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Classifying Lakes and Ponds

More about lakes and ponds:

Lakes, ponds, and reservoirs are classified many different ways. Some ways are according to the overall clarity of the water (trophic state), the parts of the water where sunlight reaches, or the temperature differences from top to bottom. Lake zones are especially useful when describing the conditions of lakes and studying their characteristics and environments in a uniform manner.

Trophic State
Secchi disk transparency, chlorophyll a, and total phosphorus are often used to define the degree of eutrophication, or trophic status of a lake. The concept of trophic status is based on the fact that changes in nutrient levels (measured by total phosphorus) causes changes in algal biomass (measured by chlorophyll a) which in turn causes changes in lake clarity (measured by Secchi disk transparency). A trophic state index is a convenient way to quantify this relationship. One popular index was developed by Dr. Robert Carlson of Kent State University. His index uses a log transformation of Secchi disk values as a measure of algal biomass on a scale from 0 - 110.

Lake Trophic Classification Nutrient Concentration Biological Productivity
oligotrophic low low
mesotrophic moderate moderate
eutrophic high high
hypereutrophic very high very high

Sunlight
Lakes are divided into zones based on amount of sunlight. There is the: 1) littoral at the lake's shoreline where sunlight can reach the bottom; 2) the limnetic, which is commonly recognized as the open waters; and 3) the profundal, or deep water where sunlight does not penetrate. The amount of light influences the life in the waters by affecting the plants, large and small, that use the light for growth through photosynthesis. If waters are too cloudy for the light to reach plants that normally live in shallow waters, then these "submerged aquatic vegetation" will die along with the important habitat they provide.

Snowbank Lake in Boundary Waters - photo by Wayne Davis

Temperature
The water column of deep-water lakes is further divided into three zones called the epilimnion (surface water), hypolimnion (bottom water), and the metalimnion (or thermocline), which is the transitional area between the bottom and surface water. Temperature affects whether lakes freeze over in the winter or if they have cool water at the bottom to support fish in the hot summer.

In many lakes or waters with a hypolimnion, all of the oxygen may be used up in the summer because the thermocline does not allow mixing of the layers. Mixing occurs when the temperature of the lake becomes more uniform from top to bottom, usually in the spring and/or fall. The mixing replenishes oxygen to the bottom waters and releases nutrients and food to the top waters. Lake Erie mixes this way in both the spring and fall. To learn more about how and why lakes stratify like this, check out the discussion on "lake processes - stratification and turnover" in the Great Lakes Atlas from EPA's Great Lakes National Program Office (GLNPO).

Special Habitats of Lakes and Ponds

The two main habitats of lakes and ponds are their open waters and bottom. The open water habitat exists within the limnetic and profundal zones whereas the benthic habitat is found at the very bottom of the lake. The formation of each habitat type is directly related to the lake's origin, however this is one of the only commonalties.

Open-water; the Limnetic and Profundal Habitats

Boat on a Lake - photo by Wayne Davis
We can identify lakes and ponds by their physical characteristics that fall under 3 general categories: 1) water conditions, 2) interactions between the water and outside elements such as air and land, and 3) drainage. Water conditions may be described by temperature, light intensity, seasonal variations, and chemical composition. Chemical composition includes dissolved nutrients, minerals and other organic substances. The air-water interaction is measured by the turbulence and gas exchanges resulting from the mixing of the water column by wind. The land-water interaction is described by instances of enclosing surrounding land formations, turbidity, erosion, runoff including pollutants, and flood cycles. Drainage simply describes a waterbody's tendency to lose/transport water.

