Spectral absorption and fluorescence excitation properties of phytoplanktonic populations at a mesotrophic and an oligotrophic site in the tropical North Atlantic (EUMELI program)

Abstract

Measurements of in vivo spectral absorption, a( λ), and fluorescence excitation, F m ( λ), of phytoplankton were performed in two contrasted situations (oligotrophic and mesotrophic) of the tropical North Atlantic, during October 1991. The vertical and inter-site variability of these properties, and the relative fluorescence yield ( F m a )(λ) were investigated, in relation to light conditions and to the pigment and taxonomic composition of the natural populations. The large vertical variations in the chl-specific absorption coefficients ( a ∗ ) at the oligotrophic site appear to be related to both the decrease of the non-photosynthetic pigment concentration, and the increase of the package effect with depth. At the mesotrophic site, lower a ∗ coefficients are observed, likely originating from the larger average size of the phytoplanktonic organisms, which induces a larger package effect. Both vertical and intersite variation of the chl-specific fluorescence ( F m ∗ ) shows opposite trends compared to a ∗ . The variable presence of non-photosynthetic carotenoids (mainly zeaxanthin) is revealed to be the most clearly identifiable source of variation for the relative fluorescence yield, with a drop in the blue-green region. Prochlorococcus, Synechococcus and picoeukaryotes show a strong photoacclimation capacity to low irradiances. In addition there is evidence in the phytoplankton communities for a complementary chromatic adaptation process to the prevailing spectral irradiance conditions at both sites.