Distinct bacterial-production–DOC–primary-production relationships and implications for biogenic C cycling in the South China Sea shelf

Abstract

Abstract. Based on two summer spatio-temporal data sets obtained from the northern South China Sea shelf and basin, this study reveals contrasting relationships among bacterial production (BP), dissolved organic (DOC) and primary production (PP) in the transition zone from the neritic to the oceanic regions. Inside the mid-shelf (bottom depth < 100 m), where inorganic nutrient supplies from river discharge and internal waves were potentially abundant, BP, DOC and PP were positively intercorrelated, whereas these three measurements became uncorrelated in the oligotrophic outer shelf and slope. We suggest that the availability of limiting minerals could affect the couplings/decouplings between the source (i.e. phytoplankton) and sink (i.e. bacteria) of organic carbon, and thus DOC dynamics. DOC turnover times were homogeneously low (37-60 days) inside the midshelf area and then increased significantly to values > 100 days in the outer shelf, indicating that riverine (Pearl River) DOC might be more labile. The actual mechanism for this is unknown, but might relate to higher inorganic nutrient supply from river/terrestrial sources. The positive correlation of the BP / PP ratios vs. phosphate (and nitrate) concentrations in the inner shelf implies that if anthropogenic mineral loading keeps increasing in the foreseeable future, the near-shore zone may become more heterotrophic, rendering the system a stronger source of CO 2 .

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…The latter is based on bacterial uptake of dissolved organic carbon (DOC), which may be provided by primary production or by external sources, such as river discharge. Lai et al (2014) conducted two summer cruises in the northern SCS and found evidence to support the malfunctioning microbial-loop hypothesis proposed by Thingstad et al (1997) that DOC accumulation occurs when bacterial production was low. In contrast to the short DOC turnover time of 37-60 days observed in the inner shelf near the mouth of the Pearl River, it was more than 100 days in the outer shelf and basin region, where the inorganic nutrients were depleted and no correlations were found among bacterial production (BP), DOC and primary production (PP) in the shelf-to-basin transition zone (Fig.…”

“…The nutrient supply from marine origin is often actuated by intrusion onto the shelf from boundary currents, such as the Kuroshio, which carries a large nutrient load with a subsurface core . In the inner shelf where nutrients are replete, primary production (PP) and bacterial production (BP) are tightly coupled driving a rapid turnover of dissolved organic carbon (DOC), whereas, in the oligotrophic outer shelf, the two are decoupled and DOC tends to accumulate due to low turnover rate (Lai et al, 2014). in the Changjiang plume in summer appeared to have shown a decrease, which is attributed to the enhanced CO 2 uptake by phytoplankton due to the increasing load of riverine nutrients. Subsequent to the phytoplankton bloom, water column respiration led to the elevated pCO 2 in the surface water in autumn.…”

Nutrient pulses driven by internal solitary waves enhance heterotrophic bacterial growth in the South China Sea

“…While even assuming that the seawater exchange occurred, the chemicals that originated from the Atoll were poor in quality for bacterial growth at the four near-shore stations. Moreover, the total volume of seawater inside the Atoll is markedly smaller than that of surrounding oligotrophic SCS seawaters (Lai et al, 2014). Any substance carried in the outflows from the Atoll could be quickly diluted.…”

“…However, according to investigations carried out in the 1980s (Currie and Kalff, ), in oligotrophic environments, bacteria could function as nutrient assimilators and compete with phytoplankton for limiting inorganic nutrients (Azam, ; Kirchman, ; Thingstad et al ., and citations therein). Therefore, the availability of inorganic nutrients might eventually affect bacterial activity and, ultimately the dynamics (accumulation vs. depletion) of dissolved organic carbon (DOC; Thingstad et al ., ; Lai et al ., and citations therein), which has an inventory equal to that of CO 2 in the atmosphere (Benner et al ., ; Hedges, ).…”

Long Term Trends and Dynamics of Dissolved Organic Carbon (DOC) in a Subtropical Reservoir Basin

“…Cheng and Chi [38] and Aparicio et al [39] also stated that eutrophication process will cause an increase in DOC concentration. Lai et al [40] found that bacterial production, primary production and DOC were positively inter-correlated when the inorganic nutrient supplies were potentially abundant. Therefore, correlation analysis was carried out to determine the interaction between DOC and Trophic State Index (TSI) trends in the water surface layer of Fei-Tsui Reservoir.…”