freshwater Ecosystems Dr. Kim de Mutsert Fall 2011 Lecture 5 – Vertical Structure and Mixing Regimes
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In freshwater, temperature is the main determinant of density The warmer the water, the better it floats, but ice floats even better Water becomes less dense as it warms The difference in density per degree of warming increases as temperatures rise
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Stratification Stable stratification results when waters of differing densities are positioned vertically in order of their density In other words, more density (heavier) water lies below less dense (lighter) water Work is required to break down this density gradient
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D e p t h Lig ht a nd  Te m p x x x x Predicted temp profile based on light attenuation Light absorbed and heat produced highest near surface and decreases with depth
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D e p t h Temperature x x x x When the wind blows, it mixes the surface water with deeper water And its energy dissipates with depth x x When wind enters the picture So the wind mixes the upper layers, but cannot reach the lower layers which makes for a steep decline in Temp and Density Epilimnion Epilimnion Hypolimnion Metalimnion Metalimnion
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Annual Mixing Patterns Mixing occurs when the density of the water body becomes uniform or nearly uniform with depth Dimictic - Stratification twice a year (and thus mixing in between) Monomictic Stratification only in summer (mild climates) Polymictic Frequent mixing (shallow lakes) Amictic If the water body never mixes (permanent ice cover) Meromictic
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D e p t h       Temperature 0 o C Winter Dimictic lake
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D e p t h       Temperature 0 o C Just after ice-out
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D e p t h       Temperature 0 o C Early spring
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D e p t h       Temperature 0 o C Warming throughout spring
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D e p t h       Temperature 0 o C 10 o C Early summer
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D e p t h       Temperature 0 o C 10 o C Upper layer mixing
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D e p t h       Temperature 0 o C 10 o C 20 o C Summer thermocline
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Bottom water colder than surface in summer Surface water is very buoyant BIG density difference between surface and cold bottom water = resistance to mixing Mid-summer thermal stratification
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D e p t h       Temperature 0 o C 10 o C Fall
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D e p t h       Temperature 0 o C F a l l T u r n o v e r
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D e p t h       Temperature 0 o C Winter
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Dimictic Lakes – Annual Cycle Seasonal heating and cooling Wind creating turbulence
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Mixing - Lecture 5 Vertical Structure and Mixing Regimes...

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