Phytoplankton growth, grazing and production balances in the HNLC equatorial Pacific

We investigate the hypothesis that phytoplankton growth and grazing processes are strongly balanced in high-nutrient low-chlorophyll (HNLC) waters of the equatorial Pacific using euphotic-zone estimates of rates and biomass determined for 30 stations during EB04 (December 2004) and EB05 (September 2005). As predicted by the balance hypothesis, depth-averaged instantaneous rates of phytoplankton growth and grazing losses to micro- and mesozooplankton show a net growth difference of zero. Contemporaneous estimates of phytoplankton biomass and specific rates from flow cytometry, microscopy and taxon-specific accessory pigments allow determination of constrained production-consumption trophic balances for the phytoplankton community as a whole and for major component populations. The magnitude of growth-based production (867 mg C m−2 d−1) is consistent with measured 14C primary production, given methodological differences. 70% of production is utilized by protistan herbivores within the microbial community; 30% is consumed by mesozooplankton. Among picophytoplankton (Prochlorococcus, Synechococcus and small eukaryotes), representing 40% of community biomass and 27% of daily biomass growth, microzooplankton consume almost all production. Among groups of larger eukaryote taxa, including diatoms but dominated by dinoflagellate biomass, micro-grazers consume 51–62% of production, with the remainder available to mesozooplankton. Some leakage from the balance is expected as export of sinking phytoplankton cells and aggregates, but is constrained to no more than a few percent of daily production from alternate determinations of mesozooplankton grazing. The demonstrated balance of growth and grazing processes in the equatorial Pacific is inconsistent with recent claims from inverse models that a large flux associated with ungrazed picophytoplankton production dominates euphotic zone carbon export in the region.