The Relationship between Photosynthesis and Irradiance for Gulf of California Phytoplankton

Abstract

During June 17-28, 1982, we carried on experiments to generate photosynthesis-irradiance curves for phytoplankton from five locations of the central and northern Gulf of California. Using natural sunlight, on board 14C incubations were done with samples collected from five different depths within the euphotic zone. In general there were great vertical changes of variables controlling primary productivity, even in cases where weak vertical TOC gradients indicated high instability and mixing of the euphotic zone. The assimilation number (PBm) had a two fold variation within the mixed layer. In general PBm decreased monotonically with depth due to phytoplankton conditioning to lower irradiances. Surface PBm values had a range of 8 to 15mg C. mg Chl a- 1. h-1. When a thermocline was present, PBm for the bottom of the euphotic zone was about 4 to 10% of that for surface waters. But, without a thermocline, P~ for the bottom of the euphotic zone was ~ 60% of that for surface waters, due to turbulence moving phytoplankton up and down the water column. Diatoms were abundant in three stations near Angel de la Guarda and Tiburon islands. For the station at the central gulf and the one at the very northern gulf, very few nano and microplankton were found with the inverted scope technique, but high chlorophyll concentrations and primary productivity indicated a high abundance of picoplankton. This difference in plankton size composition was not reflected in PBm values. Nutrient concentrations were high and did not limit PBm values. Our PBm values are higher than those reported for the gulf's winter phytoplankton; and they are about two times higher than those for winter phytoplankton of the oceanic region between San Diego and Acapulco. Our Gulf of California integrated primary productivity values had a range of 1.3 to 4.4gC. m-2. d-1.