Lakes: Physical Processes
Most lakes in temperate regions, due to a combination of solar heating and wind-mixing of surface layers, undergo a fairly predictable seasonal pattern, alternating between stratification (layering) and complete mixing.
Thermal Processes
As light energy is absorbed by water, it is converted to heat energy, which results in the warming of the lake-surface water. During summer, thermally stratified lakes are warmer at the top and cooler at the bottom. The warm surface layer is called the epilimnion, and the cooler bottom layer the hypolimnion. The zone of rapid temperature decline in the water column from shallow to deep water is called the thermocline or the metalimnion.
Once formed, the thermocline is a thermal barrier to the mixing of surface and bottom waters. Energy is required to mix water of differing densities, and the amount of energy required is related to the difference in density. In the case of lake water-column mixing, this energy is provided primarily by wind. In stratified lakes, the depth of the epilimnion is limited by the ability of wind to consistently mix the surface water.
The depth of the thermocline is generally related to transparency (clarity) of the lake water and the degree of exposure of the lake to wind. A clear lake will have a deeper thermocline than a turbid lake. A wind-exposed lake will have a deeper thermocline than a protected lake.
Stratification usually lasts all summer. During this time, the thermocline creates a strong and effective barrier to water-column mixing, isolating the hypolimnion from gas exchanges with the atmosphere. In addition, the hypolimnion often is too poorly lit for photosynthetic production of oxygen by green plants. Without an influx of oxygen, the hypolimnion can become depleted of oxygen during summer stratification due to bacterial decomposition of organic matter, which uses oxygen.
In the late summer and fall, the epilimnion begins to cool and the temperature zonation begins to break down. Once the thermal barrier is gone, the lake reaches a uniform temperature, and it completely mixes, or "turns over," from top to bottom. This destratification process often is called fall overturn.
As the lake surface begins to freeze in the winter, water's unique temperature–density relationship comes into play. In general, water becomes more dense as it cools, reaching its maximum at 3.94°C (39.2°F). In summer, this allows the warmer, less dense, epilimnetic (upper) waters to float on top of the cooler, more dense, hypolimnetic (lower) waters. But in autumn, as the water temperature falls below 3.94°C, its density suddenly
At 0°C, ice forms and its density has decreased substantially while adding about 9 percent more volume; hence, ice forms on the surface of the lake rather than at the bottom or within the water column. Once formed, an ice cover becomes a barrier to wind-induced mixing, and the relatively warm water remains protected beneath the ice. In this way, the ice cover allows fish to overwinter in the lower layers of the lake, as long as there is water beneath the ice. (At cold enough temperatures, however, shallow lakes will eventually freeze solid from top to bottom.)
In the spring, the ice melts, water temperature again becomes uniform from top to bottom, and the water column mixes completely in a process called spring overturn. Lakes that mix twice a year (fall and spring) are known as dimictic.
The stratification and mixing regimes of lakes depend on climate patterns and on the lake's shape, depth, and water chemistry. Some lakes never stratify and hence never mix (amictic), while others mix only once during the year (monomictic). Some lakes, commonly in tropical regions, stratify and mix several times a year (polymictic).
Oxygen Levels
Euthrophic lakes are rich in nutrients and highly productive. Decomposition of the abundant organic matter often may reduce the dissolved oxygen in poorly lit or unlit zones, leading to either hypoxia or anoxia in the hypolimnion. Under anoxic conditions, nutrients such as phosphorus and nitrogen are released from the bottom sediments to the overlying water, where they ultimately promote additional algal production, organic matter decomposition, and hypolimnetic oxygen reduction over a greater area. Most fish require relatively high dissolved oxygen levels and cannot survive in an oxygen-deficient hypolimnion.
Fall overturn begins with the complete loss of summer stratification. The complete mixing of the upper warm water and oxygen-depleted colder dense bottom water results in an overall decrease in oxygen levels within the lake, which in some circumstances can be severe enough to cause a fishkill.
Fishkills can occur both in the summer and winter as a result of low dissolved oxygen. Usually, summer fishkills occur in euthrophic lakes following rapid algal die-off (such as during a bloom), when the resulting bacterial-mediated decomposition depletes the dissolved oxygen. In winter months when lakes freeze at the surface, the ice and snow limit both water mixing and light penetration, and the available dissolved oxygen supply can, over the winter season, be progressively reduced to levels that cause a winter fishkill.
Sedimentation
Waters entering a lake generally carries suspended particles, including organic matter, clay, and silt washed from the drainage basin and carried by streams to the lake. In addition, sediments can carry substantial quantities of absorbed nutrients and other chemical contaminants. These suspended particles begin to settle once they reach the relatively quiescent (quiet) lake environment. Much of the sediment entering a lake, together with organic particulate matter produced internally, eventually settles to the lake bottom. The rate of sedimentation and types of materials deposited determine the physical characteristics of the bottom substrate, which in turn has tremendous influence on oxygen levels in and near the bottom sediments and thus influences the types and productivity of organisms that live there.
The physical factors commonly associated with the distribution of sediment in a lake are lake volume in relation to amount of inflow, water currents, properties of the sediment, lake mixing mechanisms, and lake shape. Normally, the distribution of bottom sediments exhibits a gradation in particle size. The coarsest sediments extend from the lake inflow point (stream), where the carrying capacity of the stream begins to diminish due to increased surface area, to where the backwater effect of the lake becomes negligible as far out into the lake that the currents will carry them. These deposited sediments form point bars and deltas .
Deposition deltas and point bars will increase in height and extend upstream as these depositional features proceed into the lake. Such features may be temporary and severely eroded by inflowing water and sediment if the water level is below the level at which the delta or point bar was formed. Resuspension and gravity flows will move some of this material along the bottom far into the lake. Finer particles, silts and clays, are carried out into the lake in suspension and settle slowly and somewhat uniformly over the bottom. Density currents will move some of the fine material along the bottom far into the lake and will produce an additional accumulation near the lake outflow, or in the center of a lake with no outflows.
SEE ALSO Algal Blooms in Fresh Water ; Fresh Water, Physics and Chemistry of ; Lake Management Issues ; Lakes: Biological Processes ; Lakes: Chemical Processes .
Thomas E. Davenport
Bibliography
Hutchinson, George E. A Treatise on Limnology: Vol. 1. Geography, Physics, and Chemistry. New York: John Wiley & Sons, Inc., 1957.
New York State Department of Environmental Conservation and Federation of Lakes Associations, Inc. Diet for a Small Lake: A New Yorker's Guide to Lake Management. Albany, New York: New York State Department of Environmental Conservation and Federation of Lakes Associations, Inc., 1990.
North American Lake Management Society. Lake and Reservoir Restoration Guidance Manual, 2nd ed. EPA–440/4–90–006. Washington, D.C.: U.S. Environmental Protection Agency, Office of Water, 1990.
Phillips, Nancy et al. The Lake Pocket Book. Alexandria, VA: Terrene Institute, 2000.
Wetzel, Robert G. Limnology, 2nd ed. Philadelphia, PA: W. B. Saunders Co., 1983.
Wetzel, Robert G. Limnology: Lake and River Ecosystems, 3rd ed. San Diego, CA: Academic Press, 2001.
LESSENING LAKE-ICE COVER
Historic records on the formation and breakup of ice cover for many lakes and rivers suggest that freeze dates over the past 100 years have become several days later, and breakup dates several days earlier. In other words, the duration of winter ice cover appears to be shorter. Whether this is suggestive of, or caused by, global warming is under debate.