Zonal and meridional patterns of phytoplankton biomass and carbon fixation in the Equatorial Pacific Ocean, between 110°W and 140°W

Primary production (Pprim) and calcification (Ccalc) were measured in the eastern and central Equatorial Pacific during December 2004 and September 2005, between 110°W and 140°W. The design of the field sampling allowed partitioning of Pprim and total chlorophyll a (B) between large (>3 ?m) and small (0.45-3 ?m) phytoplankton cells. The station locations allowed discrimination of meridional and zonal patterns. The cruises coincided with a warm El Niño Southern Oscillation (ENSO) phase and ENSO-neutral phase, respectively, which proved to be the major factors relating to the patterns of productivity. Production and biomass of large phytoplankton generally covaried with that of small cells; large cells typically accounted for 20-30% of B and 20% of Pprim. Elevated biomass and primary production of all size fractions were highest along the equator as well as at the convergence zone between the North Equatorial Counter Current and the South Equatorial Current. Ccalc by >0.4 ?m cells was 2-3% of Pprim by the same size fraction, for both cruises. Biomass-normalized Pprim values were, on average, slightly higher during the warm-phase ENSO period, inconsistent with a “bottom-up” control mechanism (such as nutrient supply). Another source of variability along the equator was Tropical Instability Waves (TIWs). Zonal variance in integrated phytoplankton biomass (along the equator, between 110° and 140°) was almost the same as the meridional variance across it (between 4° N and 4° S). However, the zonal variance in integrated Pprim was half the variance observed meridionally. The variance in integrated Ccalc along the equator was half that seen meridionally during the warm ENSO phase cruise whereas during the ENSO-neutral period, it was identical. No relation could be observed between the patterns of integrated carbon fixation (Pprim or Ccalc) and integrated nutrients (nitrate, ammonium, silicate or dissolved iron). This suggests that the factors controlling integrated Pprim or Ccalc are more complex than a simple bottom-up supply model and likely also will involve a top-down grazer-control component, as well. The carbon fixation within the Equatorial Pacific is well balanced with diatom and coccolithophore production contributing a relatively steady proportion of the total primary production. This maintains a steady balance between organic and inorganic production, relevant to the ballasting of organic matter and the export flux of carbon from this important upwelling region