Laser method for Remote Sensing of Phytoplankton photosynthetic activity in - Situ
Alexander M. Chekalyuk, Maxim Yu. Gorbunov
Moscow State University, Physics Department
Lenin Hills, 119899 Moscow, USSR
Introduction and abstract
Due to an unique feature - the possibility of photosynthesis, phytoplankton may be considered as a keycomponent of sea, ocean and freshwater basin ecosystems. It forms the base of many tropic chains and provides effective regulation of oxygen and carbon concentration in water and atmosphere. In connection with many ecological problems there exists a need for the development of modern methods for the measurements of phytoplankton characteristics with adequate possibilities.
In the present communication of Remote laser method for estimation of phytoplankton photosynthetic activity is described. Using this method it's possible to carryout express measurements in-situ from a board of a moving carrier (a vessel, a helicopter, an aircraft) with high special resolution.
Method for Remote Sensing of Photosynthetic Activity
A method of measuring the relative yield of chlorophyll "a" variable fluorescence is widely used in the labs for quantitative estimating of photosynthesis efficiency. The method is based on the fact that transient of photosystem II reaction centres to the closed state with reduced primary quinon acceptor QA (for example, by adding DCMU or under exposure of saturated light flash) causes the increase of chlorophyll "a" fluorescence from the original level I0 to the maximum one Imax due to the addition of variable fluorescence. The difference Imax - I0 by Imax (or I0) characterizes normalized efficiency of light energy conversion in primary photosynthetic reactions(1,2). For the majority of algae species under the normal conditions the value of h=(Imax - I0)/Imax is about 06..0.7 and it decreases down to 0.1….0.2 under the influence of unfavourable environmental factors such as nutrients limitation, excessive irradiance, Pollutants etc. (2).
The method we develop for Remote Sensing of phytoplankton photosynthetic the variable fluorescence relative yield h by lamp pump-and-probe technique (2,3). For the determination of the original chlorophyll "a" fluorescence level (I0) single probe laser pulses are used as in the case of phytoplankton fluorescence intensity Remote monitoring (4). For measurement of the maximum level (Imax) phytoplankton fluorescence is excited by the probe pulses following on the pumping ones with certain delay time. The pumping pulses cause the transitions of photosystem II reaction centers top the close state. Imax is determined form the back-scattered light spectrum detected in response to the probe pulses.
The backscattered light spectra from water detected during Remote laser monitoring include bands of chlorophyll "a" fluorescence and water Raman scattering. Their intensities are comparable with each other when chlorophyll "a" concentration in water is between 0.01 and
10 mg/1. In these cases it's handy to use the water Raman scattering band as an internal standard and the fluorescence parameter F= If1/ IRS as a quantitative measure of the fluorescence intensity, (4-6)(If1 is the chlorophyll "a" fluorescence intensity, IRS the Raman scattering one). It's obvious that the value of relative yield of chlorophyll "a" variable fluorescence is determined by the expression h=(Fmax - F°)/Fmax in these terms. The values Fmax and F° are calculated form spectra obtained in the excitation mode accordingly with and without pumping pulses.
Laser technique for the measurements of the chlorophyll "a" variable fluorescence relative yield was developed and tested in the laboratory during sea and fresh water algae species experiments as well as at the expeditions in the Northwestern Atlantic (spring 1990). The measurements were carried out on the laser fluoremeter = lidar that has been modified by adding second YAG:Nd3+ - laser for generation of pumping pulses. It has been shown that for correct measurements of the photosynthetic activity by means of laser Remote Sensing technique the laser excitation should answer the following demands.
- The value of the pumping pulse flux in probing volume of water must be more than 5 x 1014 cm-2 in order to ensure the transitions to the closed state for the majority of photosystem II reaction centers;
- The delay time between pumping and probing pulses should be about 40…50 ms for quenchers of fluorescence, produced by the pumping pulse, to have decayed, but the reduced primary quinine acceptor of photosystem II QA to be stabilized.
- The probing pulse photon flux density in the water should be less than 1022 sm-2s in order not to take into consideration laser induced fluorescence saturation (7,8) which may distort the measuring value of variable fluorescence relative yield.
The demands mentioned above have been taken into account when developing the special LIDAR equipment and the method for Remote measurement of photosynthetic activity.
