An algal photoprotection index and vertical mixing in the Southern Ocean

Abstract

We explored the relationship between mixing dynamics in the surface mixed layer and the photoprotective response of phytoplankton. The distribution of photoprotective pigments was determined at 15 stations in subantarctic waters southeast of New Zealand during austral autumn. The first-order kinetics of the phytoplankton photoprotective response to both light and dark was determined using in situ simulated (deck) incubations. The vertical mixing regime was deduced from two physical parameterizations based on CTD and ADCP profiles. A photophysiological index based on the vertical profile of de-epoxidation state (DES) of the diadinoxanthin-cycle to changes in light intensity was compared with the two physical estimates of vertical eddy diffusivity. For short time scales (<12 h) and within a similar water mass, the DES index provides within one order of magnitude an estimate of average vertical velocity representative of the bulk vertical eddy diffusivity. From the results, a conceptual model is presented of the quantitative relationship between vertical mixing and phytoplankton photoprotection in the water column. This relationship can provide additional insights into the effects of changes in vertical eddy diffusivity in the surface mixed layer on the photoprotective response of phytoplankton.