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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 <0.06 to >11.7 μg chl a l−1. Prochlorococcus and dia toms 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(<2 μm) and nanophytoplankton codominated at low chl a, all picophytoplankton (Pro chlorococcus, Synechococcus, picoeukaryotes) declined significantly in richer coastal waters. Their decrease was consistent with a previously proposed mechanism linking Pro chlo r o coccus decline to increased productivity and grazing pressure on heterotrophic bacteria, termed here the enhanced microbial loop hypothesis. Generalized additive models further indicated that biotic variables explained more of picophytoplankton variability than abiotic variables in CCE coastal waters. Density-independent grazing may be a strong driver of picophytoplankton selection across trophic gradients, with implications for strategy trade-offs in growth rate and grazing resistance, and for representing mortality in marine ecosystem models. OPEN ACCESS

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