In addition to the following glossary, we have links to other great on-line glossaries and water quality sites in our LINKS section.
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A	Abiotic:
		Not alive; non-biological; for example, temperature and mixing are abiotic factors that influence the O2 content of lake water whereas photosynthesis and respiration are biotic factors that affect O2 solubility.
	Acid:
		A solution that is a proton (H+) donor and has a pH less than 7 on a scale of 0-14. The lower the pH the greater the acidity of the solution.
	Acidity:
		A measure of how acid a solution may be. A solution with a pH of less than 7.0 is considered acidic. Solutions with a pH of less than 4.5 contain mineral acidity (due to strong inorganic acids), while a solution having a pH greater than 8.3 contains no acidity.
	Acid rain:
		Precipitation having a ph lower than the natural range of ~5.2 - 5.6; caused by sulfur and nitrogen acids derived from anthropogenic emissions.
	Acidification:
		The process by which acids are added to a water body, causing a decrease in its buffering capacity (also referred to as alkalinity or acid neutralizing capacity), and ultimately a significant decrease in pH that may lead to the water body becoming acidic (pH < 7).
	Adhesion:
		The molecular force of attraction between unlike bodies that that acts to hold them together.
	Algae:
		Simple single-celled, colonial, or multi-celled, aquatic plants. Aquatic algae are (mostly) microscopic plants that contain chlorophyll and grow by photosynthesis, and lack roots and stems ((non-vascular), and leaves. They absorb nutrients (carbon dioxide, nitrate, ammonium, phosphate and micronutrients) from the water or sediments, add oxygen to the water, and are usually the major source of organic matter at the base of the food web in lakes. Freely suspended forms are called phytoplankton; forms attached to rocks, stems, twigs, and bottom sediments are called periphyton.
	Alkalinity:
		Acid neutralizing or buffering capacity of water; a measure of the ability of water to resist changes in pH caused by the addition of acids or bases and therefore, the main indicator of susceptibility to acid rain; in natural waters it is due primarily to the presence of bicarbonates, carbonates and to a much lesser extent occasionally borates, silicates and phosphates. It is expressed in units of milligrams per liter (mg/l) of CaCO3 (calcium carbonate) or as microequivalents per liter (ueq/l) where 20 ueq/l = 1 mg/l of CaCO3. A solution having a pH below about 5 contains no alkalinity.
	Anaerobic:
		Technically this means "without air" but in limnology it is used synonymously with "anoxic."
	Angle of incidence:
		Angle between direction of motion of waves and a line perpendicular to surface the waves are striking.
	Angle of reflection:
		Angle between direction of motion of waves and a line perpendicular to surface the waves are reflected from.
	Anions:
		Negatively charged ions.
	Anoxia:
		Condition of being without dissolved oxygen (O2).
	Anoxic:
		Completely lacking in oxygen.
	Anthropogenic:
		Human caused.
	Aquatic respiration:
		Refers to the use of oxygen in an aquatic system including the decomposition of organic matter and the use of oxygen by fish, algae, zooplankton, aquatic macrophytes, and microorganisms for metabolism.
	Atmospheric (Barometric) Pressure:
		Measure of the pressure of the earth's atmosphere per unit area. It is 760 mm Hg at sea level and decreases with increasing elevation.
	Attenuation:
		Decrease.
	Aufwuchs:
		The community of algae and other microorganisms that attach to surfaces such as rocks, twigs, and aquatic plants; essentially the same as "periphyton" that means "attached algae."
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B	Base:
		A substance which accepts protons (H+) and has a pH greater than 7 on a scale of 0-14; also referred to as an alkaline substance.
	Basin:
		Geographic land area draining into a lake or river; also referred to as drainage basin or watershed.
	Benthic:
		Refers to being on the bottom of a lake.
	Benthic zone:
		Lake bottom sediment.
	Bicarbonate:
		The anion HCO3-.
	Bicarbonate Buffering Equilibrium Equation:
		See Carbonate Buffering System.
	Bioaccumulation:
		The increase in concentration of a chemical in organisms that reside in environments contaminated with low concentrations of various organic compounds. Also used to describe the progressive increase in the amount of a chemical in an organism resulting from rates of absorption of a substance in excess of its metabolism and excretion. Certain chemicals, such as pcbs, mercury, and some pesticides, can be concentrated from very low levels in the water to toxic levels in animals through this process.
	Bioavailable:
		Able to be assimilated (absorbed) by organisms.
