Empirical and mechanistic models for the particle export ratio

TLDR

The study introduces empirical and mechanistic models to predict organic carbon export by sinking particles, including size‑based formulations of phytoplankton grazing and export to explain pe‑ratio variability. The mechanistic model, calibrated with global field data, employs power‑law grazing dynamics for small and large phytoplankton, partitions grazing into temperature‑dependent sinking detritus, and includes a biomass‑modulated switch between pathways. Both the empirical and mechanistic models explain 61 % of the variance in the particle export to primary production ratio, with the mechanistic model attributing higher export efficiency to large phytoplankton in high‑biomass, cold regions.

Abstract

We present new empirical and mechanistic models for predicting the export of organic carbon out of the surface ocean by sinking particles. To calibrate these models, we have compiled a synthesis of field observations related to ecosystem size structure, primary production and particle export from around the globe. The empirical model captures 61% of the observed variance in the ratio of particle export to primary production (the pe ratio) using sea‐surface temperature and chlorophyll concentrations (or primary productivity) as predictor variables. To describe the mechanisms responsible for pe ‐ratio variability, we present size‐based formulations of phytoplankton grazing and sinking particle export, combining them into an alternative, mechanistic model. The formulation of grazing dynamics, using simple power laws as closure terms for small and large phytoplankton, reproduces 74% of the observed variability in phytoplankton community composition wherein large phytoplankton augment small ones as production increases. The formulation for sinking particle export partitions a temperature‐dependent fraction of small and large phytoplankton grazing into sinking detritus. The mechanistic model also captures 61% of the observed variance in pe ratio, with large phytoplankton in high biomass and relatively cold regions leading to more efficient export. In this model, variability in primary productivity results in a biomass‐modulated switch between small and large phytoplankton pathways.