Spatio-temporal variability of phytoplankton photosynthetic activity in a macrotidal ecosystem (the Strait of Dover, eastern English Channel).

Abstract: Photosynthetic parameters, phytoplankton assemblages and physicochemical parameters were analysed in the Strait of Dover along an inshore–offshore transect between September 2008 and December 2009. The samples were collected weekly at 9 stations only at the surface or at both surface and depth. The photosynthetic activity was obtained by measuring Rapid Light Curves (RLC) using Pulse Amplitude Modulated (PAM) fluorometry. Temporal variability of the maximal light utilization efficiency (α), maximum electron transport rate (ETRm) and light saturation coefficient (E k) were greater than spatial variability (in horizontal and vertical dimensions) whereas temporal variability of the maximum quantum yield (F v/F m) was of the same order of magnitude as its spatial variability within the water column (in the vertical dimension). ETRm and α were positively correlated and the slope of the relation varied significantly between the different depths and seasons. The highest values of ETRm and α were obtained in summer (June) and late autumn – early winter (September–October & December) and the lowest in spring (March–April). The spatial variability of these parameters was greater vertically than horizontally. Differences between depths were particularly pronounced at the station nearest to the coast where photosynthetic response seems to be controlled by the light history of the cells. Seasonal variability in photosynthetic activity was mostly related to phytoplankton assemblages, light and temperature whereas the light history of the cells within the water column could be the most important factor in depth variability. [Copyright &y& Elsevier]