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There is excess nitrate (NO₃) in the Pearl River coastal plume in the southern waters of Hong Kong in summer. We hypothesize that phosphorus (P) limitation controls the utilization of excess NO₃ due to the high N:P ratio in the Pearl River. To test this hypothesis, we conducted two 1-day cruises on July 13 and 19, 2000 to examine the response of the phytoplankton to P additions with respect to changes in biomass, uptake of nutrients and nutrient uptake ratios using a batch incubation of natural water samples collected from the Pearl River estuary and adjacent coastal waters. At a station (E1, salinity=5) in the Pearl River estuary, the N/P ratio at the surface was 46:1, (64μM DIN: 1.3μM PO₄) and decreased to 24:1 (12μM DIN: 0.5μM PO₄) downstream at a station (Stn 26, salinity=26) in the coastal plume south of Hong Kong. Without a P addition, NO₃ in the water samples collected at E1 could not be depleted during …