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Oceanography
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Laser Remote Sensing of Sea-Water Plankton
Victor V. Fadeev, Andrey A. Demidov,Alexander M. Checkalyuk
Moscow State University, Physics Department
Moscow 119899, Lenin Hills, USSR
Abstract
Our group have been working the problem of laser remote sensing of sea-water phytoplankton since 1975. In this way we have investigated the fundamental problems of a laser light propagation in natural waters and its interaction with the intact algae. Now we have a great experience in this field in theoretical as well as in practical approaches n about 20 shipboard expeditions in Pacific, Atlantic, India oceans and in a lot of seas. Our methods are based on an analyzing of chlorophyll fluorescence of intact phytoplankton and Raman signal of water molecules which is used as internal calibration parameter. Both signal are detected by lidar (laser fluorosensor) consisted of impulse YAG: Nd-laser (l=532 nm) and Optical Multichannel Analyzer (OMA). Lidar work in the remote mode as well as in the probe testing mode. This lidar was successfully used in quantitative search of concentrations of a phytoplankton chlorophyll in different areas of World Ocean down to mesotrophic water (medium biological productivity) and we got a log of fine structure maps (space resolution on a broad areas. Now we develop some new promising methods of detecting of the photosynthetic activity and shown the rate of its influence on the lidar detecting of phytoplankton fluorescence. High sensitivity of our fluorometric methods (up to 0.1 mg/1 of chlorophyll detection in natural waters and up to 0.1 mg/1 in acetone extracts of plankton) enabled us to develop the new method of zooplankton feed process analysis up to the level of individual sample.
Introduction
In the ocean investigations, one of the prameters broadly used in practice is the phytoplankton chlorophyll a (chl a) fluorescence. This fluorescence can be used as a quantitative parameter for the estimation of a sea-water biological productivity.
Recently in the ocean logy practice, laser fluorometers have been involved [1-16]. They are mainly used in the laser remote sensing of a sea-water. This can be explained by the specific features of the laser sources that can produce a very powerful monochromatic beam with a low divergence. Among the groups working in this area is the Group of Water Laser Remote Sensing of the Physics Department of Moscow State University [1-7]. The researches of this group have developed some sea-water Remote Sensing methods as well as investigated the laser induced fluorescence of intact algae in the sea-water probes [1-5], fluorescence of phytoplankton pigments in extracts [2,7] and fluorescence of pigments in the zooplankton extracts [6].
The specific properties of the laser light open up some new fundamental potentialities of fluorometry that are hardly possible in the “classic” lamp fluorometry. One of these properties is the fixed light wavelength that provides for a stability of absorbance and
fluorescence in the case of using different laser fluorometers but with lasers of the same type. It also enables an easy detection of the Raman scattering of the laser light by the solvent molecules that can be used as an internal standard (1, 3, 10, 14, 15).
Regarded as the pioneers in suing the Raman signal as an internal standard in sea-water analysis are D.A. Leonard and C.H. Chang (1974) (14), M. Brown (1974) (15) and V.V. Fadeev (1975) (1). In their works they investigated the fluorescence of intact phytoplankton with using water Raman signal. Then in 1978 this method was surely proved mathematically by Klyshko and Fadeev (3). Since 1974 it has been widely used in the laser remote sensing of natural sea-water.
Laser Fluorometer (LIDAR) Construction
In our investigations, we have used the shipboard mounted lidar system developed n the Moscow University which can be easily accommodated to the Remote Sensing mode or to the probe analysis (1-7). Used as the laser source is the impulse YAG laser (l=532 nm). t~ 10 ns, power p~500 KWt. Lidar was positioned in the ship laboratory. Distance from the lidar to the sea surface was about 10-15 m.
The laser beam illuminates the sea surface after reflection from the mirror mounted on the ship false board or the cuvette with the water probe of phytoplankton to excite the phytoplankton fluorescence (lf1@685 nm ) and water Raman scattering (lR=651 nm). Both signals are detected by an Optical Multichannel Analyzer (OMA, PARC production, USA). The OMA detector is gated by high voltage pulses of 0.04-1 ms duration synchronized with the laser light pulses. This approach enables one to do measurements under natural conditions of the sunshine and it would not influence the results of measurement.
The whole spectrum is stored in the OMA memory to be processed by a computer interfaced with it. Besides, this computer presets the mode of operation of the laser fluorometer as a whole. OMA operates in the mode of parallel detection of the spectrum (l=275 nm) and it is effective in detection of fluorescence intensity that is 1012 - 1013 times less than the laser light intensity. In fact, in the remote sensing mode we measure the average value of fluorescence of the upper 2-5 m of water because of a great absorbance of response signal (l=650-690 nm) by a natural sea-water.
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