PRIMARY PRODUCTION AND PHOTOADAPTATION OF PHYTOPLANKTON IN RELATION WITH TIDAL MIXING IN COASTAL WATERS

In the eastern English Channel (a coastal and shallow system), photosy nthetic parameters of the phytoplankton population were measured durin g 36 h at four depths, in a neap tide period. The vertical distributio ns of chlorophyll a (chl a), nutrients, and hydrodynamic properties we re monitored during the cruise. Despite vertical homogeneity in salini ty, chlorophyll a and nutrient profiles, a vertical gradient in both p hotosynthetic parameters (P-MAX(B) and alpha(B)) and in vivo fluoresce nce/chl a ratios was observed. Photosynthetic adjustment occurred in t he water column: low vertical current shear at mid-depth favoured shad e adaptation of phytoplankton, as compared to the photoinhibited cells in the upper layer. This was consistent with the observed similar lig ht saturation parameters on vertical profiles, suggesting the presence of shade-type phytoplankton. Synchronous circadian variations were ob served in both photosynthetic capacity (P-MAX(B)) and in vivo fluoresc ence/chl a ratios. Estimates of daily primary production, computed wit h and without the observed daily and depth variability in P-MAX(B) and alpha(B), showed large differences (from -40 to +33%) between them. T he strong light penetration in the water column, the photoadaptation p rocesses occurring at neap tide and the presence of shade-type phytopl ankton (prymnesiophytes) could be major factors inducing a high daily rate of 2.1 g C m(-2) day(-1).