Variability of phytoplankton light absorption in stratified waters of the NW Mediterranean Sea: The interplay between pigment composition and the packaging effect

Abstract

The Variability of chlorophyll-specific phytoplankton light absorption [a*ph (λ)] was examined over depth and time in stratified offshore waters of the North-Western Mediterranean Sea. Coherent water patches were tracked with Lagrangian drifters during two oceanographic cruises in September (late summer) and May (post-spring bloom phase). By simultaneously analysing the phytoplankton absorption and pigment measurements, we explicitly separated the impact of pigment composition from that of pigment packaging on a*ph (λ). We further partitioned the packaging effect by comparing the variation of the packaging index [Qa*(440)], the phytoplankton community size structure (derived from diagnostic pigment analysis), and the chlorophyll-specific beam attenuation by particles [cp* (660)] as an optical index of phytoplankton photophysiology. In the ensemble of cruises, around 50% of the a*ph (440) vertical variation was explained by changes in the pigment composition (ruled by the decrease of photoprotective pigments with depth). The remaining vertical and inter-cruise variation of a*ph (440) was attributed to the packaging effect. We found that differences in the cp*(660) index (most likely indicating changes in the intracellular pigment concentration due to photoacclimation) mainly explained the observed variation in the packaging effect. Differences in cp* (660) were coincident with either the vertical gradient of light availability or the lower mean daily PAR irradiance in the euphotic layer of the September cruise. These explained the stronger packaging and lower a*ph (440) values observed with increasing depth in both cruises, and in September relative to May. On the other hand, differences in the phytoplankton community size structure did not explain the observed patterns in the packing effect. Our results highlight the importance of phytoplankton short-term acclimation to the prevailing light conditions, determining the vertical and temporal variability of a*ph (λ). A better understanding of the a*ph (λ) variability and its main drivers are key to improve different bio-optical applications.