Relation between total quanta and total energy for aquatic photosynthesis1

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 ±10% and is 2.5 ± 0.25 × 1018 (quanta sec−1 watt−1). Within this range Q : W varies systematically and thus it may be predicted within ±5%, 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.