The quantity of chlorophyll that can be extracted from unit volume of seawater depends upon the number of plant cells present, and it is possible to calibrate a scale of pigment concentration against quantities of plant tissue (Harvey, 1950). A measured volume of raw seawater is filtered and/or centrifuged to collect all cells. These are then treated with a standard volume of acetone or alcohol to extract the chlorophyll. The intensity of colour in the extract is measured colorimetrically or absorptiometrically to determine the concentration of pigment, and the results are expressed as chlorophyll concentration or arbitrary units of plant pigment (UPP).
Surveys of chlorophyll concentration in the surface layers of the sea can also be based on measurements made by photometric equipment mounted on aircraft or satellites. This apparatus analyses the spectral composition of light reflected from the sea surface. Chlorophyll content can be estimated from the relative intensities of green and blue light or from measurements of chlorophyll a fluorescence. Modern methods of satellite recording of sea-surface chlorophyll measurements, have been in place for about fifteen years. Computers are used to produce maps with various colours indicating the chlorphyll concentration in milligrams of chlorophyll pigment per cubic metre of surface water.
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