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 community production (NCP) and the biologically active gases O₂ and CO₂. In the winter dry season, when monsoon-induced downwelling was dominant, NCP was negative (−84 ± 50 mmol C m⁻² d⁻¹) in all three regions. The negative NCP corresponded to O₂ influxes of 100 ± 50 mmol O₂ m⁻² d⁻¹ and CO₂ effluxes of 24 ± 10 mmol C m⁻² d⁻¹. In the summer wet season, when upwelling brought the deep oceanic waters to the coast due to the southwest monsoonal winds, 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. NCP in the mixed layer reached up to 110 ± 48 mmol C m⁻² d⁻¹ in the wet season. However, accompanying the high positive NCP, we observed an O₂ influx of 100 ± 60 mmol O₂ m⁻² d⁻¹ and CO₂ efflux of 21 ± 15 mmol C m⁻² d⁻¹. The contradictory observation of positive NCP and CO₂ release and O₂ uptake in the mixed layer could be explained by the influence of the southwest monsoon-induced upwelling along with the influence of the Pearl River, as the upwelling brought cold, low dissolved oxygen (DO, 160 ± 30 μM) and high dissolved inorganic carbon (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 during both the dry-heterotrophic and wet-autotrophic seasons.