Phytoplankton productivity and biomass by size classes, in the central Gulf of California during spring 1985

Abstract

An oceanographic cruise was performed in the central region of the Gulf of California from March 31 to April 9, 1985. Nineteen hydrographic casts to 0, 25, 50, 75 and 100 m were made in four transects to estimate the size-fractionated biomass of the phytoplankton (chlorophyll a). Five size-fractionated 14C assimilation experiments were also done. A thermal front was recorded during the cruise period, with similar characteristics to those previously reported for this area. The contribution of nannoplankton to the biomass was higher at most stations, although maximum values of chlorophyll a and primary productivity were found near the coast, where the abundance of microphytoplankton was higher. The higher values of the assimilation ratios corresponded to the stations with the higher nannoplankton percentage, indicating higher efficiency in their production rates. The assimilation ratios and primary productivity rates were lower than those reported in March of 1983 and 1984, when Gulf waters were under the influence of the El Niño event, but of the same order of magnitude as during normal years.