Role of particle stock and phytoplankton community structure in regulating particulate organic carbon export in a large marginal sea

In this study, we utilize Th-234/U-238 disequilibrium to determine particulate organic carbon (POC) export from the euphotic zone in the South China Sea. Depth profiles of Th-234, total chlorophyll, pigments, and POC were collected during four cruises from August 2009 to May 2011, covering an entire seasonal cycle of spring, summer, autumn, and winter. The extensive data set that was acquired allows for an evaluation of the seasonal variability of upper ocean POC export and its controls in a large marginal sea. The results show that Th-234 fluxes from the euphotic zone fall in the range of 528-1550, 340-2694, and 302-2647 dpm m(-2) d(-1) for the coastal, shelf, and basin regimes, respectively. In these regimes, POC/Th-234 ratios at the base of the euphotic zone fall in the range of 5.7-58.2, 4.6-44.0, and 2.5-15.5 mol dpm(-1), respectively. Accordingly, for the coastal, shelf, and basin regimes, the mean POC export fluxes from the euphotic zone are 24.3, 18.3, and 6.3 mmolC m(-2) d(-1), respectively. Seasonal variations in POC export flux are remarkable in the study area, and POC export peaks were generally observed in autumn. We use a simple linear regression (LLS) method to examine the correlation of POC export versus POC stock and versus plankton community structure. We found a strong correlation (R-2=0.73, p<0.005) between POC export flux and the fraction of diatom in the coastal area, indicating that POC export flux in this province is driven by large phytoplankton, in particular, diatoms. In the shelf area, a relatively strong correlation (R-2=0.54, p<0.0001) was noted for POC export flux and POC stock in the euphotic zone. This indicates that POC export flux in the South China Sea shelf is primarily controlled by POC stock. In contrast, in the South China Sea basin, we identified a weak but intriguing correlation (R-2=0.26, p<0.0001) between POC export flux and the fraction of haptophytes and prasinophytes that are typically <5 m in size. This suggests that mechanisms controlling POC export flux in the South China Sea basin are complicated. However, small phytoplankton may play a significant role in controlling POC export flux since they dominate the phytoplankton community structure in this region.