Information provided by the Wisconsin DNR
Types of Lakes
Lake types are divided according to the source of the water and the outflow of the water. To get a larger view of any of the pictures, simply click on them.
A seepage lake is a naturally occurring lake fed by precipitation, groundwater, and some runoff. There is no stream outlet. The water leaves a seepage lake through evaporation or groundwater flow. The flow of groundwater in and out is about equal. Water levels fluctuate seasonally to reflect the amount of precipitation and groundwater in the system. These are the most common lakes in Wisconsin.
Groundwater Drainage Lake
A groundwater drainage lake is fed primarily by groundwater and precipitation, and some runoff. A stream outlet accounts for most outflows, with limited amounts of water leaving through evaporation and groundwater. These lakes may also be called Spring lakes, for a spring is usually the source of the groundwater inflow. These occur at the headwaters of many streams.
A drainage lake is fed mainly by a stream or streams, and also limited groundwater, precipitation, and runoff. A stream or set of streams drains the lake, with little groundwater flow or precipitation. Most major rivers have a drainage lake along their course.
An impoundment is a manmade lake created by installing a dam on a stream. The stream is the main inflow, with precipitation, groundwater, and runoff accounting for the rest of the inflow. The major outflow is the stream beyond the dam, with some evaporation and groundwater outflow.
Chemistry of Lakes
Most sunlight that enters a lake converts directly to heat. At different depths, temperature influences the physical, biological, and chemical aspects of lake dynamics differently. The rate of decomposition, nutrient recycling, lake stratification, and dissolved oxygen concentration are all affected by lake temperature. Changes in temperature also affect the distribution of fish in a lake. Cold-water fish, like the trout and cisco, need cold lake water in which to survive.
Water temperature is the major factor behind stratification patterns in a lake. When a lake mixes, cold water from the bottom of a lake is brought to the surface, while warm water is mixed downward. Nutrients from the bottom sediment, like phosphorus, are brought up the water column. There is little temperature variation from top to bottom in the lake. When a lake stratifies, distinct temperature layers are formed, and the mixing of the water is restricted. The surface water, the epilimnion, (about the first 30 feet) warms the entire summer and continuously mixes, while the bottom layer, the hypolimnion, does not mix or get any warmer all summer. Shallow lakes (those about 20 feet deep) usually will not stratify, and will continuously mix from top to bottom.
Phosphorus is by far the most important nutrient in most of Wisconsin's lakes. Phosphorus promotes excessive aquatic plant growth. In over 80% of Wisconsin lakes, phosphorus is the limiting nutrient affecting the amount of algae or plant growth in a lake. This is because phosphorus is rapidly recycled and changes from dissolved to particulate form. Dissolved phosphorus can be used by phytoplankton (floating algae) and macrophytes (rooted plants) to grow. Particulate phosphorus is unavailable for plants to use. Also, once living organisms die, they sink to the bottom and the phosphorus becomes unavailable.
Phosphorus is mainly introduced to lakes due to human activities. Farmland runoff, lawn fertilizer, soil erosion due to construction, sewage, animal waste, and detergents all account for excess phosphorus entering a lake system. Once they enter a lake, it may take a long time until they move out of the system, and this depends on the retention time of the lake. Usually after a heavy rainfall, a eutrophic lake will exhibit an algae bloom due to increased phosphorus amount in the lake due to the above reasons.
Oxygen is the most important gas in the water, since all aquatic organisms need it to survive. The amount of dissolved oxygen water can hold depends on temperature. The colder water is, the more oxygen it can hold. The warmer the water, the less oxygen it can hold. When lakes become stratified, the epilimnion (top) and metalimnion (middle) get most of the oxygen available in the lake, while the hypolimnion (bottom) gets very little or none. Eventually, as the lake has been stratified for long enough, all of the oxygen in the hypolimnion gets used up in respiration by small organisms, plants, or fish. This is called oxygen depletion. As a result, fish that need high oxygen levels and cool water disappear from these lakes.
Photosynthesis, respiration, and the amount of oxygen entering or leaving the water at the surface usually determine oxygen levels. If the lake mixes completely, the oxygen levels will be higher than those lakes that do not entirely mix. Photosynthesis occurs when green plants use sunlight to produce oxygen and simple sugars as their energy source. This process only occurs in daylight. Chlorophyll, which gives plants their green color, serves as the oxygen production site. The more nutrients there are in the water and the warmer the water, the more photosynthesis occurs. Respiration occurs when plants and animals need to break down sugars to obtain energy, which is basically the reverse of photosynthesis.
Chlorophyll is the pigment found in all green plants that is responsible for their color. During photosynthesis, chlorophyll absorbs sunlight that is used as the energy source to convert carbon dioxide and water to oxygen and sugars. Chlorophyll data is collected because the green pigment is found in algae and can be used to estimate how much phytoplankton (floating algae) there is in the lake.
There are many more chemistry-related items that occur within lakes. However, since Temperature, Phosphorus, Oxygen, and Chlorophyll are the only ones that are used by Self-Help, they are the only ones currently featured.