Phytoplankton Pigment Concentrations Research Articles

Atmospheric effects in the remote sensing of phytoplankton pigments

We investigate the accuracy with which relevant atmospheric parameters must be estimated to derive phytoplankton pigment concentrations (chlorophyll a plus phaeophytin a ) of a given accuracy from measurements of the ocean's apparent spectral radiance at satellite altitudes. The analysis is limited to an instrument having the characteristics of the Coastal Zone Color Scanner scheduled to orbit the Earth on NIMBUS-G. A phytoplankton pigment algorithm is developed which relates the pigment concentration (C) to the three ratios of upwelling radiance just beneath the sea surface which can be formed from the wavelengths (λ) 440, 520 and 550 nm. The pigment algorithm explains from 94 to 98% of the variance in log10 C over three orders of magnitude in pigment concentration. This is combined with solutions to the radiative transfer equation to simulate the ocean's apparent spectral radiance at satellite altitudes as a function of C and the optical properties of the aerosol, the optical depth of which is assumed to be proportioned to λ-n. A specific atmospheric correction algorithm, based on the assumption that the ocean is totally absorbing at 670 nm, is then applied to the simulated spectral radiance, from which the pigment concentration is derived. Comparison between the true and derived values of C show that: (1) n is considerably more important than the actual aerosol optical thickness; (2) for C 0299-1 0.2 Μg l-1 acceptable concentrations can be determined as long as n is not overestimated; (3) as C increases, the accuracy with which n must be estimated, for a given relative accuracy in C, also increases; and (4) for C greater than about 0.5 Μg 1-1, the radiance at 440 nm becomes essentially useless in determining C. The computations also suggest that if separate pigment algorithms are used for C ≲ 1Μgl-1 and C ≳ 1 Μgl-1, accuracies considerably better than ±± in log C can be obtained for C ≲ 1 Μg l-1 with only a coarse estimate of n, while for C ≳ 10 Μgl-1, this accuracy can be achieved only with very good estimates of n.

Ocean color spectra measured off the Oregon coast: characteristic vectors

The ocean color spectrum is defined as the ratio of the spectrum of light upwelled from the sea to the spectrum of light incident on the sea surface. Ocean color spectra, observed from an airplane flown over waters off Oregon, are analyzed. The original spectra are resolved into fifty-five wavelength bands, each 5 nm wide. The shapes of these spectra are parametrized by, and shown to be accurately recoverable from, their first four principal components Y(i)(i = 1, 2, 3, 4). These Y(i) are the scalar projections of each spectrum on the first four characteristic vectors of the sample covariance matrix. Empirically determined characteristic vectors like these, but derived from a much larger and more globally representative sample of color spectra, may someday provide an efficient, standard basis for parameterizing ocean color spectra. Regression equations are found with which phytoplankton pigment concentration and water transparency may be estimated as linear functions of the parameters Y(i). Pigment concentration estimates thus obtained are imprecise. The poor fit is at least partly due to the inappropriateness of the linear regression model and the neglect of other optically important substances typically present in sea water.

The influence of phytoplankton pigments on the colour of sea water

By summing the absorbency of phytoplankton pigments with that of pure water, the absorption of the blue portion of the spectrum is markedly increased. As the concentration of phytoplankton pigments increases, the diminution of blue light gradually shifts the wave length of maximum transmission toward the green. At the concentration of phytoplankton pigments normally found in the open ocean, the red chlorophyll band has little influence on water colour. Inadequancies in present methods for detection of absorption bands in natural waters is attributed to wide band widths of filters used in submarine photometers. Poor spectral curves applicable to colour analysis in particulate systems are obtained by conventional spectrophotometric techniques because a large portion of the scattered light never reaches the photo-detector. Improvements for spectral analysis are suggested.