Relationships between picophytoplankton and optical properties in the Azores Front region in the Atlantic Ocean

The picophytoplankton evolved to become extremely effective harvesters of light and, thus, dominating productivity in the open ocean. In this study, their distribution in relation to the underwater light field was examined in the Azores Front region of the North Atlantic. In this region, attenuation coefficients of downward irradiance varied between 0.038 and 0.065 m− 1. Maximum absorptions were at the red and green parts of the light spectrum, typical of the oligotrophic ocean. The euphotic zone ranged from 70 to 120 m. The deep chlorophyll maximum (DCM) was found at depths where subsurface light ranged between 0.1 and 1% of its surface values. Prochlorococcus was the dominant phytoplankton group while Synechococcus (cyanobacteria) and picoeukaryotes were much less abundant. The ability of chlorophyll to absorb light, i.e., the absorption coefficient of chlorophyll (a⁎), was found to be dependent on the photoacclimation of the cells and was lower at low-light intensities. Due to a packaging effect and probable pigment changes, a⁎ at the DCM was the lowest while the chlorophyll per Prochlorococcus cell was the highest. This is a major adaptation of the picophytoplankton to low light (less than 1% surface light), which allows them to bloom at the DCM. This study indicates that the Atlantic Ocean models for the estimation of phytoplankton concentration and/or primary production should not use a constant carbon biomass-to-chlorophyll ratio for phytoplankton based on phytoplankton size, and should take into account the variation in chlorophyll-absorption ability.