Hydrographic features in the Gulf of California are conducive to high productivity. Waters low in oxygen and high in nutrients are very shallow,and relatively little energy is required for these nutrients to reach the euphotic zone. Upwelling occurs on the west coast during summer and is much stronger on the east coast during winter and spring. Tidal mixing is strong in the northern and inner Gulf, mainly in the region of Angel de la Guarda and Tiburon Island. It seems that moderate turbulence in the Gulf results in higher assimilation of phytoplankton than strong turbulence or stratification. Reported integrated primary productivity values are commonly >1 gC · m^-2 · day^-1, and may reach >4 gC · m^-2 · day-1. Phytoplankton abundance tends to increase from the mouth to the interior of the Gulf, and the highest reported abundances are in the Guaymas Basin (up to >3 106 cells · 1^-1). But integrated primary productivity values for the southern region are similar to those for the northern region because of deeper euphotic zones in the south. Because primary productivity data are scarce, it is not possible to properly describe the seasonal or interannual variability, although it seems that highest primary productivity is during spring and at the beginning of summer. There is an intense phytoplankton patchiness and great changes with time in the Gulf. El Nino events apparently do not cause decreasing rimary productivity in the Gulf. In the central and northern Gulf, surface nutrient concentrations remain high during El Nino events because of strong upwelling and mixing phenomena associated with the tides. Possibly these events only affect the southern region of the Gulf, which behaves more like the open Pacific Ocean.

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