Growth rates, grazing, sinking, and iron limitation of equatorial Pacific phytoplankton

Abstract

Concentrations of phytoplankton and NO3 are consistently low (0.2–0.4 µg Chl a liter‒1) and high (4–12 µM) in surface waters of the oceanic eastern and central equatorial Pacific, and phytoplankton populations are dominated by small solitary phytoplankton (<10 µm). Growth rates of natural phytoplankton populations, needed to assess the relative importance of many of the processes considered important in maintaining the high-nutrient, low-phytoplankton biomass condition observed in the equatorial Pacific, were estimated by several methods. The growth rates of natural phytoplankton populations were found to be ∼0.7 d‒1 or 1 biomass doubling d‒1 and were similar for all methods. To keep this system in its observed balance requires that loss rates approximate observed growth rates. Grazing rates, measured with a dilution grazing experiment, were high (∼0.5 d‒1), accounting for a large fraction of the daily production. Additions of various forms of Fe to 5–7-d incubations utilizing ultraclean techniques resulted in significant shifts in autotrophic (from picoplankton to diatom dominated) and heterotrophic assemblages between initial samples, controls, and Fe enrichments, which were presumably due to Fe, grazing by both protistan and metazoan components, and incubation artifacts. Estimated growth rates of small pennate diatoms (2 × 10 µm) showed increases in Fe enrichments (mean ± SD = 1.60±0.04 d‒1) with respect to controls (mean ± SD = 1.21±0.33 d‒1). The growth rates of the pennate diatoms were similar to those estimated for the larger (>5 µm) size fraction of the natural populations (mean = 1.42 d‒1). We hypothesize that biomass regulation of the larger size fraction of phytoplankton is by a combination of grazing, Fe limitation, and sinking and that understanding the controls over the larger phytoplankton is fundamental to unraveling the high-nutrient, low-biomass paradigm.