Dynamics of picophytoplankton, ultraphytoplankton and bacteria in the central equatorial Pacific

Pico- and ultraplankton are known to contribute significantly to overall biomass and primary productivity in the “high nutrient-low chlorophyll” waters of the equatorial Pacific. In order to understand the dynamics of this community on ecologically relevant time-scales, we examined the abundance, distribution and cellular characteristics of Prochlorococcus, Synechococcus, eukaryotic ultraphytoplankton and heterotrophic bacteria during two 20-day time-series at 0°N, 140°W in the spring and fall of 1992 (JGOFS time-series cruises, TS-I and TS-II). Prochlorococcus was numerically dominant among the autotrophic groups considered, with mean cell concentrations in surface waters on the order of 1.4 × 105 cells ml−1. Synechococcus and ultraphytoplankton abundances were 17–30-fold lower than those of Prochlorococcus, and heterotrophic bacterial abundances were 5–7-fold higher (during TS-I and TS-II, respectively). Daily cell abundances for all groups varied by factors of 1.5-2 within each time-series. Depth-integrated Prochlorococcus abundance averaged over each time-series was 25% lower during TS-II relative to TS-I; ultraphytoplankton abundance was 42% higher during the same period. Prochlorococcus and ultraphytoplankton both contributed significantly to the estimated total autotrophic biomass; Synechococcus contributed relatively little. Estimated total photosynthetic pico- plus ultraplankton biomass was on average 30% higher than heterotrophic bacterial biomass. s in the fluorescence and light scatter properties of individual Prochlorococcus cells were observed during the passage of a tropical instability wave during TS-II, and are hypothesized to reflect a physiological response among these cells to that event. Examination of bulk properties alone (e.g. cell numbers or total red fluorescence) would not have revealed these physiological changes. bounds for Prochlorococcus-specific growth rates were calculated based on the DNA distributions of these populations at dusk. These rates were maximal at 15 or 30 m depth, where they approached one doubling per day. Changes in Prochlorococcus forward angle light scatter (FALS) from dawn to dusk were well correlated with these estimates of specific growth rate, an observation that allowed us to relate measurements of FALS to cell volume for Prochlorococcus.