Estimation of chlorophyll concentration from the specific fluorescence at 685nm in the upward irradiance just below the sea surface

An algorithm for estimating the chlorophyll concentration from the fluorescence intensity at 685nm in upward irradiance observed just below the sea surface is presented and discussed. The observed chlorophyll concentration is closely related to the ratio of the fluoresced quanta to the total incident quanta (downward plus upward quanta) in the spectral range between 350nm and 600nm. The correlation between them, however, is not good and the rate of increase of the ratio decreases as the chlorophyll concentration increases. This is due to the fact that the number of fluoresced quanta varies with attenuation of the incident light by various materials in the sea.A better correlation is obtained between the ratio of the fluoresced quanta to the upward quanta at wavelengths around 480nm. This is explained by the fact that dividing by the upward quanta at the appropriate wavelength compensates for apparent variation of fluorescence intensity with attenuation by the sea water. In this method, upward quanta at the four wavelengths of 480nm, 640nm, 685nm and 740nm are necessary for estimation of chlorophyll concentration. The quanta at 640nm and 740nm are used for interpolating the quanta at 685nm in the absence of fluorescence.It is also shown that the correlation of the observed chlorophyll concentration with the ratio of upward quanta at the wavelengths of 685nm and 480nm is also good. Use of this ratio is recommended for remote sensing of chlorophyll concentration since the number of spectral channels required by the remote multispectral imager can be minimized.