Phytoplankton growth and microzooplankton grazing in high‐nutrient, low‐chlorophyll waters of the equatorial Pacific: Community and taxon‐specific rate assessments from pigment and flow cytometric analyses

Phytoplankton growth and microzooplankton grazing rates were investigated using the seawater dilution technique during a French Joint Global Ocean Flux Study cruise focusing on grazing processes in the high‐nutrient, low‐chlorophyll equatorial Pacific at 180° (Etude du Broutage en Zone Equatoriale, October–November, 1996). Raw rate estimates based on spectrofluorometric and high‐performance liquid chromatography pigment analyses were typically in close agreement, but most showed substantial imbalances in growth and grazing. Flow cytometric (FCM) analyses were used both as an alternate approach for distinguishing populations and as a means for adjusting pigment‐based growth estimates for changes in cellular chlorophyll content and biovolume. Total chlorophyll a (Tchl a ) gave mean community growth rates of 0.76 d −1 at 30 m and 0.27 d −1 at 60 m. Grazing rates averaged 0.56 and 0.15 d −1 at the two depths, respectively, and 69% of phytoplankton growth overall. For the prokaryotic picophytoplankter, Prochlorococcus (PRO), rate estimates from dv‐chl a and FCM cell counts generally indicated balanced growth and grazing and therefore close grazing control by microzooplankton. At the equator, rate estimates from dv‐chl a averaged 0.6–0.7 d −1 at 30 m and 0.25–0.26 at 60 m and were consistent with inferences based on diel pigment variations in the 30–70 m depth range. Phytoplankton production estimates from experimentally determined rates and microscopical assessments of autotrophic carbon at 30 m (mean = 19 mg C m −3 d −1 ) agreed well with contemporaneous measurements by 14 C uptake. Diatom growth rate estimates (1.0–1.6 d −1 ), constrained by contemporaneous measurements of silicate uptake, implied a relatively small biomass (10–45 nmol C L −1 ) with high rates of turnover and recycling.