The concept of constant elemental ratios in plankton communities—the Redfield ratio—is of central importance to ocean biogeochemistry. Recently, several studies have demonstrated regional differences in the plankton C:P and N:P ratio. However, less is known about potential systematic variations in the C:N ratio. Here we present an analysis of the particulate organic carbon to nitrogen ratio of 40,482 globally distributed samples from the upper 200 m of the ocean water column. Particulate organic carbon and nitrogen concentrations are highly correlated (R2 = 0.86) with a median value of 6.5. Using an artificial neural network analysis, we find regional variations in the C:N ratio linked to differences in environmental conditions. The ratio is lower in upper latitude cold water as well as upwelling regions in comparison to the warm oligotrophic gyres. We find substantial differences between ocean gyres that might be associated with differences in the nutrient supply ratio. Using cell sorting, we also quantified the C:N ratio of Prochlorococcus, Synechococcus, and picoeukaryotic field populations. The analysis demonstrates that picophytoplankton lineages exhibit a significantly higher ratio than the bulk particulate material but are only marginally significantly different from each other. Thus, the dominance of picophytoplankton in ocean gyres may contribute to the elevated ratios observed in these regions. Overall, the median C:N ratio derived from 40,482 samples is close to the canonical Redfield ratio, but significant regional deviations from this value are observed. These differences could be important for marine biogeochemistry and the regional coupling between the ocean's carbon and nitrogen cycles.
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