Phytoplankton assemblage structure and primary productivity along 170°W in the South Pacific Ocean

Phytoplankton pigments were measured using HPLC during non-ENSO conditions in mid-summer along a South Pacific transect from 67degreesS to the equator along 170degreesW. Highest concentrations of chlorophyll a (chl a) occurred in the Polar and the Subtropical Fronts (PF and STF, respectively) with concentrations exceeding 500 ng l(-1). In the STF, there was a distinct subsurface chl a maximum (SCM) at 40 m, which gradually deepened northward to 120 m in the Subtropical Convergence Zone. Northwards, the SCM shoaled to about 30 m in the Equatorial Zone (EZ). Relatively high concentrations of fucoxanthin and 19'-butanoyloxyfucoxanthin occurred in the nutrient-rich waters south of the Subantarctic Front, and CHEMTAX analyses indicated that diatoms, chrysophytes, pelagophytes, and haptophytes dominated the phytoplankton assemblage. Northward of the PF to the STF, where silicate concentrations were <1 muM, pelagophytes and coccolithophorids dominated the water column; diatoms were virtually absent, and Phaeocystis, prasinophytes, cryptophytes, and chlorophytes contributed significantly to the total algal biomass. Phaeocystis populations were dominant at or below the 1% light level. In the South Pacific Gyre (SPG), Synechococcus (Syn) and Prochlorococcus (Pro) were major components of the phytoplankton assemblage with Pro dominant as indicated by both flow cytometry and by the ratio of divinyl chl a:total chl a (0.43 +/- 0.07). Photoacclimation by Pro in the SPG was pronounced, with a higher average divinyl chl a per cell ratio in the SCM (1.1) relative to values (0.1) in the upper waters (0 to 100 m). Primary production rates exceeding 1 mug C l(-1) h(-1) occurred at the surface in the STF. Surface primary production rates were generally <0.4 mug C l(-1) h(-1) across the SPG, but exceeded 1.4 mug C l(-1) h(-1) at the equator. In the EZ, Pro dominated the phytoplankton assemblage, but Phaeocystis and prasinophytes were also major constituents of the assemblage.