	Biochemical Oxygen Demand (BOD):
		Sometimes referred to as Biological Oxygen Demand (BOD). A measure of the amount of oxygen removed (respired) from aquatic environments by aerobic microorganisms either in the water column or in the sediments. The parameter BOD uses the maximum rate of O2 consumption over a 5 day period in the dark at 200 to estimate the total amount of "biodegradable" organic matter in the system. Typically too insensitive to be useful for pristine lakes and so is used primarily for wastewater "streams" or systems impacted by organic pollution.
	Biomanipulation:
		Reducing algal blooms by altering the fish community to reduce predation on certain zooplankton (cladocerans such as daphnia) that can most efficiently graze on algae.
	Biomass:
		The weight of a living organism or assemblage of organisms.
	Biotic:
		Referring to a live organism; see also abiotic.
	Birgean Heat Budget:
		See Heat Budget.
	Buffer:
		A substance which tends to keep pH levels fairly constant when acids or bases are added.
	Buffering Capacity:
		Ability of a solution to resist changes in ph when acids or bases are added; the buffering capacity of natural waters is mostly due to dissolved carbonate rocks in the basin; equivalent to acid neutralizing capacity (anc). Typically considered to be exhausted.
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C	Calorie:
		A basic measure of energy where 1 calorie is equal to the total amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius.
	Capillary Action:
		The action by which water is drawn around soil particles (or any other solid substance like a small bore tube) because there is a stronger attraction between the soil or solid particles and the water molecules themselves.
	Carbon Cycle:
		The circulation of carbon atoms through the earth's whole ecosystem.
	Carbon Dioxide:
		A gas which is colorless and odorless; when dissolved in water it becomes carbonic acid; CO2 is assimilated by plants for photosynthesis in the "dark" cycles of photosynthesis.
	Carbonate ion:
		The CO3-2 ion in the Carbonate Buffer System the collective term for the natural inorganic chemical compounds related to carbon dioxide that exists in natural waterways. Combined with one proton, it becomes Bicarbonate, HCO3- and with two protons, Carbonic Acid. The carbonate ion forms a solid precipitant when combined with dissolved ions of calcium or magnesium.
	Carbonate Buffering System:
		The most important buffer system in natural surface waters and wastewater treatment, consisting of a carbon dioxide, water, carbonic acid, Bicarbonate, and Carbonate ion equilibrium that resists changes in the water's pH. If acid (hydrogen ions) is added to this buffer solution, the equilibrium is shifted and carbonate ions combine with the hydrogen ions to form bicarbonate. Subsequently, the bicarbonate then combines with hydrogen ions to form carbonic acid, which can dissociate into carbon dioxide and water. Thus the system pH is unaltered (buffered) even though acid was introduced.
	Carnivores:
		"Meat" eaters; organisms that eat other organisms.
	Cations:
		Positively charged ions.
	Chemical Equilibrium:
		Concentrations of reactants and products at which a reaction is in balance; there is no net exchange because the rate of the forward reaction is taking place at the same rate of the reverse reaction.
	CHEMetrics Water Quality Test Kits:
		CHEMetrics, Inc. (website: http://www.chemetrics.com/) is one of a number of companies that market a variety of test kits and field and lab instruments for water quality testing. Additional companies commonly cited are Hach and LaMotte, and there are probably numerous others accessible to the reader through various educational resources or scientific lab products catalogues. Water on the Web does not endorse any particular company’s products. Some test kits have been "approved" by state or federal agencies for certain types of tests in specific types of water or wastewater.
	Chemocline:
		Sharp gradient in chemical concentration; the boundary in a meromictic lake separating an upper layer of less-saline water that can mix completely at least once a year (mixolimnion) from a deeper, more saline (dense) layer (monimolimnion) that never is mixed into the overlying layer.
	Chlorophyll:
		Green pigment in plants that transforms light energy into chemical energy in photosynthesis.
	Clarity:
		Transparency; routinely estimated by the depth at which you can no longer see a sechi disk. The Secchi disk is a 20 cm (8 inch) diameter weighted metal plate with alternating quadrants painted black and white that is used to estimate water clarity (light penetration). The disc is lowered into water until it disappears from view. It is then raised until just visible. An average of the two depths, taken from the shaded side of the boat, is recorded as the Secchi depth.
	Coefficient of Heat Transfer:
		The ratio of the temperature of an object to the temperature of its surroundings. The change in temperature of an object is directly proportional to the difference between its temperature and the temperature of its surroundings.
	Cohesion:
		The molecular force between particles within a substance that acts to unite them.
	Cohesive Forces:
		All the forces of attraction among particles of a liquid.
