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Nonliving factors. Abiotic factors in an ecosystem are things such as sunlight, temperature, wind patterns and precipitation.

Alkalinity of waters refers to the quantity and kinds of compounds present which collectively shift the pH to the alkaline side of neutrality. It can also be described as the quantitative capacity of water to neutralize and acid; that is, the measure of how much acid can be added to a liquid without causing a significant change in pH. Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have high alkalinity.

Water that does not contain oxygen.

Asian Carp
Two species of Asian Carp, silver and bighead, were originally imported to control algae in southern catfish farms. Following flooding, they escaped into the Mississippi River in the early 1990s. They now inhabit the Illinois River, which connects to the Great Lakes via the Chicago Sanitary and Ship Canal.

Silver and bighead carp consume vast amounts of food and are extremely prolific. They can weigh up to 100 pounds and grow more than four feet long. Silver carp can jump from the water when agitated and have been known to injure boaters.

These species pose a significant threat to the Great Lakes ecosystem due to their large size, voracious eating habits, and rate of reproduction. Natural resource managers fear they would upset the food web, decimate native species, and damage the Great Lakes sport fishery.


Bathymetry is the study of the underwater depth of lake or ocean floors.

The bottom of a lake, sea or river.


Conductivity, or specific conductance, is a measure of the resistance of a solution to electrical flow and therefore a measure of water’s ionic activity and content. The higher the concentration of ionic (dissolved) constituents, the higher the conductivity. Specific conductance is generally found to be a good measure of the concentration of total dissolved solids and salinity, and can vary across lakes due to the differing geologies of their watersheds.

A line on a map or chart that joins equivalent points. On the NOAA-CoastWatch images, lines join points of equal temperature. Contour lines on Sea Surface Temperature (SST) maps indicate temperature breaks and also reveal weather fronts, warm or cold eddies, and the position of large currents.


Dead Zones
Dead Zones are hypoxic (water with dissolved oxygen levels below 2 mg/l) or anoxic (water that does not contain dissolved oxygen) areas without enough dissolved oxygen to support fish and/or zooplankton.

Determined by water temperature. Cold water is more dense and will sink below a less dense layer of warm water.


Ekman Transport
Ekman transport is the movement of water caused by wind blowing steadily over the surface. It occurs at right angles (90 degrees) to the wind direction. The direction of transport is dependent on the hemisphere. In the northern hemisphere this transport is at a 90 degree angle to the right of the direction of the wind, and in the southern hemisphere it occurs at a 90 degree angle to the left of the direction of the wind.

(Pronounced: ep-uh-lim-nee-on) The surface layer of water that is constantly mixed by wind and waves and is warmed by the sun, from late spring to late fall. Also see: Metalimnion and Hypolimnion

(Pronounced: yoo-trof-ik) A lake characterized by an abundant accumulation of nutrients that support a dense growth of algae and other organisms, the decay of which depletes dissolved oxygen in the summer.

The process by which lakes and streams are enriched by nutrients (usually phosphorus and nitrogen) which leads to excessive plant growth. Eutrophication can occur naturally or may be accelerated by human influences such as agriculture, urbanization and shoreline development.


The distance over which the wind blows without a significant change in direction. Fetch is an important factor in the development of wind waves.

Fluorescence is a physical property which can be used as a proxy for chlorophyll or for the amount of algae in a system.

The rate of volume flow across a unit area (m3•m−2•s−1), or discharge per unit area. Volumetric flux is often used to describe nutrient loading, or the amount of a nutrient flowing through a unit area per unit time.


Great Lakes Observing System
The Great Lakes Observing System (GLOS) is an effort dedicated to providing wide internet access to real-time and historic data on the hydrology, biology, chemistry, geology and cultural resources of the Great Lakes, its interconnecting waterways and the St. Lawrence River.

Water held below the surface of the land, underground.


Heat Sink
A substance or object that is absorbing more heat than it is radiating.

Heat Source
A substance or object that is radiating more heat than it is absorbing.

Hydrography is the mapping (charting) of water topographic features through the measurement of depths, tides and currents of a body of water. The purpose of charting a body of water is typically for shipping safety and navigation. Hydrography does not include water quality or composition, which are part of the broader field of hydrology.

(Pronounced: hi-puh-lim-nee-on) The deepest layer of uniformly cold water that does not mix with the upper layers and has low circulation. The colder water within the hypolimnion is at its maximum density at a temperature of 4 degrees Centigrade.

Hypoxia is the condition in which dissolved oxygen is below the level necessary to sustain most animal life. Hypoxic waters have dissolved oxygen levels below 2 milligrams per liter (mg/l) or 2 parts per million (ppm).


