Glossary of terms
absorption coefficient: an numeric measure of the amount of light energy that water itself and the dissolved and suspended substance within the water take up, which results in less energy or less light penetrating down into a water body. It can be measured for some or all of the wavelengths of a light ray and is reported per meter. It is an inherent optical property.
anthropogenic: related to humans
apparent optical properties (AOP): those properties of water that depend on the ambient light field and the inherent optical properties (see below). They depend on the angle of the sun, whether it is day or night and cloud coverage. Some AOPs are: (see own definitions)
- Irradiance attenuation coefficient
- Irradiance reflection coefficient
- Secchi disk depth
attenuation: to reduce in intensity
beam attenuation: decrease in light energy from a collimated beam which is passing through a water sample with a particular pathlength, which is an inherent optical property.
compensation point/depth: the depth at which CO2 uptake equals O2 output, so there is no net growth of phytoplankton at this point
constituents: a part of the whole, serving to compose or make up
diffraction: when light waves bend around an obstruction, i.e. suspended particle, and move in a new direction.
eutrophication: the over-enrichment of a water body with nutrients causing increased phytoplankton growth. Secchi disk depth can be a good measure of eutrophication, especially in lakes. Secchi disk depths for eutrophic lakes ranges from 1.5 - 3 meters, while depths for oligotrophic (nutrient-poor) waters are greater than 6 meters. Ultraoligotrophic waters can have Secchi disk depths greater than 12 meters!
inherent optical properties: properties of water that are depend only on the content of the water. They are the same regardless of light conditions, i.e. what time of day it is or if its night, that can be tested in a laboratory. They are: (see own definitions)
- beam attenuation
- volume scattering function
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irradiance: the radiance, or the amount of light energy measured during a specified time period in a given area, from all directions.
irradiance attenuation coefficient: a measure that estimates the amount of light attenuated, which is absorbed or scattered, over an entire water column. It is an apparent optical property.
irradiance reflection coefficient: an indicator of scattering in the water, which relates to the brightness of the water. It is an apparent optical property.
littoral zone: an area of a water body defined by the shoreline and the limit of submerged aquatic vegetation growth, which usually corresponds with Secchi disk depth.
optical medium: a substance which is acted upon or has an effect on light or optics (what you see)
optical water quality: the extent to which water quality is affected or determined by the waters optical properties (Kirk, 1988)
optics: pertaining to the eye or sight
photosynthesis: the formation of carbohydrates, or energy usable to non-photosynthesizing organisms (for example humans), from light energy by chlorophyll-containing plants, like phytoplankton and terrestrial plants.
radiance: the amount of light energy or photons measured in a specified time period in a unit area from a given direction
reflection: when light bounces off of a new medium (for example, when light is travel through the air and hits water) and moves in a new direction
refraction: a change in the direction or path of light as a result of a change in the medium through which it is traveling.
scatter coefficient: a numerical measure of the amount of light that is diffused, or spread away from the downward direction, in water by water itself and its contents. Its is an inherent optical property and is measured per meter.
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Secchi disk depth: the Secchi disk is an instrument used to measure the transparency of the water. It is a plate sized disk painted black and white, which is lowered into the water until it disappears. It is then pulled up and the depth at which it reappears is called the Secchi disk depth. It can also be used as a estimate of the compensation point/depth. It is an apparent optical property. Typical Secchi disk depths are:
- < 5 m in shallow, turbid waters both fresh and marine
- > 10 m in deep clear marine waters
- > 17 m in deep clear fresh water
An a reference, the Chesapeake Bay Secchi disk depth = 1- 2 m, which has shallow, moderately to highly turbid waters.
terrestrial: on the land
turbidity: a lack of clarity or purity. Turbid waters appear muddy or cloudy. It is used as a water quality parameter that estimates the transparency, or clarity of the water. It have been found to correlate with the scattering coefficient.
volume scattering function: an estimate of the new direction of scattered light for a particular substance or a water body dominated by a particular substance. It incorporates the dominant angles light is scattering away from the object in respect to the incident beam of light, or the direction the sun is coming from. The function may be thought of as the probability of light being scattered in a particular direction relative to its initial direction, the path of sunlight.
water quality: the physical, biological and chemical properties of the water that determine its suitability for human use or for its role in the biosphere (see above). Bodies of water play many roles in the biosphere, including use as a habitat for millions of species and as a part of the hydrologic cycle.
- Physical properties-temperature, salinity, light penetration
- Biological-flora (plants) and fauna (animals like fish, invertebrates and mammals)
- Chemical-pH, nutrients, chemicals (further examples: nitrates, salts, ammonium, pesticides)
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Smithsonian Environmental Research Center
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