Bacterial distribution and nutrient limitation in relation to different water masses in the coastal and northwestern South China Sea in late summer

Abstract

In order to study heterotrophic bacterial responses to upwelling in the northern South China Sea (SCS) and the influence of the Pearl River estuarine coastal plume, two cruises were conducted to investigate the distribution of bacterial abundance (BA) in September-October 2004 and 2005, along with measurements of inorganic nutrients, particulate and dissolved organic carbon (POC and DOC) in 2004. Surface BA was 10Თ108 cells l-1 near the Pearl River estuary and 6᱗108 cells l-1 in oligotrophic offshore waters of the SCS in both 2004 and 2005. In contrast, BA was 15᳗108 cells l-1 in western coastal waters during the upwelling period in 2004, and decreased to 10Თ108 cells l-1 in 2005 when upwelling was absent, indicating that upwelling exerted a significant influence on BA (p<0.05). Nutrient addition experiments were conducted and showed that phosphorus availability limited bacterial growth in coastal upwelled waters and near the Pearl River estuary, while bacteria in offshore waters were mainly C limited. The upwelled waters brought up considerable amounts of nutrients to the surface (e.g. DOC ~70 卬 DIN ~4 占and PO4 ~0.1 卩. However, P addition increased BA and bacterial production (BP) by 20ᵥ and 30ᵥ, respectively, in the upwelled water, which was higher than those near the Pearl River estuary (2ᱥ and 20᳥, respectively) (p<0.05). In the upwelled waters, phosphorus was low relative to nitrogen, which resulted in a high N:P ratio of 40:1 at the surface and hence potential P deficiency in bacteria. Consequently, there was a higher increase in BP in response to a PO4 addition.