Algae: One-celled or multi-celled plants that are either suspended in water or attached to rocks or other materials. Their abundance is measured by the amount of chlorophyll a in water samples.  Algae are essential parts of aquatic ecosystems, providing the food base for most organisms, including aquatic invertebrates and fish.  However, too much algae can degrade water quality by diminishing transparency or, when it decomposes, lowering dissolved oxygen. 

Aquatic invertebrates: Aquatic animals without internal skeletal structures, such as insects, mollusks, and crayfish.

Biomass: Amount of plant or animal matter, the total of which indicates the degree of a lake’s eutrophication or productivity.

Blue-green algae: Algae that often cause problems in lakes because some produce chemicals that are toxic to animals, including humans.  They often form thick floating mats of blue-green scum as they die.

Chlorophyll a: Green pigment present in all plant life and necessary for photosynthesis.  The amount in water samples depends on the amount of algae and is therefore used as a common indicator of water quality.

Conductivity: Measure of the ability of water to conduct electrical current.  It is directly related to the total dissolved substances in the water.

Dissolved oxygen: Oxygen in water that is produced by aquatic plants and is mixed from the air.  Used by fish and other organisms to live, dissolved oxygen (DO) is measured either as a weight concentration per volume of water (milligrams per liter) or as parts per million (ppm).  Ice and snow cover blocks sunlight and air contact with waterbodies during many Eagan winters, lowering DO in shallow lakes and ponds.  When DO concentrations fall below 5 ppm, sensitive aquatic organisms are at an increased risk of mortality.

Drainage basin: Total land area that drains to a lake or pond.

Eutrophication: Natural process by which waterbodies are enriched with nutrients, increasing the production of algae and macrophytes.  Urbanization can accelerate this process when lakes, ponds, and wetlands are incorporated into the stormwater system.

Filamentous algae: Algae that forms filaments or mats that are attached to sediment, weeds, piers, etc.

Gun Club Lake Watershed Management Organization: A public agency comprised of the respective governmental units of the cities of Eagan, Inver Grove Heights, and Mendota Heights.  Because it comprises most of the Gun Club Lake watershed, the City of Eagan coordinates the activities of the organization.  The general purpose of the organization is to regulate the natural water storage and retention of the Gun Club Lake watershed to: 1) protect, preserve, and use natural surface and ground water storage and retention systems; 2) minimize public capital expenditures needed to correct flooding and water quality problems; 3) identify and plan for means to effectively protect and improve surface and ground water quality; 4) establish more uniform local policies and official controls for surface and ground water management; 5) prevent erosion of soil into surface water systems; 6) promote ground water recharge; 7) protect and enhance fish and wildlife habitat and water recreational facilities; and 8) secure the other benefits associated with the proper management of surface and ground water.

Impervious surface: Exterior covering that does not allow rainwater or snowmelt to infiltrate into the soil but rather causes it to run off and enter the stormwater system via street drains.  Rooftops, driveways, sidewalks, and streets are common impervious surfaces in cities.

Macrophytes: Multi-celled plants that grow in or near water.  Generally, macrophytes are beneficial to lakes and ponds because they produce oxygen and provide habitat for aquatic invertebrates and fish.  However, overabundance of such plants, especially problem species (e.g., curlyleaf pondweed) is related to shallow water depth and high levels of nutrients, such as phosphorus.

Nutrient removal rates: Essentially, estimates of the proportion of the nutrients entering a stormwater treatment basin that are retained and not exported to a receiving waterbody.  In 1987, W. W. Walker developed for the U.S. Environmental Protection Agency’s (EPA) National Urban Runoff Program computer models that are used by the City to design and define the dimensions of such treatment basins.

pH: Within a scale from 1 to 14, a measure of acidity, where low numbers are more acidic than high numbers and 7 is neutral.

Phosphorus: Key nutrient influencing plant growth in most lakes and ponds, where it is the least available nutrient and therefore limits the growth of algae and aquatic vegetation.  Phosphorus is abundant in plant and animal matter and attaches to fine soil particles.  Stormwater transports phosphorus, increasing levels in waterbodies and causing algae populations to increase.  This in turn causes a decline in water transparency and can accelerate eutrophication.  With very high phosphorus concentrations, intense "blooms" of algae may occur, coloring the water green and releasing strong odors when they decay.

Photosynthesis: Process by which green plants convert carbon dioxide to sugar and oxygen using sunlight for energy.  In lakes and ponds, photosynthesis is essential to producing a food base, and is an important source of oxygen.

Respiration: Process by which aquatic organisms convert organic material into energy.  It is the reverse reaction of photosynthesis.  Respiration consumes oxygen and releases carbon dioxide.  It also takes place as organic matter decays.

Runoff: See Stormwater.

Secchi disk: An 8-inch diameter plate (usually all-white or with alternating black and white quadrants) used to measure transparency.  Attached to a length-calibrated cord, the disk is lowered into the lake until it disappears.  It is then raised until it becomes visible.  The average of these depths is the Secchi disk reading.  About 30 citizen lake monitors regularly collect Secchi disk readings as part of the Eagan citizen lake monitoring program.  Invented by an astrophysicist and scientific advisor to the Pope, the first disk was lowered into the Mediterranean Sea by Fr. Pietro Angelo Secchi on April 20, 1865.

Sediment: Accumulated organic and inorganic matter on the bottom of a waterbody.  It includes decaying algae and weeds and soil and organic matter from the drainage basin.

Stormwater: Rainwater and snowmelt that runs off impervious surfaces rather than infiltrate into the soil.  Through a drainage system of underground pipes, stormwater carries nutrients, fine soils, plant debris, drippings from vehicles, and other substances from the drainage basin.  Most of Eagan lakes, ponds, and wetlands are connected to the stormwater system.

Suspended solids: Algae, dirt particles, grass clippings, and other material that floats in water or is carried in stormwater without dissolving.

Transparency: The extent to which lake water is clear, indicating the amount of light penetration into a lake.  Transparency is typically referred to as a depth measured with a Secchi disk.  Water transparency tells a lot about a lake's water quality.  It provides an indirect measure of the amount of suspended material in the water, which in many cases includes algae.  Long-term transparency monitoring helps detect signs of degradation to a lake.  Generally, the sooner water quality problems are detected, the easier and less expensive it is to restore the lake to its previous state.

Trophic State Index: A water quality index developed by R. E. Carlson in 1977 that uses three parameters (total phosphorus, chlorophyll a, and transparency) to characterize nutrient levels of waterbodies on a scale from 1 to 100.  The index is widely used because data collection for it is easy and economical, there is a relatively large existing database, and it takes into account both algae growth and suspended solids.

Watershed: See Drainage basin.

Wetland: Land that is transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water.  According to Minnesota law, a wetland is defined by the presence of: 1) soils with characteristics developed under wet conditions, 2) surface or subsurface water, and 3) vegetation that is growing where at least periodically deficient oxygen exists as a result of excessive water content.

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