Assessing the Dynamics and Ecology of Marine Picophytoplankton: The Importance of the Eukaryotic Component

We assess population dynamics of picophytoplankton groups (≤2 μm diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument-specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 ± 12% of the total picoplankton cells over time. It had moderately high growth rates $(0.52-0.86\ \text{d}^{-1})$ and was subject to low grazing mortality (-0.14 to $-0.39\ \text{d}^{-1}$). Prochlorococcus growth and mortality rates were roughly balanced ($0.33\pm 0.14\ \text{d}^{-1}$ and $-0.36\pm 0.06\ \text{d}^{-1}$, respectively). Picoeukaryotes had the highest growth rates $(0.71-1.29\ \text{d}^{-1})$ and were responsible for, on average, 76% of net carbon production (NCP), amounting in up to $32.05\pm 1.31\ \mu \text{g C L}^{-1}\ \text{d}^{-1}$ produced and $28.31\pm 2.61\ \mu \text{g C L}^{-1}\ \text{d}^{-1}$ consumed. In order to better define the eukaryotic component of these populations, an isolate was characterized via small subunit rRNA gene sequencing, transmission electron microscopy, and growth experiments and was identified as the prasinophyte Ostreococcus, not previously known to the Pacific Ocean. Our results show that although picoeukaryotes do not stand out as particularly important players in this system on the basis of cell abundance, they dominate in terms of picophytoplankton biomass and trophic transfer potential of carbon in this size class.