	Conductivity (electrical conductivity and specific conductance):
		Measures water's ability to conduct an electric current and is directly related to the total dissolved salts (ions) in the water. Called EC for electrical conductivity and is reported in micromhos per centimeter (umhos/cm) which has been recently renamed as uS/cm (microSiemans per centimeter). EC is temperature sensitive and increases with increasing temperature. Most modern probes automatically correct for temperature and standardize all readings to 25°C and then refer to the data as specific EC.
	Conduction:
		Thermal conduction is the transfer of heat between two solid materials that are physically touching each other.
	Consumers:
		Organisms that must eat other organisms for their energy metabolism; organisms that cannot produce new organic matter by photosynthesis or chemosynthesis (producers).
	Convection Currents:
		Air or water movement caused by changes in density or thermal (temperature) gradients.
	Covalent:
		Refers to the chemical bond formed by the sharing of one or more electron pairs between two atoms.
	Cyanobacteria:
		Bluegreen algae; phylum or organisms that are biochemically bacterial in nature but perform plant photosynthesis.
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D	Decomposition:
		The breakdown of organic matter by bacteria and fungi.
	Denitrification:
		Anaerobic bacterial process metabolism in which nitrate is used instead of oxygen during the oxidation of organic carbon compounds to yield energy (respiration). The process oxidizes organic carbon and (chemically) reduces nitrate to the gaseous end products N2 (nitrogen gas) or N2O (nitrous oxide). This is the major process used in wastewater treatment plants to ultimately convert combined nitrogen to a non-polluting state.
	Density:
		The mass of a substance or organism per unit volume (kg/cubic meter; grams/liter).
	Density Stratification:
		Creation of layers in a water body due to density differences; controlled by temperature, dissolved solids concentration and particle concentration.
	Detritus:
		Dead or decaying organic matter; technically called organic detritus to distinguish it from the mineral detritus classified by geologists.
	Diatom:
		Group of algae characterized by glass (silica) cell wall, beautifully ornamented; often the brown stuff attached to rock surfaces.
	Diel:
		A 24 hour period of time.
	Diffusion:
		The movement of a substance from an area of high concentration to an area of low concentration. Turbulent diffusion, or mixing, results from atmospheric motions (wind) diffusing water, vapor, heat, and other chemical components by exchanging parcels called eddies between regions in space in apparent random fashion. Molecular diffusion, which operates in stagnant zones, such as at the bottom sediment-water boundary in a deep lake, occurs much, much more slowly and so is important only on a very small scale such as right at the bottom.
	Dimictic:
		Having two mixing periods, typically in spring and fall.
	Dipole:
		A molecule that has two opposite electrical poles, or regions, separated by a distance.
	Dipole - Dipole Forces:
		Intermolecular attraction between the oppositely charged poles of nearby molecules.
	Dipole - Induced Dipole Forces:
		Very weak forces between a dipole and non-polar molecule that acts like a dipole in the presence of a dipole molecule.
	Dipteran:
		True flies.
	Dissolved Oxygen (DO or O2):
		The concentration of free (not chemically combined) molecular oxygen (a gas) dissolved in water, usually expressed in milligrams per liter, parts per million, or percent of saturation. Adequate concentrations of dissolved oxygen are necessary for the life of fish and other aquatic organisms and the prevention of offensive odors. DO levels are considered the most important and commonly employed measurement of water quality and indicator of a water body's ability to support desirable aquatic life. Levels above 5 milligrams per liter (mg O2/L) are considered optimal and most fish cannot survive for prolonged periods at levels below 3 mg O2/L. Levels below 1 mg O2/L are often referred to as hypoxic and when O2 is totally absent anoxic (often called anaerobic which technically means without air). Secondary and advanced wastewater treatment systems are generally designed to degrade organic matter to ensure adequate dissolved oxygen in waste-receiving waters (from North American Lake Management Society.
	Dissolved Oxygen Profile:
		A graph of the amount of dissolved oxygen per unit depth; where the depth is on the z (vertical) axis and dissolved oxygen is on the x (horizontial) axis. Limnologists plot graphs this way but be sure to note that the depth (z) axis is really for the independent variable and the horizontal (x) axis is really for the dependent variable.
	Dissolved Solids Concentration:
		The total mass of dissolved mineral constituents or chemical compounds in water; they form the residue that remains after evaporation and drying. Often referred to as the total dissolved salts (TDS) concentration or dissolved ion concentration. In seawater or brackish water this is approximated by the salinity of the water. All of these parameters are estimated by the electrical conductivity (EC).
	Drainage lakes:
		Lakes having a defined surface inlet and outlet.
