Microzooplankton grazing in the gulf of california

Abstract

Microzooplankton (organisms < 200 μm) grazing rates were determined using the dilution technique at nine stations in the central region of the Gulf of California and one in the sill of the Guaymas Basin, during spring, summer, and autumn 1995. The microzooplankton was numerically dominated by protozoa, the most important being the dinoflagellates; the most frequent genera were Amphidinium , Oxytoxum ,  Gyrodinium , and  Peridinium . Ciliates were the second most important group and the most abundant genera were Mesodinium ,  Strombidium , and Strombilidium . Mean phytoplankton growth rates were 1.58, 1.64, and 1.82  d–1 during spring, summer, and autumn, respectively. Mean grazing rates were 1.25, 1.93, and 1.84  d–1 during spring, summer, and autumn, respectively. There were no significant differences in grazing rates among seasons (P > 0.05). Based on the grazing rates we estimated that the microzooplankton can consume about 89%, 106%, and 100% of the potential primary production during spring, summer, and autumn, respectively. Our results suggest that in Guaymas Basin there are two seasons in the year: spring, when a small fraction of the nanophytoplankton is not removed and may be exported to the bottom of the ocean; and summer/autumn, when nanophytoplankton biomass is totally consumed in the pelagic system.