Glossary of Water Quality Terms

Alkalinity: A measure of the buffering capacity of a lake, or the capacity of water to neutralize acids. It is a measure of naturally-available bicarbonate, carbonate, and hydroxide ions in the water, which is largely determined by the geology of soils and rocks surrounding the lake. Alkalinity is important to aquatic life because it buffers against changes in pH that could have dire effects on animals and plants.

Chlorophyll-a (Chl-a): A measurement of the green pigment found in all plants, including microscopic plants such as algae. It is used as an estimate of algal biomass; higher Chl-a equates to greater amount of algae in the lake.

Color: The influence of suspended and dissolved particles in the water as measured by Platinum Cobalt Units (PCU). A variety of sources contribute to the types and amount of suspended material in lake water, including weathered geologic material, vegetation cover, and land use activity. Colored lakes (>25 PCU) can have reduced transparency readings and increased TP values. When lakes are highly colored, the best indicator of algal growth is chlorophyll-a.

Dissolved Oxygen: The concentration of oxygen that is dissolved in the water. DO is critical to the healthy metabolism of many creatures that reside in the water. DO levels in lake water are influenced by a number of factors, including water temperature, concentration of algae and other plants in the water, and amount of nutrients and organic matter that flow into the waterbody from the watershed. Too little oxygen severely reduces the diversity and abundance of aquatic communities. DO concentrations may change dramatically with lake depth. Oxygen is produced in the top portion of a lake (where sunlight drives photosynthesis), and oxygen consumption is greatest near the bottom of a lake (where organic matter accumulates and decomposes).

E. coli: A type of bacteria that lives in the intestines of warm-blooded animals, including humans. The non-pathogenic form of E.coli is monitored in freshwater systems as an indicator of fecal contamination from wildlife, pets, or humans (e.g., malfunctioning septic systems). The State of Maine sets water quality criteria for bacteria in surface waters to protect designated uses, including primary contact recreation (e.g., swimming) and aquatic life. Levels of E.coli that exceed these criteria indicate the likely presence of harmful pathogens also found in fecal matter. Exposure to or consumption of these pathogens may cause gastrointestinal, respiratory, eye, ear, nose, throat, and skin infections.

Flow: The measure of discharge (area of stream cross-sectional profile multiplied by the average velocity of water moving through that profile). The amount of water flowing through a particular point in a stream is a result of the size of the drainage area (e.g., larger drainage areas feed larger streams), the type of land cover and soils within the drainage area (e.g., forests and loamy soils are able to absorb more water than developed areas and sandy soils), and the local climate (e.g., amount of rain falling within the drainage). Climate change is predicted to increase the frequency and intensity of precipitation, causing greater and more frequent fluxes in discharge; thus, stream flow is a critical indicator of climate change impacts to ecological systems.

pH: The standard measure of the acidity or alkalinity of a solution on a scale of 0-14. Most aquatic species require a pH between 6.5 and 8. As the pH of a lake declines, particularly below 6, the reproductive capacity of fish populations can be greatly impacted as the availability of nutrients and metals changes. pH is influenced by bedrock, acid rain or snow deposition, wastewater discharge, and natural carbon dioxide fluctuations.

Secchi Disk Transparency (SDT): The vertical measure of the transparency of water (ability of light to penetrate water) obtained by lowering a black and white disk into the water until it is no longer visible. Transparency is an indirect measure of algal productivity and is measured in meters (m).

Temperature: The measure of the degree of heat in water. Temperature affects the density of water (e.g., cooler water sinks), the solubility of gases (e.g., cooler water holds more dissolved oxygen), the rate of chemical reactions, and the activity of aquatic organisms (e.g., metabolic growth rates peak at different temperatures for different species; some species such as trout and salmon prefer cooler, more oxygen-rich waters; others such as bass prefer warmer waters). Humans can alter temperature by removing shade-providing trees near surface waters, constructing dams or other impoundments that restrict free flowing waters, and causing soil erosion (e.g., turbid water absorbs more heat from the sun). Climate change is predicted to increase surface water temperatures at a much faster rate than the observed increase in air temperature; thus, water temperature serves as a critical indicator of climate change impacts to ecological systems.

Total Phosphorus (TP): The total concentration of phosphorus found in the water, including organic and inorganic forms. TP is one of the major nutrients needed for plant growth. It is generally present in small amounts and limits plant growth in freshwater ecosystems. As phosphorus increases, the amount of algae generally increases. Humans can add phosphorous to a lake through stormwater runoff, lawn or garden fertilizers, and leaky or poorly maintained septic tanks.