Microzooplankton grazing in the central equatorial Pacific during February and August, 1992

Dilution studies were conducted on EqPac cruises in the central equatorial Pacific (2°N to 2°S, 140°W) during February–March and August–September 1992 to determine phytoplankton growth rates and mortality rates attributable to microzooplankton grazing. Instantaneous growth rates (μ) based on bulk chlorophyll measurements were highly variable from day-to-day, but averaged 0.83 day−1 for the upper (10–20 m), 0.34 day−1 for the mid (40–50 m) and 0.22 day−1 (70–80 m) for the lower euphotic zone on the first cruise. Corresponding rate estimates for microzooplankton grazing (m) were 0.72, 0.22 and 0.21 day−1, respectively. During the second cruise, growth estimates strongly exceeded grazing estimates for the two upper (μ = 0.98 and 1.00 day−1; m = 0.57 and 0.42 day−1), but not the lower depth strata (μ = 0.32 day−1; m = 0.27 day−1). Grazing losses accounted for about 83% of depth-integrated phytoplankton growth in February–March experiments and only about 55% in August–September experiments. In addition, growth rates in the presence of added nutrients (including iron) showed evidence of limitation in February–March, which coincided with a major El Niño event, but not in August–September, which was more representative of climatological mean conditions. Differences in growth rates, implied nutrient limitation, and the balance of phytoplankton growth and microzooplankton grazing were consistent with a greater abundance of large diatoms in August–September. Despite the disparity between chlorophyll-based estimates of growth and grazing rates for this cruise, flow-cytometric analyses of specific populations (Prochlorococcus, Synechococcus, and autotrophic nanoeukaryotes) in a subset of experiments conducted in August demonstrated that microzooplankton grazing was still sufficient to balance growth rates of the smaller components of the phytoplankton assemblage.