Phytoplankton Community Composition, Growth and Mortality Rates in the North Pacific During May 2002

The IOC North Pacific expedition of May 2002 from Osaka, Japan to Honolulu, Hawaii, surveyed the region from roughly 22 - 50°N 147°E - 158°W. This survey included nine stations, at which we collected samples for analysis with microscopy and flow cytometry to characterize the abundance and biomass of phytoplankton and microheterotroph populations. We also conducted deckboard-incubated dilution experiments to determine the growth and mortality rates of the phytoplankton community at eight of the nine stations occupied. Preliminary results suggest that the stations studied can be distinguished based on phytoplankton community composition and growth response to macronutrient additions during incubations. Station 1, located in the Kuroshio Current (34°22.76N 147°11.84E), was characterized by a centric diatom bloom (~20°C, surface chlorophyll 0.7 \mug l^-1) and phytoplankton growth rates were significantly enhanced after the addition of ammonium and phosphate. Phytoplankton communities at four oligotrophic stations (<0.5 μM nitrate, <0.1 μM phosphate) also exhibited enhanced growth rates after macronutrient additions (Station 6: 30°N 170°E; Station 7: 24°N 170°E; Station 8: 26°N 175°W; and Station 9: 22°45N 158°W). Surface chlorophyll at these stations ranged from 0.05-0.10 μg chlorophyll l^-1 with surface temperatures ranging from 22-27°C. The phytoplankton communities at these oligotrophic stations were dominated by picoplankton (Prochlorococcus and Synechococcus) and small (<5 μm) autotrophs. Finally, no nutrient limitation was observed at three of the stations (Station 2: 44°N 154.6°E; Station 3: 50°N 167°E, and Station 4: 39°N 170°E). Surface chlorophyll at these stations ranged from 0.3 to 0.7 μg l^-1 and surface temperatures ranged from 4-13°C. The phytoplankton at these stations were dominated by autotrophs measuring 2-10 μm, including haptophytes and pennate diatoms. In addition, 10-20 μm autotrophic dinoflagellates were common at Station 2. Phytoplankton growth and mortality rates at each of these stations will be discussed.