Carbon Transport
Carbon transport/Transport flux of carbon
• Lateral transport flux of carbon
Vertical export flux of organic carbon

1. Lateral transport flux of carbon
Previous studies have reported that about 50% of carbon in marginal seas is transported to deep seas by the continental shelf pump. Accurate estimates of lateral carbon transport flux from marginal seas to the deep ocean are of great significance for studies on marginal sea carbon fixation capacity and ecological environment changes. There are currently two main methods for estimating lateral transport flux. The most commonly used box model treats the study area as a black box, uses salt balance and other conditions to estimate the proportion of water in and out of the box, and then calculates the exchange of substances at each interface according to the material concentration of each water mass. This method generally estimates the average material concentration of each water mass over a period of time (usually the annual scale) and then multiplies the corresponding water flux to estimate the mass exchange flux. However, material concentration and water flux vary significantly, leading to greater uncertainty in estimated results. The second method is numerical model simulation. However, given the complexity of marine ecological models and the difficulty in the quantification of biological processes, it can be challenging to accurately simulate DOC concentration and transport flux of complex marginal seas using a numerical model.
According to Liu et al. (2014), upper-ocean organic carbon storage, that is, the 3D distributions of POC and DOC concentrations, can be estimated based on satellite remote sensing data. Combined with 3D flow data provided by hydrodynamic numerical simulation, the lateral flux of organic carbon at any interface of a marginal sea can be estimated. Cui et al. (2018) estimated the lateral transport flux of DOC in the East China Sea using satellite-derived 3D DOC and numerical simulation flow field provided by ROMS.

Representative articles
Cui, Q., X. He, Q. Liu, Y. Bai, C. T. A. Chen, X. Chen and D. Pan. Estimation of lateral DOC transport in marginal sea based on remote sensing and numerical simulation. Journal of Geophysical Research: Oceans, 2018, doi: 10.1029/2018JC014079.

2. The vertical export flux of organic carbon
Oceans contain the greatest actively cycled carbon in the world (Falkowski et al., 2000), and play an important role in regulating atmospheric CO2 as they absorb about 26% of anthropogenic CO2 emissions (Quéré et al., 2012). The biological carbon pump is essential for carbon sequestration in the ocean (Boyd & Trull, 2007; Ducklow et al., 2001; Neuer et al., 2014; Siegel et al., 2014), converting inorganic carbon into organic carbon and exporting POC to the deep ocean, where it can be sequestered on time scales ranging from seasons to centuries (DeVries et al., 2012).
For the euphotic layer, biogenic POC is mainly generated through the photosynthetic processes of phytoplankton. Furthermore, the ratio of biogenic POC export flux to ocean net primary production (NPP) in the euphotic zone (namely, POC export efficiency) is often used to represent carbon sequestration efficiency of the biological carbon pump. Generally, POC should reach a certain density (e.g., through zooplankton grazing and formation of aggregates) before being exported to the deep ocean, which may introduce a time lag between NPP and subsequent POC export (Henson et al., 2011). However, current field observation data are insufficient to reveal their relationships and regulation mechanisms. Therefore, several satellite-based empirical algorithms for POC export efficiency of the euphotic layer have been established. Li et al. (2018a) investigated the relationship between POC sinking export efficiency and NPP in the euphotic layer of the northern South China Sea in the context of satellite-derived 16-day-composited NPP. Similar to that generally observed in the global oceans, POC export efficiency in shelf areas appears to be strengthened with the increase in NPP. In basin areas, however, the opposite relationship is observed, that is, POC export efficiency significantly decreases with the increase in NPP. Seasonal decoupling of NPP from POC export, phytoplankton size structure, grazing by zooplankton, and DOC export may account for the observed negative relationship between POC export efficiency and NPP in the euphotic layer of basin areas.
While these empirical algorithms can predict inter-regional variability in carbon export, they often have limitations for assessing temporal variation in carbon export (Stukel et al., 2015) and for application in other regions (Maiti et al., 2013, 2016). Recently, Siegel et al. (2014) established a food-web model combined with satellite data to estimate POC export flux, which consisted of direct sinking flux of large phytoplankton and associated aggregates by gravity and flux of zooplankton feces by grazing. Based on this model, Li et al. (2018b) estimated POC export flux in the northern South China Sea. As the food web model involves important input parameters, such as primary productivity, phytoplankton particle size and structure, and phytoplankton carbon content, as well as material transport efficiency parameters, remote sensing of POC export flux needs further improvement. However, compared with in-situ sampling, remote sensing estimates have great potential for improving spatiotemporal resolution and reducing regional estimation uncertainty.

Representative articles
1. Li, T., Bai, Y., He, X., Chen, X., Chen, C.-T., Tao, B., et al. (2018). The Relationship between POC Export Efficiency and Primary Production: Opposite on the Shelf and Basin of the Northern South China Sea. Sustainability, 10(10), 3634.
2. Li, T., Bai, Y., He, X., Xie, Y., Chen, X., Gong, F., & Pan, D. (2018). Satellite‐based estimation of particulate organic carbon export in the northern South China Sea. Journal of Geophysical Research: Oceans.