Stratification

An illustration of lake stratification.

Water temperature is important in lakes, affecting nearly every aspect of the lake ecosystem.

Aquatic life is affected directly and indirectly by water temperature. Some fish species cannot survive in warm water, while others cannot survive in cold.

The amount of dissolved oxygen in the water is related to temperature, with more dissolved oxygen found in colder water. Dissolved oxygen decreases as water warms.

LAKES HAVE LAYERS

Water temperatures also play a role in water density. Warm water is less dense meaning it is lighter and stays toward the top of the lake. The colder, heavier water is found at the bottom.

Deeper lakes create layers based on water density. This is called stratification. During summer months, these lakes have three defined layers.

Epilimnion: This is the upper, less dense layer of warmer water, that is readily mixed by wind. Oxygen producing plants and algae typically live in this zone.

Metalimnion: Also called the thermocline, this is the middle layer characterized by a steady drop in water temperature, that prevents water mixing between the epilimnion and hypolimnion. Where this layer begins is dependent on the how far down into the water the sun’s rays reach.

Hypolimnion: This is the bottom, denser layer of cold water. Low light levels in the hypolimnion prevent growth of aquatic plants and algae.

In lakes with high nutrients, a summer thermocline layer can create issues. Once the thermocline is formed, it prevents mixing between the epilimnion and hypolimnion.

The oxygen plants and algae in the epilimnion produce oxygen, but the oxygen-laden water cannot mix with the lower hypolimnion.

As the summer draws on, oxygen decreases in the hypolimnion due to bacterial decomposition. This creates anoxic conditions, and releases phosphorus bound to the lake bottom sediments back into the water, leading to algal blooms.

MONITORING

Carver County Water Management Organization staff monitor water temperature, dissolved oxygen, pH, and specific conductivity in 19 lakes. Measurements are taken starting at the water’s surface and every meter down to the lake bottom. This occurs every other week from April to October. Winter measurements are taken once during winter series.

Of the 19 lakes monitored, nine are deep enough to stratify. Staff are studying those nine lakes to determine if thermocline depth is changing as a result of changes to temperature and precipitation.

Staff calculate thermocline depths and create heat maps which show how lake temperature changes throughout the spring, summer, and fall.

Trend analysis on thermocline depth was performed on lakes with at least seven years of data. Of the lakes with enough data, only one, Courthouse Lake, is showing a significant change in thermocline depth over the last nine years. The thermocline is getting deeper within Courthouse Lake. All other lakes are showing no trend in thermocline depth currently.

Andrew Edgcumbe is a water resources technician with the Carver County Water Management Organization.

Community Editor

Mark Olson, the Chaska and Chanhassen community editor who has worked in Carver County for 20 years, makes any excuse to write about local history. In his spare time, Mark enjoys perusing old books, watching blockbusters and taking Midwest road trips.

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