Diurnal Variations of Optical Properties of Four Species of Oceanic Phytoplankton and their Co-Varying Variables

With the recent development of geostationary satellites equipped with ocean color sensors and the increasing availability of moored sensors, temporal resolution of ocean color data is higher than ever. This allows the investigation of diurnal variations of phytoplankton ecology and physiology in situ and from space, potentially providing new insights into their growth and physiology. This study aims to characterize the diurnal variations of phytoplankton optical properties and possible co-varying variables in the lab, to understand what can be observed in situ and from space. In a laboratory experiment, we grew triplicate semi-continuous cultures of four species of micro-phytoplankton (E. Huxleyi, D. Tertiolecta, T. Pseudonana and P. Tricornutum) under a sinusoidal light regime. We measured the optical scattering and absorption properties of the cultures every two hours of during daylight with an ACS and a BB9. Other analysis included Chla, in vivo fluorescence, POC, cell cycle with a flow cytometer, Coulter Counter cell counts and size distributions and flowcam imaging. Amongst other things, we found that the beam attenuation coefficient (c_p) showed diurnal variations both in biomass normalized amplitude and in shape as expected, but that the backscattering coefficient (b_bp) did not, suggesting that diel variations in b_bp often observed in the field may be linked to other non-phytoplanktonic particulate pools. Diurnal variations of c_p also differed between species.