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Physical Measurements


The greatest source of heat in water is from the sun by direct absorption. Accurate temperature data are important to the limnologist.

Temperature directly and indirectly exerts many effects on limnological phenomena such as lake stability, gas solubility and biotic metabolism.

Thermometers are useful for measuring surface temperatures by direct immersion but are limited in use for subsurface measurements. Electrical thermistor thermometers and reversing thermometers are used for temperature recordings when various depths are sampled.


Color in water may result from the presence of natural metallic ions such as iron and manganese, humus and peat materials, plankton, weeds and industrial wastes.

Color is removed to make water suitable for general applications. Colored industrial wastewaters may require color removal before discharge into the waste water.

True color of water is the color from which turbidity has been removed.

The term apparent color includes not only color due to substances in solution, but also that due to suspended matter. Apparent color is determined on the original sample without filtration or centrifugation.


Illumination at a particular depth can be measured by a submarine photometer. Transmission of light through water is of special interest for many studies, especially those related to photosynthesis. Several factors affect the transmission of light in water: the water itself, suspended particles and dissolved material.


Visibility is a measure of the depth to which one may see into the water. The Secchi disk is a simple device used to estimate this depth.


Turbidity in water is the presence of suspended solids, which affects the transmission of light either through scattering or absorption. Clarity of water is important in producing products destined for human consumption and in many manufacturing processes.


Conductivity is a measure of the ability of an aqueous solution to carry an electric current. This ability depends on the presence of ions; on their total concentration, mobility and valence and on the temperature. Solutions of most inorganic compounds are relatively good conductors. Molecules of organic compounds that do not dissociate in aqueous solution conduct a current very poorly, if at all.

Stream Flow

Stream velocity is best measured at several portions of the stream by using a flowmeter. Velocity and volume of discharge are important measurements for the limnologist who wishes to determine the downstream transport of dissolved and suspended matter, and stream biota.

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