The sun provides the energy which drives the world's wind patterns. Wind energy generates waves which lead to the vertical mixing of water in lakes. The light energy transmitted directly to the aquatic environment through solar radiation also influences the distribution of organisms and water temperature, as well as powering plant photosynthesis. Physical structural components of lakes include their shape, distribution of light, distribution of heat, and movement of water.
Lake Morphology (Shape)
The shape of a lake basin is largely determined by its mode of origin. The depth and contour of lake bottom can be determined by lowering a weighted line or much more quickly with an echo sounder. The hydraulic retention time (time required for all the water in the lake to pass through its outflow) is an important measure for lake pollution studies and calculations of nutrient dynamics. The hydraulic retention time is mainly determined by the interplay between inflow of water into the lake and the basin shape.
The approximate retention times for the five Great Lakes are:
- Lake Superior 191.0 years
- Lake Michigan 99.0 years
- Lake Huron 22 years
- Lake Ontario 6.0 years
- Lake Erie 2.6 years
Distribution of Light: The Photic Environment
The elements of lake structure that involve light distribution are often used to describe conditions within the influence of the shore or lake bottom. Primary production in large, deep lakes is entirely dependent on light transmitted through the water to enable photosynthesis.
The following depth zones are recognized in lakes:
a) littoral zone extends from the shore just above the influence of waves and spray to a depth where light is barely sufficient for rooted plants to grow.
b) photic (or "euphotic") zone is the lighted and usually well-mixed portion that extends from the lake surface down to where the light level is 1% of that at the surface.
c) aphotic zone is positioned below the littoral and photic zones to bottom of the lake where light levels are too low for photosynthesis. Respiration occurs at all depths so the aphotic zone is a region of oxygen consumption. This deep, unlit region is also known as the profundal zone.
d) compensation depth is the depth at which rates of photosynthesis and respiration are equal.
e) sublittoral zone, which is the deepest area of plant growth, is a transition between the littoral and profundal zones.
f) pelagic zone (or "limnetic zone") is the surface water layer in offshore areas beyond the influence of the shoreline.
Boundaries between these zones vary daily and seasonally with changing solar intensity and transparency of the water. There is a decrease in water transparency with algal blooms, sediment inflows from rivers or shore erosion, and surface waves.
Throughout the water column, water molecules and suspended particles reflect and absorb the various wavelenths which comprise incident light. The apparent colour (the colour that is due to light reflected from suspended particles or organisms and from bottom sediments) is further modified by the true colour (the colour of water that results from dissolved substances). True and apparent colours combine with topography, nearby vegetation, the lake bottom, and changing sunlight to produce the lake colour actually seen by an observer (actual colour).
In a series of glacial valley lakes, the lake nearest the glacier is usually white in colour due to a dense suspension of fine particles called glacial flour. The brilliant green characteristic of the second glacial lake in the chain is due to lower amounts of glacial flour as heavy particles settle out. Finally, with more settling, transparent blue lakes terminate the chain.
Distribution of heat
The elements of lake structure that involve water movement and distribution of heat are often used to describe offshore conditions in the pelagic zone. Water mass has a characteristic vertical temperature structure independent of the shape of the basin. As sunlight penetrates and is absorbed (especially light at the red end of the spectrum), it is converted to heat and if it were not for the unique nature of water and the frequent mixing of the lake, the same vertical distribution would be observed for temperature as for light.
Seasonal distribution of heat
Thermal Stratification: Summer
Thermal stratification, which contributes much to lake structure, is a direct result of heating by the sun. Thermal stratification is the phenomenon in which lakes develop two discrete layers of water of different temperatures: warm on top (epilimnion) and cold below (hypolimnion). These layers are each relatively uniform in temperature but are separated by a region of rapid temperature change (the metalimnion or thermocline).
The distribution of water as measured by temperature is a reflection of the differences in its density. Colder, denser water is on the bottom; zone of rapid change above, and warmer, less dense water at the surface of the lake. Although storms may stir the warm waters of the epilimnion into furious motion, little energy is transmitted through the thermocline to the cool quiescent hypolimnion. Consequently, the epilimnion is often called the mixed layer.
