Impact of Photoacclimatory Status and Community Structure on the Bio-optical Properties of Marine Phytoplankton

In vivo light absorption spectra of phytoplankton (a_ph(λ)) harbour information on both the size structure and the physiological state of natural phytoplankton assemblages. Using a large in situ dataset that covers a wide range of oceanographic domains (from the poles to the tropics) in the Atlantic Ocean, this study examined the basin-scale variation of a_ph(λ). Our results show that the magnitude and spectral shape (measured using the spectral slope) of a_ph(λ) were influenced strongly by the presence of non-photosynthetic pigments (NPP): a considerable fraction of the basin-scale variability in the chlorophyll a-specific absorption coefficient of phytoplankton at 440 nm (a^*_ph(440)) was explained by changes in the relative abundance of NPPs, whereas cell size estimated using the diagnostic pigments appeared to be of secondary importance. The observed relationships between the bulk chlorophyll a concentration, photoacclimatory status as indicated by the proportion of NPPs, and community structure are largely dependent on the contrasting photophysiological strategies of dominant prokaryotic and eukaryotic groups in each trophic domain. Our study suggests that the influence of NPPs on phytoplankton light absorption should be considered when using remote sensing reflectance to derive information on phytoplankton taxonomic composition and to estimate marine primary production using bio-optical models. Developing a deeper understanding of the link between marine bio-optics and the ecophysiology of marine phytoplankton will improve ocean biogeochemical models and the monitoring of open-ocean ecosystems for fisheries and marine resource management.