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Three
GoMOOS buoys (B, E & I) monitor PAR in coastal waters within a few
miles of shore. Each buoy monitors three depths: above the surface at
the top of the buoy, 3 meters depth in the sunlit part of the water
column, and 18 meters depth where productivity tends to be lower
because of reduced light levels. We use these three measurements to
determine the depth of PAR penetration and the color of penetrating
radiation that helps us quantify water color and clarity.
Phytoplankton use the energy of visible light with wavelengths from
400 to 700 nanometers (1 nanometer = one billionth of a meter),
corresponding to the blue through red parts of the visible spectrum.
PAR is defined as the flux (amount per unit area over time) of light
energy in this wavelength range. The units for PAR are micro-Einsteins
per square meter per second (µE/m2/s). One Einstein corresponds to a mole (6.023 x 1023) of photons. At noontime on a clear summer day, PAR at the surface of the ocean is about 2000 µE/m2/s.
Phytoplankton acclimate to remarkably low light levels: some can photosynthesize and grow below 1 µE/m2/s.
They do this by increasing the concentration of chlorophyll and other
pigments in their cells. The surface layer in the ocean that has
sufficient light for plant life is called the euphotic zone, which can
vary in depth from less than a meter to more than 100 meters, depending
upon water clarity.
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