Phytoplankton biomass and production in the Gulf of California: a review

Abstract

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 (PTint) 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). PTint values for “summer” are ∼30% of those for “winter.”