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
