Planktonic community respiration in the East China Sea: Importance of microbial consumption of organic carbon

Planktonic community respiration (PCR) rates were measured using the oxygen method in autumn 1998 in order to evaluate the respective roles played by microbes (heterotrophic bacteria and ciliates) in organic carbon consumption on the continental shelf of the East China Sea (ECS). For comparative purposes, the ECS shelf was divided into mesotrophic ([NO₃^-]>0.3 μM) and oligotrophic ([NO₃^-]≤0.3 μM) systems. Bacterial biomass (23.4±28.4 mg C m^-3) and production (4.9±6.8 mg C m^-3 d^-1) as well as particulate organic carbon concentrations (129.3±40.4 mg C m^-3) were significantly higher in the mesotrophic system, while protozoa (95.6±74.9 mg C m^-3) were more abundant in the oligotrophic system. PCR rates ranged from 127.6 to 4728.6 mg C m^-2 d^-1, and the rates were either linearly related to protozoan biomass or multiply regressed with both bacterial and protozoan biomass. Further analysis showed that PCR were dominated by distinct microbial components in different trophic systems, with bacteria and protozoa contributing 72% and 85% of PCR in meso- and oligotrophic systems, respectively. The low primary production to PCR ratio (0.33±0.30) suggests that the ECS was net heterotrophic during the study period. Allochthonous supplies of organic carbon, in addition to in situ production, are required to support these high respiration rates. Riverine inputs and/or resuspension from superficial sediments are potential sources of this allochthonous organic carbon.