The remarkable mixing of the epilimnion is explained by the greater density change per degree of temperature change in warm water than in cold. Thirty times as much energy is required to completely mix equal volumes of 24 degrees Celcius and 25 degrees Celcius water as it takes to mix the same volumes of water at 4 degrees Celcius and 5 degrees Celcius. The resistence to mixing linked to these density differences maintains the stratified structure in lakes, which is important to the distribution of dissolved chemicals, gases, and biota. Thermal stratification is most characteristic of deep lakes. Shallow lakes never stratify for more than short periods of time.
The position of the thermocline is not fixed in depth as it gradually descends during the summer. As less solar radiation reaches the water and there is greater heat loss at night during the fall, convection and wind mixing begin to break down the thermocline. As autumn progresses, the epilimnion increases in depth as it decreases in temperature. Eventually the density and temperature difference between overlying water and water below it is so slight that a strong wind in late fall overcomes the remaining resistance to mixing. Thermal stratification is lost and the lake becomes a uniform temperature with depth. This is known as "fall overturn".
Winter: Freezing and inverse stratification
Lakes freeze over when surface waters reach 0 degrees Celcius on a windless, cold winter night. The presence of ice cover prevents further wind mixing and conserves heat remaining in the lake. Ice forms on top of the lake rather than on the bottom due to the unique density shifts in water. The maximum density of water is reached at 4 degrees Celcius, not at its freezing point. As winter progresses, ice increases in thickness and may acquire a layer of insulating snow. A characteristic thermal structure known as inverse stratification develops where water at 0 degrees Celcius is in contact with the ice but is warmer a few centimeters below. The water is warmest near the bottom where it is usually 4 degrees Celcius.
In spring, the lake ice melts and wind mixes cold water until lake temperature increases enough from the sun for thermal stratification to be re-established.
The Great Lakes
In a few of very large cold lakes, such as the Great Lakes, the thermocline does not form all over the lake at the same time. In the spring, the Great Lakes' waters are divided into two sections, offshore unstratified water at 4 degrees Celcius and a warmer weakly stratified mass nearshore. The thermal bar enhances early algal growth by effectively trapping both heat and nutrients from spring meltwater. The 4 degrees Celcius water between the two is the densest and sinks. The zone of dense sinking water is called the thermal bar - it gradually moves offshore until the entire lake stratifies. The nearshore water becomes warmer than the main water mass because it is relatively shallow and the heat is contained in a small volume. In smaller lakes, the nearshore waters are quickly wind-mixed horizontally into offshore waters.
Lake Classification According to Frequency of Mixing Events
Many lakes in Canada follow a cycle of summer stratification coupled with mixing twice a year - once during the fall and once during the spring. These lakes are known as dimictic and an example is Lake Erie. Other lakes, such as Lakes Superior, Huron, and Ontario, are called monomictic because they have one long mixing period all winter since they do not freeze. Amictic lakes, found high in the mountains or in the Arctic, have year-round ice cover and never mix. Oligomictic lakes are also found in cold climates but may thaw every few years. Holomictic is an umbrella term that refers to the types of stratification which describe lake circulation that occurs throughout the entire water column.
Meromix: Chemical stratification
A number of lakes never undergo complete circulation, with surface and deep waters never mixing. Their resistance to mixing derives from varied levels of salt concentration with depth. Meromixis occurs in many of the saline lakes of the prairies. The mixolimnion is the surface layer that contains fresh water and is able to turn over or mix. The mixolimnion often stratifies thermally as in a normal lake so that the meromictic lake is divided into three isolated areas. The chemocline is the layer of rapid water density change with depth. It is analogous to the thermocline and functions in the same way to isolate the upper and lower layers. The monimolimnion consists of the deep stagnant waters below the chemocline. This water contains little dissolved oxygen, having been depleted by decomposition of organic matter by bacteria.
Modifying effects of lakes on environment
Water has an enormous capacity for heat storage since it has the highest specific heat of any naturally occurring substance. As a result in deep lakes, the water takes a long time to heat. A deep lake with a large water volume can contain more heat than a shallow lake at the same location merely because it has more water. Local effects such as exposure to sunlight, protection from wind, and warm submerged springs are important in determining the thermal regime of smaller lakes. Shallow lakes stratify, destratify, freeze, and thaw more rapidly than deeper lakes in the same vicinity.
