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

Abstract

Mechanisms in controlling the growth of heterotrophic bacteria have seldom been explored in the tropical South China Sea (SCS). This study reports the tempospatial distribution patterns and the controlling mechanisms of bacterial biomass (BB), production (BP), and specific growth rate (B mu) from one summer (Jun 2010; 4 transects) and two winter (January and December 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 (B mu), with primary production (PP), chlorophyll-alpha, 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, B mu, changed proportionally with temperature up to ca. 22 degrees C. The Q(10) (the increase of reaction rate for a temperature rise of 10 degrees C) for B mu, was similar to 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 one 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.