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

Abstract

Chlorophyll‐specific absorption coefficients of particles, a*p(λ), and of phytoplankton, a*ph(λ), were determined using the glass‐fiber filter technique along 150°W in the equatorial Pacific (13°S–1°N). 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*ph into the contributions of photosynthetic pigments (a*ps) and nonphotosynthetic pigments (a*nps). Both a*ph and a*nps values were observed to decrease from the oligotrophic waters of the subequatorial area (13°–1°S) to the mesotrophic waters of the equatorial area (1°S‐1°N) and from the surface to deep waters. The a*ph variations were primarily, but not exclusively, caused by changes in the concentrations of nonphotosynthetic pigments. The level of pigment packaging was also variable both horizontally and vertically, as a result of changes in populations and photoacclimation. In comparison with a*ph, a*ps exhibited a reduced range of variation with depth and along the latitudinal gradient. The variations in a*ps originating from the package effect were partly compensated by variations in the concentrations of photosynthetic pigments. We extended this analysis to include data collected in other areas with different trophic states. The a*ps values varied over a factor of 4 at 440 nm, instead of 8 for a*ph, for chlorophyll a concentrations covering 2 orders of magnitude (0.02–2 mg m−3). In agreement with a previous study performed off California with a different method [Sosik and Mitchell, 1995], we conclude that a*ps is less dependent on environmental parameters than a*ph. In addition, our results provide evidence that the variability in a*ps cannot be neglected. The use of a*ps instead of a*ph in light‐photosynthesis models (in conjunction with a quantum yield for carbon fixation defined with respect to the photosynthetically active absorbed amount of quanta) presents the advantage of removing the variability associated with nonphotosynthetic pigments.