Influence of seasonal monsoons on net primary production and CO2 in subtropical Hong Kong coastal waters.

Data from seven cruises in three different environments including the Pearl River estuary, sewage discharge outfall and eastern coastal/shelf waters, were used to examine the seasonal variations in net primary production (NPP) and the biologically active gases O₂ and CO₂. In the winter dry season, when monsoon-induced downwelling was dominant, NPP was low (-60±50 mmol C m^-2 d^-1) in all three regions. The negative NPP corresponded to low O₂ influxes (-100±50 mmol O₂ m^-2 d^-1) and CO₂ effluxes (24±10 mmol C m^-2 d^-1). In the summer wet season, when upwelling brought the bottom oceanic waters to the nearshore due to the southwest monsoonal wind, there was a 2 to 15-fold increase in integrated primary production (IPP) compared to winter. The increase in IPP was likely due to the favorable conditions such as stratification and the nutrient inputs from upwelled waters and the Pearl River estuary. NPP reached up to 240±100 mmol C m^-2 d^-1 in the wet season. However, accompanying the high positive NPP, we observed an influx of O₂ (-100±60 mmol O₂ m^-2 d^-1) and efflux of CO₂ (25±15 mmol C m^-2 d^-1). The high positive NPP corresponding to a CO₂ source and O₂ sink could be explained by the influence of the southwest monsoon-induced upwelling, as the upwelling brought cold, low DO (160±30 μM) and high DIC (1960±100 μatm) water to the surface in the wet season. Hence, the subtropical Hong Kong coastal waters are generally a CO₂ source due to the monsoonal influence in both the dry and wet seasons. [ABSTRACT FROM AUTHOR]