Phytoplankton biomass and size structure across trophic gradients in the southern California Current and adjacent ocean ecosystems

We used a combination of epifluorescence microscopy and flow cytometry to investigate variability of phytoplankton biomass and size structure with ocean richness, measured as autotrophic carbon (AC) or chlorophyll a (chl a). Samples were collected from mixed layers of 4 North Pacific ecosystems: the California Current (CCE), Equatorial Pacific, Costa Rica Dome, and subtropical gyre (station ALOHA). Both nano- (2−20 μm) and microphytoplankton (20–200 μm) in creased biomass as power functions with increasing richness, with a sharper slope leading to microphytoplankton dominance at high biomass. The AC:chl a ratio (μg:μg) was also well fit by a power function, varying from ~170 to 20 over the range of 11.7 μg chl a l−1. Prochlorococcus and diatoms were major biomass contributors at poorer and richer extremes, respectively, but mixed flagellates (prymnesiophytes, dinoflagellates, others) comprised a surprisingly consistent half of AC over the richness range. While pico- (