	Dry deposition:
		Fine particulate matter and aerosols settling from the atmosphere onto lake and land surfaces during periods with no precipitation.
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E	Ecological pyramid:
		Conceptual scheme whereby the amount of biomass or energy at each level of the food "chain" decreases as you move from primary producers through the different levels of consumers.
	Ecoregion:
		An environmental area characterized by specific land uses, soil types, surface form, and potential natural vegetation.
	Ecosystem:
		All of the interacting organisms in a defined space in association with their interrelated physical and chemical environment.
	Electrical Conductivity (EC):
		See Conductivity.
	Electromagnetic Radiation:
		Radiation that travels through space at the speed of light that includes light, radio waves, x-rays, and gamma rays.
	Endothermic Reaction:
		A reaction which absorbs heat; see also  exothermic reaction.
	Epilimnion:
		The upper, wind-mixed layer of a thermally stratified lake. This water is turbulently mixed throughout at least some portion of the day and because of its exposure, can freely exchange dissolved gases (such as O2 and CO2) with the atmosphere.
	Equilibrium:
		See Chemical Equilibrium.
	Euphotic zone:
		Layer of water where sunlight is sufficient for photosynthesis to occur.
	Eutrophic Lake:
		A very biologically productive type of lake due to relatively high rates of nutrient input. See Eutrophication.
	Eutrophication:
		The process by which lakes and streams are enriched by nutrients (usually phosphorus and nitrogen) which leads to excessive plant growth - algae in the open water, periphyton (attached algae) along the shoreline, and macrophytes (the higher plants we often call weeds) in the nearshore zone. See the Lake Ecology Primer Biology section for more information about this problem; it remains the biggest pollution problem for Minnesota‘s (and in fact for the rest of our country as well) lakes . The extent to which this process has occurred is reflected in a lake's trophic classification: oligotrophic (nutrient poor), mesotrophic (moderately productive), and eutrophic (very productive and fertile). The less productive a lake is naturally, the more sensitive it is to increased nutrient loads from human-caused disturbances in the watershed.
	Evaporation:
		The process of converting liquid to vapor.
	Excel:
		Refers to Microsoft’s Excel spreadsheet software.
	Exothermic Reaction:
		A reaction which gives off heat; see also endothermic reaction.
	Export rates:
		Amount of a particular nutrient or contaminant annually transported from its source to a lake or stream; usually related to land uses and expressed per unit area per year.
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F	Fetch:
		Distance the wind blows over water without appreciable change in direction; relates to intensity of turbulent mixing.
	Fix:
		Convert CO2 to carbohydrate or N2 to NH4+ (carbon fixation and nitrogen fixation);
	Flagella:
		Whiplike structure that enables motility in certain groups of algae.
	Flow Rate:
		The rate at which water moves by a given point; in rivers it is usually measured in cubic meters per second (m3/sec) or cubic feet per second (cfs).
	Flushing Rate:
		The retention time (turnover rate or flushing rate), the average length of time water resides in a lake, ranging from several days in small impoundments to many years in large seepage lakes. Retention time is important in determining the impact of nutrient inputs. Long retention times result in recycling and greater nutrient retention in most lakes. Calculate retention time by dividing the volume of water passing through the lake per year by the lake volume.
	Food Chain:
		The transfer of food energy from plants through herbivores to carnivores. An example: insect-fish-bear or the sequence of algae being eaten by zooplankton (grazers; herbivores) which in turn are eaten by small fish (planktivores; predators) which are then eaten by larger fish (piscivores; fish eating predators) and eventually by people or other predators (fish-eating birds, mammals, and reptiles).
	Food Web:
		Food chains hooked together into a complex interconnected web.
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G	Gas Solubility:
		The ability of a gas to dissolve into another substance.
	Geographic Information System (GIS):
		A computer system which allows for input and manipulation of geographic data to allow researchers to manipulate, analyze and display the information in a map format.
	Grazers:
		Herbivores; zooplankton in the open water zone.
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H	Hach Water Quality Test Kits:
		Hach, Inc. (Website: http://www.hach.com/) is one of a number of companies that market a variety of test kits and field and lab instruments for water quality testing. Additional companies commonly cited are LaMotte and CHEMetrics, and there are probably numerous others accessible to the reader through various educational resources or scientific lab products catalogues. Water on the Web does not endorse any particular company’s products. Some test kits have been "approved" by state or federal agencies for certain types of tests in specific types of water or wastewater.
	Hardwater:
		Lakes that have a high buffering capacity and are not generally sensitive to acid deposition. These lakes have dissolved salt concentrations greater than 120 mg/L.
