Using satellite derived data from 1997–2010, sea surface temperature (SST), chlorophyll a (Chl) and primary productivity (PP) variations are described at different temporal and spatial scales in the Gulf of California. Interannual signals of El Niño (EN) and La Niña (LN) were more evident at the entrance to the gulf. Once EN or LN events were over, there was a delayed response of three to six months for the reestablishment of “normal” phytoplankton activity levels. Phytoplankton response to high concentrations of nutrients due to LN was not immediate. At regional scale, EN significantly decreased the Chl mean with regard to “normal” conditions at the entrance, central gulf, and Midriff Islands region (MIR); however, PP under EN conditions decreased significantly only at the entrance. The increase in Chl and PP due to LN relative to “normal” conditions was significant only at the entrance. SST, Chl, and PP annual signals are the main source of variation, particularly for the central gulf and entrance, but in the MIR, “winter” PP (1.96 g C m–2 d–1) was not significantly different from that of “summer” (1.62 g C m–2 d–1). Highest PP was found in the central gulf and MIR, where inferences of production attributable to new production were estimated at more than 60% of PP. At the gulf entrance, primary productivity was mainly attributable to regenerated production, particularly during summer and autumn.

The Gulf of California has three main natural fertilization mechanisms: upwelling, tidal mixing, and water exchange with the Pacific Ocean. Waters high in nutrients occur at very shallow depths in the gulf, and little energy is required for these nutrients to reach the euphotic zone. Upwelling off the eastern coast is strong, chlorophyll a concentration (Chl) can exceed 10 mg m^-3, and because of eddy circulation it increases the phytoplankton biomass across the gulf. Because of strong stratification during summer, upwelling off the western coast causes Chl to increase only to ∼0.5 mg m^-3. The annual cycle is the dominant mode of Chl variability in most of the gulf. El Niño events cause the suppression of Chl mostly in areas on the eastern side of the central and southern gulf, with the effect decreasing from the mouth to the central gulf. ¹⁴C data show that highest productivities occur during winter–spring, and in the Guaymas Basin (up to >4 g C m^-2 day^-1). Averages of total integrated production (P_Tint) for “winter” and for whole regions within the gulf estimated from satellite imagery are in good agreement with averages of ¹⁴C estimates (∼1.8 g C m^-2 day^-1). P_Tint values for “summer” are ∼30% of those for “winter.”