This study aims to understand the influence of hydrochemical and nutrient dynamics on the metabolism of organic carbon, and to explore the relationship between the metabolism of organic carbon and air-sea fluxes of CO2 in the Kaoping coastal zone and the northern South China Sea (NSCS). Distributions of nutrients in the Kaoping Canyon increased generally with the increase of freshwater input from the Kaoping River that discharged the highest rate during the summer season. In the northern SCS, the enhanced nutrient distributions were caused by freshwater input or upwelling in coastal and shelf zones, and by vertical mixing in the central basin in winter. During the study periods, the integrated gross production (IGP) ranged from 1389 to 8918 mgC m-2d-1 in the Kaoping Canyon, and from 851 to 5032 mgC m-2d-1 in the NSCS. The integrated dark community respiration (IDCR) ranged from 919 to 5848 mgC m-2d-1 in the Kaoping Canyon, and from 435 to 10707 mgC m-2d-1 in the NSCS. The higher IGP was found in summer than in winter for both study areas, primarily due to greater inputs of freshwater from the Kaoping River and/or from the Pearl River. The deeper euphotic depth may be also responsible for higher IGP in the central basin during the summer season. Positive correlations are significant between GP (DCR) and temperature, PAR and nutrients, and negative correlations are also significant between GP (DCR) and salinity, showing the significant impacts of freshwater inputs and climatic changes on GP (DCR). However, GP was determined largely by DCR, and DCR was attributed mainly to BR (bacteria respiration) for both the Kaoping Canyon (ave., 78%) and the NSCS (ave., 65%). In addition, the ratio of IBR/IDCR ranged from 48 to 88% for the Kaoping Canyon and from 58 to 88% for the NSCS. The ratio of IGP/IDCR is an indicator of net ecosystem production, with >1 for the autotrophic system and <1 for the heterotrophic system. The ratio was greater than 1.0 for most stations during summer but was <1.0 away from the nearshore station during winter in the Kaoping Canyon. The ratio was <1.0 for all but stations near the Pearl estuary (H and H1 stations) during both summer and winter in the NSCS, indicating a year-round heterotrophic around the slope and basin of NSCS. However, this ratio was higher in winter than in summer in the NSCS, possibly resulted from higher GP in winter than in summer. The IGP/IDCR may not be the sole factor in determining the air-sea fluxes of CO2. The physical forcing such as temperature and wind velocity may be also important in determining the source or sink of CO2 in the study areas.

Biogeochemical processes of reefs have been studied for over fifty years, however, information is still lacking on several fundamental reef processes. This lack of information has been limited essentially by techniques that cannot repeatedly sample large spatial areas. These limitations can be reduced with the use of an optical model to estimate biogeochemical processes. This project applied Monteith's light-use efficiency model to coral reef communities for determining photosynthetic and calcification efficiency of light. Gross primary production and net calcification were pooled from the peer-reviewed literature to calculate efficiency. Process efficiency was then compared across functional types of reef communities (i.e., coral, algae/seagrasses, mixed, and sand), and by year, location, season, and depth. Photosynthetic efficiency was calculated from 19 studies, showing an average of 0.039 mol O2 mol-1 photons. Photosynthetic efficiency differed significantly for mixed communities between studies, and for algae/seagrass communities among depths. Calcification efficiency averaged at 0.007 mol CaCO3 mol-1 photons. Significant differences were found in calcification efficiency of algae/seagrasses and mixed reef communities among studies and localities. Additionally, calcification efficiency of algae/seagrasses varied significantly in accordance with depth. Future use of the light-use efficiency model will require determining the efficiency of each functional type to estimate gross production and calcification. Additionally, further investigation of the light-use efficiency model will require long-term measurements of APAR, which is the fraction of incident light absorbed, and the incorporation of environmental parameters that reduce efficiency.