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

Abstract

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 (-60ᵰ mmol C m-2 d-1) in all three regions. The negative NPP corresponded to low O2 influxes (-100ᵰ mmol O2 m-2 d-1) and CO2 effluxes (24ᱰ 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 240ᱰ0 mmol C m-2 d-1 in the wet season. However, accompanying the high positive NPP, we observed an influx of O2 (-100ᶰ mmol O2 m-2 d-1) and efflux of CO2 (25ᱵ 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 (160ᳰ 卩 and high DIC (1960ᱰ0 塴m) 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.