Large lakes, such as the Great Lakes, actually modify the climate of the surrounding area by virtue of their great heat content. The microclimatic shifts created are important in preventing early frosts. Lakes at higher latitudes and elevations, although subject to more intense solar radiation, tend to be cooler because of lower ambient air temperatures. Winter ice in these areas may persist well into summer and the open-water period may be brief.
Inflowing sediment and nutrients as well as thermal power plant discharges may affect the heat content of a lake. Pollution may change freezing and thawing dates or the onset of thermal stratification, which will affect lake chemistry and biota.
Water Movement: What are waves and currents?
Waves consist of the rise and fall of water particles, involving some oscillation but no net flow. Currents consist of net unidirectional flows of water. Currents and waves normally occur together. Part of the wind's kinetic energy goes into the continuous formation of surface waves which lose their form and dissipate their energy as they break on the down-wind shore. Some of the wind energy is transferred indirectly via breaking waves to currents. Currents build up much more slowly than waves, depending on the forces of gravity, solar radiation, and wind, but eventually contain most of the lake's kinetic energy. In addition, wind induces internal waves in the thermocline and hypolimnion.
Surface Waves (also known as progressive waves) are wind-driven. Regular patterns of smooth, rounded waves are called swells.
Capillary Waves have wavelengths less than 6 cm and are restored to equilibrium due to the surface tension of the water.
Gravity waves have wavelengths greater than 6 cm and fall due to force of gravity.
Surface Seiches are generated when the wind blows for an extended period from one direction, driving the surface water downward. The wind piles water up in the lee shore and remains there until the wind drops, at which time the driving force is released and the accumulated water mass flows back under the influence of gravity. This produces a standing wave which rocks back and forth with gradually decreasing motion. A series of waves are produced which are called standing surface gravity waves or "surface seiches". The sloshing back and forth of the water produces a standing wave at certain resonant frequencies. Surface seiches can also result from air pressure and the pressure of rain falling.
Internal Seiches are more pronounced than surface seiches and occur at the thermocline. Internal seiches are determined by the natural resonant frequencies of the basin and involve oscillations of the stratified layers of the lake. The surface waters may not move, but the thermocline may move up and down quite dramatically.
River Inflow Currents
Rivers cause currents when they enter a lake. River water entering a lake may be of different density than the lake water. The relative densities of the river and lake waters change seasonally and depend on temperature, dissolved substances and silt load. River water may flow at the surface of the lake (less dense river water) - known as overflow, along the bottom (underflow - more dense river water) or at an intermediate depth (interflow - river water of same density as lake water). Eventual mixing occurs due to turbulence, which happens sooner when density differences are low and currents already in the lake are high.
Convection is the movement of energy by mass displacement. An example is the cooling and consequent sinking of surface water on summer nights.
Surface currents in large lakes and estuaries flow at approximately 45 degrees to the direction of the prevailing wind. This direction of flow results from the Coriolis force that derives from the earth's rotation.
Deep water currents below the surface move at progressively greater angles to the wind the greater the distance from the surface. The deepest currents flow in an opposite direction to the wind. This spiral staircase flow of currents is called an Eckman spiral.
Langmuir circulations. On a windy day, lines of foam called windrows can often be seen oriented in the same direction as the wind and at right angles to the waves. These lines mark the boundaries of pairs of Langmuir circulations, a series of adjacent horizontal clockwise and counterclockwise rotating cells of water. They produce alternate areas of upwelling and downwelling, and the foam occurs above the downwelling zones. The windrows or slicks contain algae and zooplankton as well as oily substances or naturally foaming substances from the death and decay of plankton or shoreline vegetation. Langmuir circulations result from the interaction between surface waves and wind-driven currents.
Longshore currents are produced as waves approach a shoreline. As the waves enter shallow water, they are refracted and generate a current parallel to the coast, zigzagging in the prevaling direction of the incoming waves. Longshore currents work in combination with wave action to transport large amounts of sand, gravel, and sediment along the shore - a process known as beach drift.