Seasonal and spatial variation of bacterial production in the continental shelf of the East China Sea: Possible controlling mechanisms and potential roles in carbon cycling

Surveys of bacterial biomass (38-673 mg C m^-2), bacterial production (6-179 mg C m^-2 d^-1), primary production (17-2079 mg C m^-2 d^-1) as well as other hydrographic variables within the mixed layer or euphotic zone were conducted in the East China Sea (ECS) shelf in December 1997 and March 1998 (cold seasons; SST, 10-26°C) as well as June and October 1998 (warm seasons; SST, 21-30°C). The results showed that bacterial rate parameters and biomass were regulated independently. Substrate supply and temperature might be the two most important factors interactively affecting the spatial and seasonal patterns of bacterial rate parameters. However, their relative importance shifted with season and location. During warm seasons, when SST was high (>20°C), integrated bacterial productivity (IBP; 9-179 mg C m^-2 d^-1) and turnover rate (Bμ; 0.03-0.37 d^-1) over the entire shelf were dominated by substrate supply, as judged from their positive relationships with integrated primary productivity (IPP; 18-2079 mg C m^-2 d^-1). Multiple regression analysis indicated that during cold seasons, spatial variations of IBP (6-59 mg C m^-2 d^-1) and Bμ (0.06-0.23 d^-1) were explained better by temperature inside, and substrate supply outside, the middle-shelf. Annual average of IBP:IPP ratio was 17±13%, which is close to the global average of 25%. Bacterial carbon demand (BCD) estimated by two independent approaches yielded similar values of 430 and 482 mg C m^-2 d^-1. This implies that bacteria might consume organic carbon equivalent to seasonal averaged IPP (370 mg C m^-2 d^-1; Gong et al., 2003). Substrate sources from non-algal components plays an important role in supporting BCD in the ECS shelf. For all four seasons, IBP:IPP ratios (6-86%) were negatively correlated with IPP, suggesting a greater response of phytoplankton to inorganic nutrient inputs than that of bacteria to organic substrate supply. Possible mechanisms and implications are considered.