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The relationships between photoadaptation, photoacclimation, cell size and the optical characteristics of the phytoplankton community were studied in 2 areas of the Atlantic Ocean. Pigment composition, absorption and fluorescence excitation spectra were analyzed for samples collected during 2 spring cruises: one in the Labrador Sea and the other in the Central North Atlantic. Photoadaptation (i.e. evolutionary adaptation of different species leading to acquisition of different pigment composition) was evident in the distribution of the main phytoplankton pigments in the area. Photoacclimation (i.e. temporary changes in pigment concentrations in a given species) was also noticeable in the changes in pigment composition and optical characteristics of phytoplankton with changes in depth. Size fractionation of samples from the depth of the chlorophyll a (chl a) maximum showed that, on average, chl a concentration and the values of absorption and fluorescence were dominated by the >2 pm fraction of phytoplankton. Spectral variations in absorption and fluorescence excitation were, however, similar for the small and for the large size fractions. A distinct regional distribution of algal groups was observed, in which prokaryotic picophytoplankton (size class 2 pm) dominated in coastal areas and at higher latitudes. Some significant relationships were observed between the relative abundances of pigments characteristic of certain algal groups and optical properties of the sample. For example, the ratio of absorption at 440 nm to that at 676 nm was positively correlated with the ratio of zeaxanthin to chl a, indicating high abundance of picophytoplankton, especially the cyanobacteria Synechococcus and Prochlorococcus. The ratio of absorption at 555 nm to that at 623 nm was positively correlated with the abundance of phycoerythrin-containing Synechococcus. These results show that useful information about phytoplankton group composition and photoacclimation state can be retrieved from optical properties at a regional scale.

Variability in pigment composition and optical characteristics of phytoplankton in the Labrador Sea and the Central North Atlantic