Micro- and mesoprotozooplankton at 140 degrees W in the equatorial Pacific: Heterotrophs and mixotrophs

Abundance, biomass and presence or absence of plastids in micro- (>20 to 200 Fun) and meso- (>200 mu m to 2 mm size range) protozooplankton were determined during March-April and October 1992 at 140 degrees W on the equator as part of the United States' participation in the Joint Global Ocean Flux Study (JGOFS). The March-April cruise took place during strong El Nino conditions but the October 1992 cruise was during a relaxation in El Nino. Protistan zooplankton biomass in the 20 to 64 mu m size range was dominated by planktonic ciliates, in the >64 to 200 mu m size range by ciliates, heterotrophic dinoflagellates, foraminifera and acantharia, and in the >200 pm size range by acantharia, foraminifera and polycystine radiolaria. Presence of algal plastids, indicating mixotrophic nutrition, was common. In October, plastidic cells contributed -27, 47 and 56% of the protozooplankton biomass in the 20-64, >64-200 and >200 mu m size classes, respectively. During October, the protistan micro- and mesozooplankton biomass was generally higher than in March-April 1992. Most of the increase in biomass was due to ciliates and foraminifera. These data suggest that the numerical responses of ciliates may be important in coupling changes in primary productivity of nanoplankton to their removal by grazing in the equatorial Pacific. The increased biomass of foraminifera in October may be Linked to the relaxation in El Nino. Foraminifera may be particularly important in the coupling between primary production and biogenic flux of organic carbon and carbonate in the equatorial Pacific.