	Heat:
		Energy that is transferred from one body to another because of a difference in temperature.
	Heat Budget:
		The amount of heat energy required annually to raise the temperature of a water body from its winter minimum to its summer maximum.
	Heat Energy:
		An energy form proportional to and associated with molecular motion. Conduction, convection or radiation can transfer heat from one mass of matter to another.
	Heat Reflection:
		The return of radiant heat energy by a reflecting surface.
	Heat of Transformation:
		See Latent Heat.
	Heat of Vaporization:
		The heat required to convert a substance from the liquid to the gaseous state with no temperature change. This is also called the latent heat of vaporization.
	Henry's Law:
		States that at a given temperature the solubility of a gas is directly proportional to the pressure of the gas directly above the liquid.
	Herbivores:
		Plant eaters.
	Heterogeneous:
		Not uniform; patchy.
	Holomictic:
		Typically mixes completely throughout the water column at least once a year.
	Hydrogen:
		Colorless, odorless and tasteless gas; combines with oxygen to form water.
	Hydrogen Bond:
		A type of chemical bond caused by electromagnetic forces, occurring when the positive pole of one molecule (e.g., water) is attracted to and forms a bond with the negative pole of another molecule (e.g., another water molecule).
	Hydrogen Ion:
		An individual atom of hydrogen which is not attached to a molecule and therefore has a positive (+) charge.
	Hydrology:
		The study of water's properties, distribution and circulation on Earth.
	Hydrostatic Pressure:
		Pressure exerted in a column of water.
	Hypolimnetic Oxygen Depletion:
		A condition where the dissolved oxygen in the bottom layer (hypolimnion) of a water body is gradually consumed through respiration and decomposition faster than it can be replaced over the course of the summer. A similar phenomenon may occur in the winter under ice cover. The rate at which O2 is depeleted is a measure of the productivity of the system.
	Hypolimnion:
		The bottom, and most dense layer of a stratified lake. It is typically the coldest layer in the summer and warmest in the winter. It is isolated from wind mixing and typically too dark for much plant photosynthesis to occur.
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I	Ice-out:
		Date when lake thaws.
	Impervious surfaces:
		Land surfaces such as roads, parking lots, buildings, etc that prevent rainwater from soaking into the soil. The water increases in velocity causing more erosion; it warms causing potential heat stress for downstream trout; it picks up roadway contaminants; and the loss of vegetation removes a "sink" for dissolved nutrients - plant uptake.
	Inflow:
		Water flowing into a lake.
	Inorganic:
		Substances of mineral, not carbon origin.
	Ion:
		An electrically charged particle.
	Isothermal:
		Constant in temperature.
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L	Lake Profile:
		A graph of a lake variable per depth; where the depth is on the z-axis and the variable is on the x-axis. Depth is the independent variable and the x-axis is the dependent variable.
	LaMotte Water Quality Testing Kits:
		The LaMotte Company (website: http://www.lamotte.com/) is one of a number of companies that market a variety of test kits and field and lab instruments for water quality testing. Additional companies commonly cited are Hach and CHEMetrics, and there are probably numerous others accessible to the reader through various educational resources or scientific lab products catalogues. Water on the Web does not endorse any particular company’s products. Some test kits have been "approved" by state or federal agencies for certain types of tests in specific types of water or wastewater.
	Landuse:
		The primary or primary and secondary uses of land, such as cropland, woodland, pastureland, forest, water (lakes, wetlands, streams), etc. The description of a particular landuse should convey the dominant character of a geographic area and establish the dominant types of human activities which are prevalent in each region.
	Landscape:
		All the natural geographical features, such as fields, hills, forests, and water that distinguish one part of the earth's surface from another part. These characteristics are a result not only of natural forces but of human use of the land as well.
	Latent Heat (Energy):
		The amount of heat  (energy) released from or absorbed by a substance when it undergoes a change of state; also known as Heat of Transformation.
	Le Chatelier's Principle:
		A principle of equilibrium; states that in a balanced equilibrium, if one or more factors changes, the system will readjust to reach equilibrium.
	Leach:
		To remove soluble or other constituents from a medium by the action of a percolating liquid, as in leaching salts from the soil by the application of water.
	Limnetic zone:
		Open water zone.
	Littoral:
		Nearshore out from shore to the depth of the euphotic zone where it is too dark on the bottom for macrophytes to grow.
	Loading Rates:
		The rate at which materials (typically suspended sediment, nutrients [N and P], or contaminants) are transported into a water body.
	Loricas:
		Glass cell covering.