Growth-controlling mechanisms on heterotrophic bacteria in the South China Sea shelf: Summer and Winter patterns

  • Author(s): Eleanor Austria, Chao-Chen Lai, Chia-Ying Ko, Kuo-Yuan Lee, Hsiang-Yi Kuo, Tzong-Yueh Chen, Jen-Hua Tai, and Fuh-Kwo Shiah
  • DOI: 10.3319/TAO.2018.01.19.01
  • Keywords: Bacteria, Continental shelf, DOC, Microbial loop, Primary production, South China Sea, Tropical shelf-sea
  • Bacterial growth (Bμ) in tropical shelf-sea was controlled by temperature and substrate supply
  • Bottom-up processes control Bμ in the warm season across the continental shelf
  • Temperature primarily controls Bμ in inner-shelf during cold season

Mechanisms in controlling the growth of heterotrophic bacteria have seldom been explored in the tropical South China Sea (SCS). This study report the tempo-spatial distribution patterns and the controlling mechanisms of bacterial biomass (BB), production (BP) and specific growth rate (Bm) from one summer (Jun, 2010; 4 transects) and two winter (Jan and Dec, 2011; one transect each) cruises along the northern SCS-shelf. In summer, all three bacterial variables showed strong gradients with greater readings at the inner-shelf then decreasing seaward. The positive correlations of bacterial production rate (BP) and bacterial specific growth rate (Bμ), with primary production (PP), chlorophyll-a and dissolved organic carbon observed in summer indicate a high possibility of bottom-up (substrate supply) control. Positive bacterial temperature response was observed in the inner to mid-shelf area in winter. There, Bm changed proportionally with temperature up to ca. 22 oC. The Q10 (the increase of reaction rate for a T rise of 10oC) for Bm was ~4.0, which was in the range reported by coastal studies. Very high BP/PP ratios (summer average: 89±92%; winter average: 131±88%) indicated bacteria carbon demand relied heavily on allochthonous organic carbon sources such as river input and re-suspension processes, and that the SCS-shelf might be net heterotrophic in these two seasons. In winter, BP/PP ratios changed positively with temperature in areas inside the mid-shelf, suggesting that the coastal zone might become a stronger CO2 source during cold season under a warming climate, if anthropogenic loadings of inorganic nutrients and organic matter remain high in the future.

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