algorithms for remote sensing of phytoplankton ... - Science Direct

Adv. Space Res. Vol. 13, No.5, pp. (5)197—(5)201, 1993 Printed in GreatBritain. All rights reserved.

0273—1177~3$24.00 Copyright ‘B 1993 COSPAR

ALGORITHMS FOR REMOTE SENSING OF PHYTOPLANKTON PIGMENTS IN INLAND WATERS A. Gitelson The Jacob Blaustein InstituteforDesertResearch, Ben-Gurion University of the Negev, Sede-Boker Campus 84993, Israel

ABSTRACT In order to develop algorithms for the comprehensive remote sensing of phytoplankton chlorophyll-a in inland waters simultaneous measurements of the reflectance spectra in the region of 400 to 750 nm and concentrations of chlorophyll-a, dissolved organic matter and suspended matter were carried out. Algorithms for remote estimation of chlorophyll-a content were developed and tested in a wide range of pigment concentrations. Chlorophyll-a concentrations estimated from reflectance spectra agree closely (within 3 mgm-3) with values measured at the sampling sites in the range of concentrations of 0.1 up to 300 mgm-3. The results of the research provided the possibility for the development of an appropriate methodology for remote monitoring of eutrophication processes in inland waters.. INTRODUCFION Most algorithms used extensively in the remote sensing of chlorophyll-a are considered valid only for Case 1 waters. The attempts to use these algorithms for productive waters have demonstrated the limitation that this technique places on measurements of waters that are high in chlorophyll-a and/or dissolved organic matter concentrations (e.g. /11). The present study was designed to produce a multivariate statistical approach, for the independent quantitative assessment of chlorophyll-a in inland waters, using a high spectral resolution radiometric system. First, the spectral distribution of radiance reflectance and concurrently acquired surface samples were measured in a wide region of water quality parameters. Magnitude and position of reflectance spectral features were related to relevant constituent concentrations. Second, models of both radiative transfer and optical properties of organic and non-organic matter were used for calculation of reflectance. Particular wavelengths, where the reflectance values were maximally and minimally sensitive to variation of constituent concentrations were found. Third, one set of the measurements was used to develop the relationship between reflectances and chlorophyll-a content. Finally, having found empirical and calculated algorithms for pigment estimation, the other data sets, covering a range of trophic states of the aquatic systems from oligotrophic to hypertrophic, were used to test the developed algorithms; then, an attempt was made to extrapolate them from a local to a regional scale. METHODS Several thousand spectral irradiance measurements, along with simultaneous ground-data references, were taken between 1983 and 1989 in various aquatic systems of the former USSR, Hungary, Bulgaria and Germany. In the study of individual ecosystems, the phytoplankton chlorophyll-a concentration (Cchl) ranged from 0.1 to 350 mgm3, the suspended matter ranged from 0.1 to 66 mg11, and the dissolved organic matter absorption at 380 nm was 0.1 to 12 m1 (5)197

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A. Gitelnon

/2/. A radiometer, recording in the range of 400 to 750 nm, with a spectral resolution of about 1 nm and field of view of 70 was used to measure the upwelling radiance and downwelling irradiance /3/. Radiometric readings and samples were taken from a boat. Measurements of chlorophyll-a concentration were made by fluorimetric technique. Suspended matter was determined by the gravimetric method. The dissolved organic matter concentration was estimated by using absorption of the filtrate at 380 nm. The fluorescence spectrum of each sample in the range of 640 to 750 nm with a spectral resolution of more than 0.7 nm was measured using a KVANT-5 fluorimeter /3/. To study the effect of independent components on the variability of reflectance spectrum Gordon’s fluorescence model /4/ was combined with a two-flow optical model /5/. The reflectance in the region of 400 to 750 nm was calculated for various concentrations of relevant constituents. To specify cell size distribution of phytoplankton, the Junge distribution f(r)=Ar-V was used /6/. The parameter v varied from 0.1 to 5.0. The minimum cell radius ri was assumed to be between 0.5 and 1 micron, and the maximum radius r~varied from 2.5 to 10 micron. For non-organic suspended matter, a Junge distribution with v ranged from 0.1 to 5.0 was assumed, and a refraction coefficient was taken at 1.2. Fluorescence quantum efficiency of phytoplankton was assumed to be between I and 10% /7/. RESULTS Spectral Features of Reflectance Several features of the reflectance spectra were common for waters that had different trophic states. A broad reflectance maximum of around 560 nm is caused by a low absorption by algae. A low reflectance, from 400 nm to 500 nm, is due to absorption by both chlorophyll-a and dissolved organic matter. A dip at 620 nm, attributed to absorption by phycocyanin, a minimum at 675 nm, caused by chlorophyll-a absorption, and a minor peak at 650 nm were observed. When Cchl was less than 3 mgm-3, a shoulder at 680 nm was observed; it became a distinct peak for Cchl more than 5 mgm3. The magnitude of the peak increased and approached the value of the maximum at 560 nm for Cchl > 70 mgm3. The peak height (Rmaxred) was quantified by normalizing it to the reflectance value of the maximum of the spectrum at 560 nm (R(560)). The ratio Rmaxred/R(56O) vs. chlorophyll-a concentration for the Don and the Northern Donec rivers in Russia, for the different seasons and years, was described by a power function Rmaxred/R(S6O) = 0.232 Cchl(0~34±0.009)

(1)

with a determination coefficient of more than 0.93 (Figure 1). The increase in the peak height was accompanied by a shift in the position towards the longer wavelengths; meanwhile, the peak of the phytoplankton fluorescence had a permanent position at 683 nm for all the samples measured. The peak position varied from 683 nm for Cchl < 3 mgm-3 up to 715 nm for Cchl above 100 mgm3 (Figure 2). For all the considered aquatic systems, the regression in the form Peak position

=

683.5 1+ (0.268 ±O.OO7S)Cchl, nm

(2)

was found with a determination coefficient higher than 0.93. The parameters of equation (2), obtained from model calculation for fluorescence quantum efficiency 2.5%, were 685 nm and 0.27 mgl m3 nm, respectively /8/. Detection Algorithms. On the basis of the results of above mentioned signature analysis and factor analysis of reflectance /2/ we suggest that in mesotrophic and eutrophic aquatic systems an improved estimation of chlorophyll-a concentration will be achieved by using the function of reflectance in the red region of the spectrum, where unique spectral features for phytoplankton take place (minimum at 675 nm and maximum at near 700 nm).

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Remote Sensing of Phytoplankton Pigments

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The peak height, normalized to reflectance at 560 nm, versus chlorophyll

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