Photosynthetic parameters of northern gulf of California phytoplankton

Abstract

At the end of autumn, 1981, and end of spring-beginning of summer, 1982, we generated photosynthesis-irradiance curves for phytoplankton from five locations, in each cruise, of the northern Gulf of California. In general, photosynthetic parameters, phytoplankton abundance and chlorophyll a had large vertical changes within the euphotic zone, even in cases where thermohaline vertical homogeneity indicated high instability. Nutrient concentrations were very high. The assimilation number ( P m B), in general, decreased with depth due to conditioning of phytoplankton to lower irradiances. Surface ( P m B) values had a range of 2–15 mg C ( mg Chla) −1h −1. Where the bottom of the euphotic zone was within the thermocline, P m B was 4–15% of the values for surface waters; and where it was within the mixed layer, P m B was 25–85% of the values for surface waters. This was due to greater residence time of phytoplankton at depth in the first case. Very strong turbulence by storm winds caused relatively low P m B values in a December station, possibly because of mixing of the near-surface phytoplankton with relatively deep populations conditioned to low irradiances. Our data indicate that in the Gulf moderate turbulence causes higher P m B values than strong turbulence or stratification.