Microzooplankton grazing of primary production at 140°W in the equatorial Pacific

Phytoplankton growth rates and the grazing impact by microzooplankton were estimated from dilution experiments during spring and fall time-series cruises in the equatorial Pacific as part of the U.S. JGOFS program. The Time-series I (TS-I) cruise occurred during El Niño conditions, while Time-series II (TS-II) coincided with a relaxation event. Deck incubation experiments were conducted using samples from the upper mixed layer (15 m) and depths coinciding with subsurface peaks in chlorophyll a (30–60m). Initial chlorophyll a concentrations were similar at 15m (0.1-0.2 μg 1−1) and at 60 m (0.2-0.4 μg 1−1) in both cruises (experiments at 30 m were conducted only in TS-II). Phytoplankton growth rates were highest at 15 m and decreased with depth. Growth rates in the mixed layer were lower in TS-I (0.4-0.6 day−1) than TS-II (0.8-1.1 day−1). The same trend was observed in phytoplankton growth in the subsurface chlorophyll a maxima (0.2 vs 0.6-0.7 day−1). Grazing rates, which also declined with depth, were higher in TS-II than in TS-I at 60 m (0.6-0.7 vs 0.2-0.4 day−1), but lower at 15 m (0.5-0.8 vs 0.7-1.0 day−1). HPLC pigment analyses indicated that microzooplankton grazing generally balanced the daily production by prymnesiophytes, and consumed much of the daily production of picophytoplankton. However, microzooplankton apparently consumed only about half the potential production by diatoms, implying that other loss processes (macrozooplankton grazing, sinking) regulate diatom abundance in these waters. Herbivory by microzooplankton, primarily by small microflagellates and dinoflagellates, averaged 133 (15 m) to 123% (60 m) of phytoplankton growth in TS-I, and 70 (15–30 m) to 105% (60 m) in TS-II. Thus, grazing of phytoplankton by microzooplankton represented a major pathway of organic carbon transformation at the equator during El Niño and non-El Niño conditions.