MICROZOOPLANKTON GRAZING OF PRIMARY PRODUCTION AT 140-DEGREES-W IN THE EQUATORIAL PACIFIC

Phytoplankton growth rates and the grazing impact by microzooplankton were estimated from dilution experiments during spring and fall time-s eries cruises in the equatorial Pacific as part of the U.S. JGOFS prog ram. The Time-series I (TS-I) cruise occurred during El Nino condition s, while Time-series II (TS-II) coincided with a relaxation event. Dec k incubation experiments were conducted using samples from the upper m ixed layer (15 m) and depths coinciding with subsurface peaks in chlor ophyll a (30-60 m). Initial chlorophyll a concentrations were similar at 15 m (0.1-0.2 mu g l(-1)) and at 60 m (0.2-0.4 mu g l(-1)) in both cruises (experiments at 30 m were conducted only in TS-II). Phytoplank ton 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 gr owth 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 th an 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 microz ooplankton grazing generally balanced the daily production by prymnesi ophytes, and consumed much of the daily production of picophytoplankto n. However, microzooplankton apparently consumed only about half the p otential production by diatoms, implying that other loss processes (ma crozooplankton grazing, sinking) regulate diatom abundance in these wa ters. Herbivory by microzooplankton, primarily by small microflagellat es and dinoflagellates, averaged 133 (15 m) to 123% (60 m) of phytopla nkton 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 pathw ay of organic carbon transformation at the equator during El Nino and non-El Nino conditions. Copyright (C) 1996 Elsevier Science Ltd.