Satellite-Based Estimation of Particulate Organic Carbon Export in the Northern South China Sea

Knowledge of particulate organic carbon export flux (EP) is the key to understanding the marine biological carbon pump. In this study, a satellite remote sensing algorithm based on the food-web model established by Siegel et al. (2014; ) was used to estimate EP in the northern South China Sea (NSCS), which consists of direct sinking flux of large phytoplankton and associated aggregates by gravity and flux of zooplankton feces by grazing. This is the first time that fine spatiotemporal variation of satellite-derived EP in the NSCS is unveiled. Compared with the results on 1 degrees spatial scale products, the model results based on the satellite products with 1/3 degrees and 1/12 degrees resolutions showed better consistency with the observations. Validation with in situ EP showed that the model exhibited a good performance in the NSCS basin, but the predicted EP in the shelf regions were smaller compared with measurements. The satellite-derived annual-mean EP in the shelf areas was 8.47 and 5.56mmol C m (-2) d (-1) in the NSCS basin, on the 1/3 degrees spatial scale, with relative differences from observations were about -50% and -15% for the shelf and basin, respectively. The flux of feces from zooplankton grazing, especially that from microzooplankton grazing, might account for the major fraction of the EP in the NSCS. Accurate products of ocean net primary production and empirical coefficients in the food web (e.g., export efficiency of zooplankton grazing and specific mortality rate of nongrazing phytoplankton) suitable for the NSCS, are essential for performance improvement of the satellite-based EP model. Plain Language Summary The ocean has the largest inventory of active cycling carbon in the Earth's system. At present, the global ocean absorbs about 2 billion tons of CO2 from the atmosphere every year, accounting for about one fourth of the world's annual CO2 emissions. By converting inorganic carbon into organic carbon and exporting particulate organic carbon to the deep ocean, where it can be sequestered on time scales ranging from seasons to centuries, the biological carbon pump is essential for carbon sequestration in the ocean. However, the regulatory mechanism of the biological carbon pump is complex, and remains poorly understood, especially in the marginal seas. In this study, a food-web model established by Siegel et al. (2014; ) was used to estimate the particulate organic carbon export flux in the northern South China Sea. This is the first time the fine spatiotemporal variation of satellite-derived POC export flux in the northern South China Sea is unveiled. In addition, based on the model results, we found that the flux of feces from microzooplankton grazing plays an important role in regulating the particulate organic carbon export in the northern South China Sea.