Spatial variations in the chlorophyll‐specific absorption coefficients of phytoplankton and photosynthetically active pigments in the equatorial Pacific

Abstract

Abstract. Chlorophyll-specific absorption coefficients of particles, a • (X), and of phytoplankton, a ;h(•-), were determined using the glass-fiber filter technique along 150øW in the equatorial Pacific (13øS-løN). A site-specific algorithm for correcting the path length amplification effect was derived from field measurements. Then a decomposition technique using the high-performance liquid chromatography pigment information and taking into account the package effect was used to partition a;h into the contributions of photosynthetic pigments (a The spectrophotometer automatically corrected for the baseline which was stored prior to analyses. All spectra were set to 0 at 750 nm to minimize differences between sample and reference filters. Such an approximation, which would be questionable for detrital samples, is justified here because total absorption was largely dominated by living phytoplankton (see Figures 4a, 4b, 4e, and 4f). The optical densities measured on the filter, ODf(X), were corrected for the path length amplification effect using a site-specific algorithm, as described in the Appendix (equation (8) Partitioning Absorption into Photosynthetic and Nonphotosynthetic ComponentsThe chl a-specific absorption spectrum of phytoplankton, a,oh(X), was further partitioned into its photosynthetic and nonphotosynthetic components, a ps(X) and a np•(X) (see (1) Pigment Analyses and Other MeasurementsSamples for pigment measurements were collected on Whatman GF/F filters and either analyzed immediately or stored in liquid nitrogen for later analysis. Pigments were quantified using HPLC according to the procedure described by Vidussi et al. [1996]. The algal pigments identified and quantified include the following photosynthetic pigments: chlorophylis a, b, c, DV chl a, divinyl chlorophyll b (DV chl b), peridinin, 19'-hexanoyloxyfucoxanthin (19'-HF), 19'-butanoyloxyfucoxanthin (19'-BF), prasinoxanthin, fucoxanthin, a carotene, and three nonphotosynthetic pigments: zeaxanthin, diatoxanthin, and diadinoxanthin. The /3 carotene, which is a nonphotosynthetic pigment, was not discriminated from a carotene, which results in underestimating a* and overestimat-* Analyses previously performed on oligotrophic waters ing a p•.in the tropical North Atlantic, however, showed that the /3 carotene-to-zeaxanthin ratio was <10% throughout the water column, so that the error on these coefficients is thought to be very limited.Cell number densities for Prochlorococcus, Synechococcus, and picoeukaryots were determined using a FACSort flow cy-

Bio-optical properties of the subtropical North Atlantic. I. Vertical variability

“…Hoepffner & Sathyendranath 1992, Sosik & Mitchefl 1995, Babin et al 1996, Lazzara et al 1996, Allali et al 1997). There are 2 reasons why this is so.…”

“…Therefore, these differences in the shapes of the absorption spectra can be ascribed to the high conTo examine the within-station differences in the centrations of chlorophyll b at depth (Bricaud & Stramspectral shape of aEh(h), absorption coefficients at each ski 1990, Hoepffner & Sathyendranath 1992, Lazzara et wavelength were normalised with respect to the al. 1996, Allali et al 1997)., absorption coefficient of phytoplankton at 440 nm ~eaxanthin was another pigment that exhibited a [aih (440)] (Fig. 7).…”

“…principal factors that cause variations in the optical properties of marine phytoplankton. It has been found that a significant portion of the observed vertical variation in both the shape and magnitude of the phytoplankton absorption spectrum can be ascribed to the presence of non-photosynthetic pigments (NPPs) (Bricaud & Stramski 1.990, Sosik & Mitchell 1995, Babin et al 1996, Lazarra et al 1996, Allali et al 1997.…”

“…These stratified, open-ocean waters exhibit strong vertical gradients of density, temperature, nutrients and light regime, which in turn pro-?late vertical changes in species composition (Parten-I sky et al. '1996) and in the optical (Babin et al 1996, Lazzara et al 1996,Allali et al 1997) and physiological status (Babin et al 1996) of phytoplankton. Becausethe absorption spectrum of phytoplankton is used to approximate the shape of the photosynthetic action spectrum, and to model the attenuation of light beneath the sea surface in spectral models of primary production, it is important that we improve our understanding of the.…”

“…the paucity of bio-optical data collected in subtropical waters, several recent studies have focussed on such regions (Babin et al 1996, Lazzara et al 1996, Allali et al 1997. These stratified, open-ocean waters exhibit strong vertical gradients of density, temperature, nutrients and light regime, which in turn pro-?late vertical changes in species composition (Parten-I sky et al.…”