Estimating Global Marine Size‐Fractionated Primary Productivity Over Two Decades Using a Depth‐Independent Model

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Abstract Marine phytoplankton net primary productivity (PP) is vital in the global carbon cycle and varies significantly across different phytoplankton size classes. In this study, we investigated variations in the maximum carbon fixation rate within the water column ( P B opt , mg C (mg Chl) −1 hr −1 ) among micro‐ (>20 μm), nano‐ (2–20 μm), and pico‐phytoplankton (<2 μm). Our results demonstrated that P B opt values were markedly lower in samples dominated by micro‐phytoplankton, while higher values were associated with increased fractions of nano‐ and pico‐phytoplankton. Moreover, distinct patterns in P B opt with sea surface temperature (SST) were observed across three size classes. Building on these observations, we parameterized P B opt for each size class and developed a novel size‐fractionated PP estimation model. Compared to the original Vertically Generalized Production Model (VGPM) proposed by Behrenfeld and Falkowski (1997), https://doi.org/10.4319/lo.1997.42.1.0001 , our model achieved improved accuracy, exhibiting good agreement with in situ measurements, a reduced RMSE of 733.72 mg C m −2 d −1 , and a lower bias of 112.78 mg C m −2 d −1 . Application of this model to satellite data over the past two decades revealed that micro‐ and nano‐phytoplankton primarily dominated PP in coastal and high‐latitude regions, whereas pico‐phytoplankton prevailed in oligotrophic open oceans. Notably, despite general dominance of pico‐phytoplankton in total PP in most open oceans, episodic structural shifts in the fractional contribution of different phytoplankton size classes were observed, with increasing contributions of micro‐ and nano‐phytoplankton. Our study proposes a simplified and innovative approach for estimating size‐fractionated PP and enhances the understanding of global marine primary production dynamics linked to phytoplankton size structure.

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