Laser method for Remote Sensing of Phytoplankton photosynthetic activity in

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.

Two Pulse Lidar System
The two-pulse LIDAR system has been developed on the base of the LIDAR that has been formerly used for laboratory and Remote Sensing measurements of the phytoplankton fluorescence intensity (4) and has show itself as reliable and effective tool during some ocean and sea cruises. The base do modification was an addition of the second laser providing generation of the pumping pulses and some changes in optics electronics and computer software.

The block diagram of the set up is shown on Fig. 1 The Whole system consists of the following devises systems : two YAG: Nd3+ - lasers with frequency doubling (wavelength - 532 nm, pulse width - 10 ns, divergence - 5 x 10-3 radian), working synchronously with regulated delay time of 2..150, ms; the optical system; the multichannel spectrum analyzer OMA and the computer for system control and data processing.

Fig.1 A block diagram of the LIDAR installation for laser Remote Sensing of phytoplankton fluorescence and photosynthetic activity.
The first and second lasers generate pumping and probing pulses. The pulse power values are 3 and 0.5 MW and the pulse frequency - 5 and 1-Hz. These characteristics provide modes required for Remote Sensing of phytoplankton photosynthetic activity from the board of the vessel of subsurface water layers with the thickness of 3……5 m from the distance of 15…20m. The optical system provides coaxial passing of pumping and probing laser beams through subsurface water layer and receiving backscattered light by OMA, which makes its spectrum analysis. The OMA detector is gated by short (0.2 mks) pulses synchronously with probing laser pulses, ensuring selective registration of light spectrum. This gating also excludes the influence of background daylight illumination on the registrated spectra.

Results and Discussion
The first testing of laser method for photosynthetic activity measurements was carried out in the spring of 1990 in North-Western Atlantic. As a result of the systematic measurements in the are of mixture of the warm waters of the Gulf-Stream and the cold water of the Labrador current we obtained the valuable material on the character and scales of the photosynthetic activity horizontal changeability.

A matter of special interest is a map of the chlorophyll "a" variable fluorescence relative yield distribution (fig. 2.a). It was formed on the base of the measurements carried out form the board of a vessel at the area with the sizes of 350*500km between 37° and 44° northern latitude, 42° and 46° western longitude. We don't have an aim of detailed analysis of the distribution features, but it's easy to note considerable photosynthetic activity changeability of not only synoptic but also of mezoscale. Three areas of high activity (0.5….0.7) and three are of low activity (0.2…..0.3) may be found in this region.

Fig. 2 the distributions of photosynthetic activity (a), dynamic height (b) and chlorophyll "a" concentration (c). horizontal - longitude ; vertical - latitude. North western Atlantic, May 1990.
An interesting result may be obtained by comparing this map with the distribution of dynamic height (fig. 2.b), which was calculated on the base of hydrological measurements from the depth of 0 to 2000 m. these data were courteously placed at our disposal by Dr. I.Kelkin, (IO AC USS). The analysis can show, that the areas of high photosynthetic activity coincide with the positions of anticyclonic flows of water Masses. It's necessary to note, that the correlation between the positions of hydrological and chlorophyll "a" concentration (fig. 2.c) features is not so strong. Chlorophyll "a" distribution was obtained as a result of LIDAR Remote Sensing. This data were corrected by taking into account the chlorophyll "a" fluorescence diurnal rhythm modulation, calculated from the photosynthetic activity measurements. Now we are working in order to understand the nature of that phenomena and toe develop on this base the method of hydrological structure method of hydrological structures search by Remote Sensing of phytoplanction photosynthetic activity.

When analyzing the results of the measurements the most effective way is comparing the distribution of chlorophyll "a" fluorescence and the and the distribution of chlorophyll "a" variable fluorescence relative yield. The first parameter contains an information on chlorophyll "a" concentration. The second - on phytoplankton photosynthetic activity. So this comparison gives us not only the information about the features of horizontal distributions of phytoplankton and its photosynthetic activity in subsurface water layers, but in some case is also enables to form the situation development prediction.

As a conclusion we'd like to not that the described method and setup maybe used not only for phytoplankton investigation, but they are also suitable for Remote Sensing of another kinds of photosynthetic organisms, including higher plants.

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