Isotherms are lines formed by connecting points with the same temperature. On the NOAA-Great Lakes Environmental Research Lab (GLERL) CoastWatch images, each line indicates a change in temperature of 1 degree Fahrenheit. (iso=similar; therm=temperature)


Latitude is a geographic coordinate that specifies the north-south position of a point on the Earth’s surface. It is is an angular measurement, usually expressed in degrees, minutes and seconds.

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth’s surface. It is is an angular measurement, usually expressed in degrees, minutes and seconds.


(Pronounced: meso-trof-ik) Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes.

(Pronounced: meta-lim-nee-on) The middle layer characterized by a steep gradient in temperature and demarcated by the regions above (epilimnion) and below (hypolimnion). The metalimnion is the barrier that prevents mixing and heat exchange between the epilimnion and hypolimnion (meta=between; limnio=lake)


(Pronounced: ol-i-goh-trof-ik) A lake with low nutrient concentrations.

To move back and forth between two points.


Measurement of the amount of hydrogen ions present in a substance such as water. Knowing the amount of hydrogen in a substance allows us one to determine whether it is acidic, neutral or basic.

A group of organisms that interbreed and live in the same place at the same time.


Redox (reduction-oxidation) reactions include all chemical reactions in which atoms have their oxidation state changed. The cycling of essential nutrients in lakes is complex, and is largely regulated by variations in oxidation-reduction states, which in turn are mediated by photosynthetic and bacterial metabolism.

Excess rainfall or snowmelt that flows over land into lakes and rivers because it was not absorbed by soil or plants.


Sea Grant
Sea Grant is the National Oceanic and Atmospheric Administration’s primary university-based program in support of coastal resource use and conservation. Research, education and outreach programs promote better understanding, conservation and use of America’s coastal resources. In short, Sea Grant is “science serving America’s coasts.” Environmental stewardship, long-term economic development and responsible use of America’s coastal, ocean and Great Lakes resources are at the heart of Sea Grant’s mission.

Secchi Disk
Instrument used by scientists to measure water clarity. The depth at which the disk’s black and white marks cannot be seen is called the “secchi depth.”

Soil and other particles carried by water or settled on the bottom of a water body.

Seiches are created by storm surges. The word seiche is French for ‘to sway back and forth.’ The wind blowing across the water surface causes water to pile up at the downwind side of the lake and drop on the upwind side (storm surge). When the wind stops, the water sloshes back and forth (oscillates) until it reaches equilibruim (seiche).

Specific Heat
The amount of heat in calories required to raise the temperature of one gram of a substance by one degree Celsius. The specific heat of water is one. Most other common liquids and solids have a specific heat of less than one.

Surface Water
Water that is above ground, e.g., in lakes and rivers.


The degree of hotness or coldness.

Thermal Bar
A thermal bar is a mixing of water temperatures leading to the development of a thermocline. A thermal bar may establish many miles offshore on large lakes, starting at the edge of a mass of 39°F water.

Thermal Breaks
Thermal breaks are randomly occurring surface water temperature gradients.

Thermal Fronts
Thermal fronts are where water masses with significantly different temperatures meet or interface.

Thermal Stratification
Vertical layering of water within a lake or in the ocean. Warm water being less dense tends to form a layer above colder, denser water.

A transition layer of water that separates the warm mixed surface layer of water from the cold deep water in the lake. The thermocline acts as a barrier to the mixing of water and nutrients.

Trophic Status
Trophic status is a means of classifying lakes in terms of their productivity. Eutrophication is the process by which lakes are enriched with nutrients, increasing the production of rooted aquatic plants and algae. The extent to which this process has occurred is reflected in a lake’s trophic classification or state:

  • Oligotrophic is nutrient poor
  • Mesotrophic is moderately productive
  • Eutrophic is very productive and fertile

Measurement of sediment and/or other particles stirred up or suspended in water.


The process by which water rises from a deeper to a shallower depth; contributes to lake turnover.


Water Discharge
The amount of water flowing in a channel. Water discharge is expressed as volume per unit of time.

Water Displacement
Water displacement is the volume of water that is removed when something else occupies that space. In the case of storm surges, wind is displacing water.

An area, such as a bog, swamp, or marsh that has seasonally wet soils and a distinct plant community. Wetlands provide valuable nursery areas and habitat for many plants and animals.

Wind Barbs
Symbols used on weather maps to indicate wind speed and direction.


Zebra Mussel
The zebra mussel is a small, non-native mussel originally found in Russia. In 1985, the zebra mussel was introduced into the Great Lakes most likely via the ballast water of one or more transoceanic ships. In less than 10 years zebra mussels spread to all five Great Lakes. They are voracious “filter feeders” processing up to 1 gallon of water per day per mussel. This filter feeding process depletes critical microscopic organisms necessary for a healthy food web. Zebra mussels also destroy native mussels, greatly reducing their populations.