Open Water: A Glimpse at the Structure of a Food Web
Primary producers dominate the open water of the limnetic zone. Photosynthesis occurs in this zone where phytoplankton that oxygenate the water are ingested by the slightly larger zooplankton. Insects and fish eat zooplankton, and are eaten by larger fish, insects, amphibians, and other animals. The rate of production by phytoplankton is directly related to nutrient concentrations, phytoplankton variety and the flushing rate of the waterbody. Rapid influxes of nutrients lead to blooms, or rapid, abundant growth of phytoplankton. Blooms give lakes and ponds their familiar dark green color. Of course, the presence of herbivorous zooplankton may greatly affect the growth of phytoplankton. In instances

Algal Blooms
where nutrient levels become excessively high and the waterbody's trophic classification is eutrophic, algal blooms may occur.

Algal blooms can be very dangerous to the health of their respective ecosystems. Algal blooms are the result of too much food for the algae making them grow and reproduce. The food is in the form of nutrients, often washed in from the land during a rainfall. The bad part is that they grow so fast that they become so thick, they block the sunlight from reaching algae below the surface. The algae without the sunlight begin to die and drop to the bottom of the lake or pond. When they die, bacteria help decompose the cells in a process that uses up the oxygen dissolved in the water. This long process means that fish and other animals and plants can not get what they need to live.

Plants and Animals

Vegetation - photo by Wayne Davis
Lake ecosystems are dominated by autotrophic organisms where phytoplankton, periphyton, and aquatic plant species make up the majority of species. Species type varies with trophic level. For example, diatoms are common in oligotrophic waterbodies. Zooplankton, the floating aquatic animals that may be microscopic, but are often large enough to feed upon phytoplankton, are a primary food source for numerous other open water species, and therefore dominate the species composition. Common zooplankton found in lakes and ponds include rotifers, water fleas, and copepods. Insects such as water beetles and larval mosquitoes are also common. Various freshwater fish feed upon the plankton and insect communities including brook trout and perch and amphibians are not uncommon. All sorts of birds may be found in and around open water including ducks, osprey, and raptors that feed upon the abundant food resources.

Down at the Bottom: Benthic Habitats
The benthos or benthic zone of lakes and ponds lies at the bottom where decomposition occurs. Although biological activity flourishes in the darkness, it is an extremely oxygen deficient environment dominated by anaerobic bacteria.

Physical Characteristics
The benthic zone has topography. Depressions and elevations form in the soil, which is usually a composition of silt, clay, and organic debris. Large-scale drainage may effect the benthic zone by increasing light and oxygen levels.

Plants and Animals and the Lack Thereof
The absence of light prevents any vegetation from growing in this aquatic habitat, and while life is not abundant, the organisms who subsist in the benthic zone have adapted to the low oxygen, saturated soil conditions. The animals of the benthic zone include microscopic species and many forms of what are known as macroinvertebrates.


How Were Lakes And Ponds Formed?

There are several ways in which lakes and ponds are formed. These are the most common types of lakes you may encounter.

Glacial Lakes
Glacial lakes occur where basins have been excavated by moving ice or where drainage patterns have been altered by deposition of glacial till.

Oxbow Lakes
Oxbows are formed when river meanders are cut off from the main channel and are therefore generally found in the immediate vicinity of rivers.

Levee Lakes
Levees are formed when the water levels of rivers become too high and/or flood and deposit enough sediment to form a completely separate waterbody. Levees may also be man-made obstructions along the edge of a waterbody used to restrain the flow of water out of a riverbank.

Sinkholes
A pond formed in the depression left by dissolving organic materials such as salt and limestone and may be alkaline (have a pH greater than 7) and have an unusually high salt content.

Barrier Lakes
Barrier lakes occur behind sand bars in coastal regions and have characteristically high levels of salinity. Water of this type, a mixture of fresh and salt waters, is referred to as brackish water. Brackish water may result from tidal flow or sea spray.

Beaver Ponds
Beaver ponds are small waterbodies constructed and maintained by beavers. Local vegetation includes preferred beaver foods such as alder, birch and poplar. Although beaver ponds may be sustained for several years, they are often abandoned when food supplies run low.

Learn more about lakes and ponds: Exit EPA Disclaimer

Biological Indicators | Aquatic Biodiversity | Statistical Primer

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