Passive remote sensing of phytoplankton via chlorophyll α fluorescence

The spectrum of light backscattered from the sea in the visible and near infrared and, in particular, the chlorophyll a fluorescence line at 685 nm have been observed from an aircraft under natural illumination by using a multichannel silicon diode spectrometer. The instrument was mounted in the aircraft so as to view the water surface at the Brewster angle by using a polarizer to reduce reflected skylight substantially, even under rough surface conditions. This and the relatively high red sensitivity of the silicon diode detectors explain why this line appears here but not in previous airborne observations. The observed line height has been compared with chlorophyll depth distribution measurements made from a launch and is shown to be proportional to an average of the chlorophyll concentration near the surface, weighted with depth to allow for absorption by the water of light at 685 nm. These observations were made at low (150 m) altitude, but it is shown that the observed line height is relatively insensitive to altitude up to 1200 m. Although the lowest chlorophyll concentration encountered was 2 mg/m3, the technique is expected to be useful for airborne mapping of chlorophyll at concentration several times smaller than this.