Measurements of underwater spectral irradiance have been used to investigate the behavior of the ratio of total quanta to total energy (Q : W) within the spectral region of photosynthetic activity. In data from a wide range of optical water types, the Q : W ratio varies by no more than &lo% and is 2.5 4 0.25 x 10” (quanta set-l watt-l). Within this range Q : W varies systematically and thus it may be predicted within 45%, if either the wavelength of maximum spectral irradiance or the chlorophyll concentration of the water under investigation is approximately known. The value of Q : W may be used to determine the total quanta available for photosynthesis accurately from a measurement of the total energy and vice versa. From a measurement of spectral irradiance, dE(x)/dh (watt m-2 nm-l), the total quanta, EQ (quanta m-2 set-l), or total energy, Ew (watt m-2), within a given wavelength region can be obtained using the following equations,
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