| Summary: | The aim of this work was to find statistical relationships between the concentrations of accessory pigments in natural populations of marine phytoplankton and the absolute levels and spectral distributions of underwater irradiance. To this end, empirical data sets from some 600 stations in different parts of the seas and oceans were analysed. These data were obtained from the authors' own research and from the Internet's bio-optical data base. They included the vertical distributions of the concentrations of various pigments (identified chromatographically) and the vertical and spectral distributions of the underwater irradiance measured <i>in situ</i> or determined indirectly from bio-optical models. The analysis covered a total of some 4000 points illustrating the dependence of pigment concentration on underwater irradiance characteristics, corresponding to different depths in the sea. The analysis showed that the factor governing the occurrence of photoprotecting carotenoids (PPC) is short-wave radiation λ < 480 nm. A mathematical relationship was established between the relative PPC concentration (relative with respect to the chlorophyll <i>a</i> concentration) and the magnitude of the absorbed radiative energy per unit mass of chlorophyll <i>a</i> from the spectral interval λ < 480 nm, averaged in the water layers c<i> z</i> = 60 m (or less near the surface) to account for vertical mixing. This absorbed short-wave radiation (λ < 480 nm) was given the name of Potentially Destructive Radiation (PDR<sup>*</sup>(z)). Analysis of the relationships between the concentrations of particular photosynthetic pigments (PSP), <i>i.e.</i> chlorophyll <i>b</i>, chlorophyll <i>c</i>, photosynthetic carotenoids (PSC), and the underwater irradiance characteristics indicated that these concentrations were only slightly dependent on the absolute level of irradiance E<sub>0</sub>(λ), but that they depended strongly on the relative spectral distribution of this irradiance <i>f</i>(λ)= <i>E</i><sub>0</sub>(λ)/<i>PAR</i><sub>0</sub>. The relevant approximate statistical relationships between the relative concentrations of particular PSP and the function of spectral fitting <i>F<sub>j</sub></i>, averaged in the layer Δ<i>z</i>, were derived. Certain statistical relationships between the pigment composition of the phytoplankton and the irradiance field characteristics are due to the photo- and chromatic acclimation of natural populations of marine phytoplankton. These relationships can be applied in models of the coefficients of light absorption by phytoplankton.
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