Airborne test of laser pump-and-probe technique for assessment of phytoplankton photochemical characteristics.

Initial results of the airborne LIDAR measurement of photochemical quantum yield, Phi(Po), and functional absorption cross-section, sigma(PS II), of Photosystem II (PS II) are reported. NASA's AOL3 LIDAR was modified to implement short-pulse pump-and-probe (SP-P&P) LIDAR measurement protocol. The prototype system is capable of measuring a pump-induced increase in probe-stimulated chlorophyll fluorescence, DeltaF/F(sat), along with the acquisition of ;conventional' LIDAR-fluorosensor products from an operational altitude of 150 m. The use of a PS II sub-saturating probe pulse increases the response signal but also results in excessive energy quenching (EEQ) affecting the DeltaF/F(sat) magnitude. The airborne data indicated up to a 3-fold EEQ-caused decline in DeltaF/F(sat), and 2-fold variability in the EEQ rate constant over a spatial scale a few hundred kilometers. Therefore, continuous monitoring of EEQ parameters must be incorporated in the operational SP-P&P protocol to provide data correction for the EEQ effect. Simultaneous airborne LIDAR measurements of Phi(Po) and sigma(PS II) with EEQ correction were shown to be feasible and optimal laser excitation parameters were determined. Strong daytime DeltaF/F(sat) decline under ambient light was found in the near-surface water layer over large aquatic areas. An example of SP-P&P LIDAR measurement of phytoplankton photochemical and fluorescent characteristics in the Chesapeake Bay mouth is presented. Prospects for future SP-P&P development and related problems are discussed.