Phytoplankton biomass and primary productivity by size classes in the Gulf of Tehuantepec, Mexico

From January 5 to February 6, 1989, we carried out an oceanographic cruise to the Gulf of Tehuantepec. Size-fractionated chlorophyll a and super(14)C assimilation experiments were carried out at 12 stations. Nutrient concentrations (PO sub(4) and SiO sub(2)) were also measured. In general, the nutricline was very shallow and developed within the euphotic zone depth. The size-fractionated biomass distribution and primary productivity rates for the gulf were closely coupled to the hydrographic regimes. Close to 70% of the total phytoplankton biomass and productivity was due to the nanophytoplankton size fraction. Productivity rates after a wind event were comparable to other upwelled-driven systems in temperate zones. Total integrated euphotic zone chlorophyll a varied from 4.5 to 135.5 mg m super(-2) (x super(-) = 53.2 mg m super(-2)), and productivity rates ranged from 0.07 to 1.43 g C m super(-2)/day (x super(-) = 0.70 g C m super(-2)/day). These values are comparable to the rich waters of the central Gulf of California and the upwelling area off Baja California. Then, it is suggested that at least during the "nortes" season, the Gulf of Tehuantepec acts as a nutrient and phytoplankton-carbon pump, enriching part of the offshore waters of the Eastern Tropical Pacific.