Chap.10 - Water Density and Layering in Lakes

Chap.10 - Water Density and Layering in Lakes

Lake Stratification Epilimnion upper part of lake. Even temperature due to wind mixing Hypolimnion bottom layer of lake water. No wind mixing Thermocline or metalimnion temperature decreases with depth. Located between upper and lower zones.

Stratification

Water Density Water is heaviest at 4C (40F). It is lighter above and below this temperature. Salinity increases density and decreases freezing point. Suspended Particles change water density. Make it heavier.

Lake Stability Lake Stability It takes a lot of energy to mix water of different densities. S= amount of work energy required to entirely mix a lake to a uniform temperature without adding or subtracting heat.

Lake Classification Mictic and mixis mean mixing and are terms used in lake classification to describe how the lake mixes. Some lakes mix differently every year. So they are hard to classify.

Amixis Amicic lakes never circulate. Covered with ice. Antarctica and Greenland. Some of these lakes have unusual stratification profiles due to saline stratification or temperature fluctuations due to thermal springs. Amictic lakes can also be found at high altitudes. Peruvian Andes and on Mount Kenya in Africa.

Lake Classification Holomixis - Wind circulation mixes entire lake. Oligomictic Monomictic Dimictic Polymictic Meromictic Lake Nicaragua

Lake Maggiore Italy Lake Classification Lake Geneva Switzerland Oligomictic lakes generally equatorial lakes. There is warm water at all depths. Very little seasonal change. Stratified and usually stable because water is very warm on the surface and relatively cooler at lower depths. Circulation is unusual, irregular and of short duration. Cold oligomictic lakes- large sub-alpine, European lakes (Lake Geneva Switzerland) and Lake Tahoe in U.S. Lake Tahoe CA

Lake Classification Monomictic Lakes - One regular mixing period sometime during the year. Usually exist in climates where seasonal changes are strong Elephant Butte NM Lake Oneida

Lake Oneida Cold Monomixis Frozen during winter months. Water shielded from wind. Water stagnant, no mixing in winter (amictic). 0 C just below ice to 4C at bottom (inverse stratification). Ice melts in spring Stratification stability is low and water mixes. No summer stratification. Ex. Lake Oneida NY New Quebec Crater Lake Lake Hazen Ellesmere Island

Warm monomixis Lack ice cover Circulate in winter stratify in summer Usually occurs south of 40 degrees latitude Some Oklahoma lakes are warm monomictic if they are deep enough and somewhat sheltered from the wind. Great Lakes are warm monomictic Lake Victoria, lies across the equator. Expect it to be holomictic ( in circulation all the time). However, it stratifies in the rainy season. Cool rain water flows to bottom setting up stratification from bottom up. Lake Atitlan Lake Superior

Dimictic Lakes Two mixing periods Spring and fall overturns This is the idealized, typical temperate zone lake. Stratification is strong during warm summer months Falling temperatures and wind break down stratification in the fall. Lake circulates when water reaches 4 C. Ice forms on lake in winter, no circulation Water becomes stratified, 4C on bottom, may be colder on top. Spring ice melts. Lake temperature much the same top to bottom Lake Itasca Wind mixes lake. Mixing continues until summer stratification sets in. Lake Tom Wallace

Polymictic Lakes Many mixing periods or continuous circulation. Influenced more by daily changes than seasonal changes. Some Andean Lakes stratify during the day and destratify at night. Can happen with desert lake also. Lake Nicaragua (very shallow) stratifies at night but is mixed each afternoon by wind. Ponds exposed to wind in Oklahoma can be polymictic. Lake Nicaragua

Remember! Lake categories can change with unusual seasonal changes.

Meromixis Big Soda Lake NV Meromictic lakes circulate sometimes but not completely. Dense stratum of bottom water remains stagnant and usually anaerobic.

Terms for regions in a meromictic lake. Monimolimnion permanently stagnant bottom layer. Greater concentration of dissolved substances then overlying waters. Mixolimnion more dilute, mixed by wind. Shows seasonal changes. Chemocline Between the mixolimnion and the monimolimnion. Chemical gradient. Salinity increases rapidly with depth.

Meromixis Biogenic Meromixis caused by an accumulation of substances derived from bacterial decay, diffusion from the sediment and carbonate precipitation from photosynthesis. Lake Tanganyika Fayetteville Green Lake

Biogenic Meromixis No accumulation of NaCl(salt). Substances found are what you usually see in a eutrophic lake at the end of summer. Anoxic water, high Hydrogen sulfide. Devoid of most life except for anaerobic bacteria. Biogenic meromictic lakes form when lake shape (steep sides, deep), surrounding topography (lake sheltered by woods and hills), and weather conditions that hinder overturn combine with accumulation of biologically derived substances that increase water density on lake bottom. Round Lake NY

Meromixis Ectogenic meromixis - Water density differences brought about from outside sources. Soap Lake WA

Ectogenic meromixis Judd Lake Can enter lake in 2 forms 1) dilute upper layer that lies above an existing saline layer. 2) Outside source of saline water finds its way to bottom of a freshwater lake. Topographic, morphological and meteorological factors not so important with this kind of meromixis. Soap Lake WA

Ectogenic meromixis This situation is being created in some parts of the U.S. where salt is used to de-ice roads. This creates saltwater runoff in the spring that flows to the bottom of freshwater lakes. Accumulations over time create a dense layer of salt water at the bottom of the lake. Judd Lake

Ectogenic meromixis Hot Lake WA. Lake water is rich in magnesium sulfate (epson salts). Freshwater from snow melt enters the lake each year and forms a layer of fresh water on top of the magnesium sulfate rich water. Soap Lake WA

Meromixis Crenogenic meromixis subsurface flows of saline water into a lake plus freshwater inflow from the surface when in equilibrium produce this type of meromixis. Lake Kivu Africa is an example.

Unusual Temperature Profiles Dichothermy coldest stratum of water found at an intermediate depth. Bottom water warmed by bacterial metabolism of geothermal activity.

Unusual Temperature Profiles Mesothermy Temperature increases with depth then decreases before the lake bottom is reached. Results in a warm stratum sandwiched between cooler strata of water. Seen in meromictic lakes due to temperature changes caused by salinity differences.

Unusual Temperature Profiles Poikothermy complex temperature curves that occur when a meromictic lake is beginning to warm in the spring.

Unusual Temperature Profiles

The Thermal Bar Occurs In large cold lakes where temperature falls below 4C during winter. circulation Shallow water near shore warms before deep water farther out in the lake does. Deeper water continues to circulate while the warmer inshore water begins to stratify. The two regions are separated by a narrow zone of down-welling water. This water is at 4C and the most dense water in the lake. It forms a barrier between circulating off-shore water and warmer, much less dense in-shore water.

Thermal Bar

The Thermal Bar Nutrients flowing in from the water shed cannot penetrate thermal bar so phytoplankton blooms will occur out into the water until they reach it. The 2 lake regions are very different in chemical, optical and phytoplankton species. Thermal bar disappears when off-shore area temperatures warm and stratify during the summer.

The End