Laser Remote Sensing of Sea-Water Plankton
The use of a computer enabled us to do advanced automatization of a whole lidar complex and process of measurements. As a result, we have achieved the time duration of a one measurement circle (from the start to the final result) about 1.5 min. That allowed us to obtain the space resolution of a sea-water monitoring about 450 m at a ship speed 10 miles/hours. This is not a principle limit and we know how to improve this result.
Laser Fluorometry of Phytoplankton and its Applications
In the Fig. 1 there is presented the typical spectrum of detected response signal. There, one can see two signals: (1) Raman signal (IR lR=651 nm) and (2) phytoplankton chl 1 a (If1, lf1 @685 nm). Both signals are similarly influenced by the measurement conditions (the form of water surface or the cuvette positions, illumination conditions etc.) and their ratio f=If1/IR will not be dependent on these conditions and, consequently, on the fluorometer type (3). We name f as ‘fluorescence parameter’. This parameter is a quantitative value of
pigment fluorescence and it is linearly connected with the pigment concentration f=aC (2, 3, 5), where C is the concentration and a - coefficient. In fact, Raman signal is regarded as an internal standard to be used for the fluorescence calibration.
Fig. 1. Typical spectrum of response signal detected in South Atlantic near Namibia, 1985-1986, the 43rd scientific trip of the ‘Academitian Kurchsatov’ ship (Institute of Ocean logy, USSR Academy of Sciences);
lR – water Raman,
l f1 – phytoplankton chl a fluorescence.
One of the “underwater reef” in the laser fluorometry of a phytoplankton is the effect of fluorescence saturation when any one use the powerful laser light to excite this fluorescence (2, 5). This effect is connected with a set of specific processes in the pigment light-harvesting antenna of algae, for example, with the singlet-singlet annihilation in it. So, one have to do special precautions to avoid it or to do a necessary correction procedure (2, 5).
In the sea-water monitoring there is necessary to know not only the intensity of a phytoplankton fluorescence but mainly the value of a chl a concentration C (C=
af). We have been investigating the problem of a estimation for years, now it isn’t yet solved for sure. But in a set of scientific expeditions in the Pacific, Atlantic and Indian oceans and in some of seas we have got:
Where
f0 is the fluorescence parameter f intact phytoplankton fluorescence measured in the probes in the shipboard laboratory with the specific laser excitation mode and with the correction procedure of fluorescence saturation elimination (2, 4, 5). This method allows one to measure chl a concentration as low as 0.1
mg/1.
The other one “underwater reef” in the laser Remote Sensing of a natural phytoplankton is the day time dependence of a phytoplankton fluorescence connected with a natural day tie changing of an angle photosynthetic activity. This effect causes the day time modulation of the fluorescence responses single (see Fig. 2) and if any one wants to do correct monitoring of a sea-water subsurface phytoplankton fluorescence he has to d the appropriate correction procedure. We have to mention that this day time dependence (the curve form and value of the modulation coefficient is the function of a
phytoplankton environment inhabitance and average sunshine illumination intensity. Now we have some laser methods of estimation of the algae photosynthetic activity (see materials of this conference).
Fig. 2. Day time dependence of a normalized fluorescence of subsurface phytoplankton (in fact, fluorescence quantum yield of intact algae in situ); remote sensing on station near the Elephant island, South Atlantic, 1985-1986.
A special interest of using the laser monitoring in the sea-water control is connected with its possibility to get express-information of a phytoplankton chl a distribution on the large areas in a distant mode. Our lidar enables on to do scanning of a sea surface with a high resolution (up to hundred meters) at a ship speed over 10 miles/hour and analyze the fine structure of a phytoplankton fluorescence distribution. For example, we have found that variability of phytoplankton fluorescence can achieve about 300% on the 1 mile duration. Now we have got a set of maps of phytoplankton fluorescence distributions over the large areas of some oceans and seas. One of them is presented in the Fig. 3.
Fig. 3. Subsurface phytoplankton fluorescence distribution map, near the Elephant island, South Atlantic, 1985-1986.
