We observed a phytoplankton bloom downstream of a large estuarine plume induced by heavy precipitation during a cruise conducted in
the Pearl River estuary and
the nor
thern South China Sea in May–June 2001. The plume delivered a significant amount of nutrients into
the estuary and
the adjacent coastal region, and enhanced stratification stimulating a phytoplankton bloom in
the region near and offshore of Hong Kong. A several fold increase (0.2–1.8 μg Chl L
−1) in biomass (Chl
a) was observed during
the bloom. During
the bloom event,
the surface water phytoplankton community structure significantly shifted from a pico-phytoplankton dominated community to one dominated by micro-phytoplankton (>20 μm). In addition to increased Chl
a, we observed a significant drawdown of
pCO
2, biological uptake of dissolved inorganic carbon (DIC) and an associated enhancement of dissolved oxygen and pH, demonstrating enhanced photosyn
thesis during
the bloom. During
the bloom, we estimated a net DIC drawdown of 100–150 μmol kg
−1 and a TAlk increase of 0–50 μmol kg
−1. The mean sea–air CO
2 flux at
the peak of
the bloom was estimated to be as high as
![not, vert, similar not, vert, similar](http://www.sciencedirect.com/scidirimg/entities/223c.gif)
−18 mmol m
−2 d
−1. For an average surface water depth of 5 m, a very high apparent biological CO
2 consumption rate of 70–110 mmol m
−2 d
−1 was estimated. This value is 2–6 times higher than
the estimated air–sea exchange rate.