Measured photophysiological parameters used as tools to estimate vertical water movements in the coastal Mediterranean

Abstract

Aspects of phytoplankton photophysiology were investigated in a coastal area of the Mediterranean Sea (Gulf of Naples) using high performance liquid chromatography pigment analysis, flow cytometric cellular fluorescence information and in vivo pump and probe measurements. Discrete sampling, continuous profiles and on-board kinetic experiments were used to estimate photoacclimation kinetics and to identify the time scales of these reactions in phytoplankton. Based on the relationship between the depth of the euphotic zone and the mean chlorophyll concentration, the area could be classified as Case 1 water, although, oligotrophy was not reflected in phytoplankton photosynthetic efficiencies, suggesting that it represented a transient feature induced by an eddy, originating offshore. Fronts at the edge of the eddy exhibited different photophysiological features, in which the phytoplankton showed greater physiological stress, brought about by the convergent motion within the front. Kinetic experiments allowed identification of time scales for photoacclimation. Using photoprotective pigments as tracers we estimated vertical mixing velocities in the upper layer of 0.07 cm s-1 and from this we inferred kinetic coefficients for different photophysiological processes in the mixed layer as well as significant threshold values for photoacclimation rates to be 4% h-1. From the threshold value, the vertical eddy diffusivity in the area at the time of sampling was estimated as 1.75 × 10-2 m2 s-1, a high value for vertical mixing.