SEASONAL VARIABILITY IN THE PHYTOPLANKTON COMMUNITY OF THE NORTH PACIFIC SUBTROPICAL GYRE

Time series measurements of in situ fluorescence, extracted particulat e chlorophyll a, primary productivity, extracted adenosine 5'-triphosp hate, and fluorescence per cell, as measured by flow cytometry, demons trate seasonal cycles in fluorescence and chlorophyll concentrations i n the North Pacific Subtropical Gyre (22 degrees 45'N, 158 degrees 00' W). Two opposing cycles are evident. In the upper euphotic zone (0-50 m), chlorophyll a concentrations increase in winter, with a maximum in December, and decrease each summer, with a minimum in June or July. I n contrast, chlorophyll a concentrations in the lower euphotic zone (1 00-175 m) increase in spring, with a maximum in May, and decline in fa ll, with a minimum in October or November. The winter increase in chlo rophyll a concentration in the upper 50 m of the water column appears to be a consequence of photoadaptation in response to decreased averag e mixed-layer light intensity rather than a change in phytoplankton bi omass. In the lower euphotic zone, however, the seasonal cycle in pigm ent concentration does reflect a change in the rate of primary product ion and in phytoplankton biomass as a consequence of increased light i ntensity in summer. These observations have important implications for phytoplankton dynamics in the subtropical oceans and for remote sensi ng of phytoplankton biomass.