Phytoplankton photosynthetic parameters from the Gulf of California southern region

Phytoplankton physiologic response in the euphotic zone was determined by photosynthesis-irradiance experiments carried out between June 2000 and June 2001 at Bahia de La Paz, Gulf of California, Mexico. A stratified water column was present, from June to September (stratification index > 200 J m^-3), with the assimilation number (P_m) represented by an average of 3.7 mg C m^-3 h^-1, while for the period from November through February, the water column was homogeneous (stratification index = 0 J m^-3) and the average was 9.2 mg C m^-3 h^-1. The maximum photosynthetic efficiency at low irradiances (a) of 0.017 mg C m^-3 h^-1 (µmol quanta m^-2 s^-1)^-1 was the representative average for the stratified period, while 0.063 mg C m^-3 h^-1 (µmol quanta m^-2 s^-1)^-1 was the corresponding value for the homogeneous period. The photosynthetic parameters (P_m and a) had an increase during mixing conditions, as result of phytoplankton photoacclimation to average irradiance in the euphotic zone. Diatom abundance (Pseudo-nitzschia delicatissima, Chaetoceros compressus and Coscinodiscus perforatus) were related with higher values of P_m, while lowest values of this parameter are associated with the dinoflagellate group. Average values estimated for the light saturation parameter (E_k) were 334, 173 and 91 µmol quanta m^-2 s^-1 for the surface, 10 and 1% of the euphotic zone, respectively. In general, nanophytoplankton (2-20 µm) dominated the phytoplanktonic community during the period studied.