Bacterial influence on chromophoric dissolved organic matter in coastal waters of the northern South China Sea

Chromophoric dissolved organic matter (CDOM), bacterial production (BP) and phytoplankton biomass (chl a) were investigated at 2 coastal sites in the northern South China Sea during summer 2011. Our data showed that chl a levels were higher in Hong Kong eastern waters (HKEW) than Sanya Bay (SYB), but CDOM exhibited the opposite pattern. This decoupling between surface CDOM and chl a indicates that phytoplankton biomass is not the determining factor affecting CDOM distribution. The low levels of CDOM in HKEW could be associated with upwelling of CDOM-deficient deep waters. In addition, bacteria produced more CDOM during the beginning of incubation experiments at SYB than at HKEW, although CDOM turnover time by bacteria was similar to 4 to 10 d and showed no significant difference between SYB and HKEW (p > 0.05). Addition of nitrogen shortened the CDOM turnover time by 2 fold at SYB. Upwelling likely resulted in a phosphorus deficiency for bacteria, and phosphorus enrichment increased bacterial abundance (BA) and BP at HKEW. Interestingly, nitrogen enrichment increased both BP and CDOM generation in SYB, but phosphorus enrichment did not change CDOM at either location, suggesting that CDOM distribution is more associated with nitrogen than phosphorus availability.