The Relationship between POC Export Efficiency and Primary Production: Opposite on the Shelf and Basin of the Northern South China Sea

Teng Li,Xuan Zhang,Xianqiang He,Xiaoyan Chen,Bangyi Tao,Chen-Tung Chen,Yan Bai,Delu Pan

doi:10.3390/su10103634

Abstract

Accurate estimation of particulate organic carbon (POC) export efficiency in the euphotic layer is essential to understand the efficiency of the ocean’s biological carbon pump, but field measurements are difficult to conduct and data are sparse. In this study, we investigated the relationship between POC sinking export efficiency and ocean net primary production (NPP) in the euphotic layer of the northern South China Sea (NSCS), with the help of high spatiotemporal coverage satellite-derived NPP. Annual mean POC export efficiency in euphotic zone is 34% for the shelf areas and 24% for the basin of the NSCS in the context of satellite-derived 16-day-composited NPP. Similar to what is generally observed in the global ocean, the POC export efficiency on the shelf areas appears to be strengthened with the increase of NPP. However, in the basin areas, the opposite relationship is observed. That is, the POC export efficiency significantly decreases with the increase of NPP. Seasonal decoupling between NPP and POC export, phytoplankton size structure, grazing by zooplankton, and dissolved organic carbon export might account for the observed negative relationship between the POC export efficiency and NPP in the euphotic layer of basin region. System comparison between shelf and basin would be helpful to promote understanding of the regulation mechanism of POC export in the tropical marginal seas.

Highlights

The ocean is the greatest inventory of actively cycling carbon in the world [1]
The carbon sequestration by biological carbon pump plays an important role in the global ocean carbon cycle
We utilized two datasets of seasonal 234Th-derived particulate organic carbon (POC) export flux at the euphotic depth and satellite-derived 16-day-composited net primary production (NPP) prior to the 234Th sampling time to examine the correlation between POC export efficiency and NPP in the euphotic layer of northern South China Sea (NSCS)

Summary

Introduction

A key term in the global carbon cycle is the ocean’s biological carbon pump, since its magnitude and efficiency have great impact on regulating the concentration of atmospheric carbon dioxide (CO2) [2,3,4]. The lateral transport of POC to the interior of adjacent deep ocean through transcontinental shelf processes, e.g., benthic turbidity current and horizontal particulate plume near the nepheloid layers is an important carbon fixation mechanism [5]. Understanding the mechanisms that regulate the magnitude and efficiency of POC export from the euphotic layer is crucial for assessing the role of the ocean under global climate change [8], especially in the context of sustained changes in the marine ecosystems [9,10,11,12]

Methods

Results

Discussion

Conclusion