The Pearl River in the south of China is the second largest river in China in terms of discharge volume. Two cruises were made to investigate the dynamics of nutrients and phytoplankton biomass in June and July 1998, across the Pearl River estuary to the adjacent territorial waters of Hong Kong. On-deck incubation experiments of 5 mixtures of freshwater from the surface with seawater from below the halocline were conducted to simulate time scales of phytoplankton blooms for each freshwaterheawater mixture and to examine uptake of nutrients. In July, phytoplankton growth rates increased with salinity of the mixtures, with the lowest growth rate (0.81 d-') in freshwater and the highest (2.41 d-l) in 100% seawater ( s a h i t y = 29). PO4 was lower in freshwater (0.3 PM) than in seawater (1.2 pM), whereas concentrations of NO3+ NH4 + urea (-80 FM) and SiO, (150 FM) were higher in freshwater than those in seawater (25 pM for nitrogen and 26 pM for SO,). During the incubation PO, disappeared first, indicating that P was limiting the phytoplankton biomass. All mixtures reached the maximum in phytoplankton biomass in 3 to 4 d. There was a regional maximum of phytoplankton biomass that occurred at the seaward edge of the estuarine plume during June The region moved eastwards (away from the estuary) to the southern waters between Lantau Island and Hong Kong Island during July. Mixing dagrams of NO3 and Si04 showed conservative behaviour with sabnity in the estuary. These observations suggest that dilution by freshwater outflow was a controlling factor in determining the distribution of nutrients and phytoplankton biomass in the estuary due to high flow during June and July. The regional maximum of phytoplankton biomass was comparable to that resulting from the incubation and coincided with the exhaustion of PO, during July. On the estuanne (west) side of the regonal maximum, chl a fluorescence increased during 24 h incubations, but decreased at the station with the maximum and on the east side, suggesting the possible hutation of nutrients to the phytoplankton community. In the same eastern waters, both PO, and SiO, were very low. However, NO, and NH, remained abundant, suggesting possible CO-limitation by phosphorus and silicon. We hypothesize that considerable seasonal rainfall in June and early July might have contributed an additional source of nitrogen to the water column, which resulted in the exhaustion of PO, and SiO, before nitrogen.

Anthropogenic loading of nutrients in rivers often increases disproportionally among N, P, and Si, and thus may shift the type of phytoplankton nutrient limitation in the coastal receiving waters. The effect of anthropogenic nutrient loading has rarely been addressed in the Pearl River estuary along the southern coast of China, even though it is one of the largest rivers in the world. We conducted a cruise along the Pearl River estuary and adjacent coastal waters south of Hong Kong during July 17 to 18, 1999. Samples were taken for salinity and nutrients (NO 3 , SiO 4 , PO 4 , NH 4 and urea) and nutrient addition experiments were conducted on board. Vertical profiles of salinity showed a salt-wedge estuary and the coastal plume covering the waters south of Hong Kong. Concentrations of NO 3 were very high (ca 90 µM) upstream of the Pearl River estuary, and much of the riverine NO 3 was not utilized in the estuary until depletion at the edge of the coastal plume on the east side of Hong Kong. SiO 4 was 120 µM upstream and its utilization was similar to that of NO 3 . PO 4 was low in surface waters (< 0.5 µM) and higher below the halocline in the estuary. NH 4 and urea were generally < 4 and 1.5 µM, respectively. In the estuary, N:P ratio was 200 :1, indicating potential P limitation, while N:Si was below 1:1. Beyond the coastal plume to the east of Hong Kong, N:P and N:Si ratios were < 5 :1 and 1:0.3, respectively, indicating potential N limitation. Nutrient limitation was shown in nutrient addition experiments and was consistent with the ratios of nutrients. Therefore, nutrient limitation shifted across the coastal plume from P limitation in the estuary to N limitation in the oceanic waters. Potential P limitation was observed in the estuary; P and Si co-limiting occurred at the edge of the coastal plume, and N was limiting in the oceanic side. This spatial shift in nutrient limitation has great implications for nutrient pollution control and coastal management of Hong Kong waters.

The magnitude of El Nino in 1997-98 was one of the strongest of the century. A series of red tides occurred in Hong Kong territorial waters between mid-March and rnidApnl 1998, resulting in the loss of HK$250 (US$32) million in fish kill damage. The causative species was Gyrodinium aureolum. We used a time series of satellite images to track the development of the harmful algal bloom and relate its movement to physical oceanographic conditions. Prior to the red tide event in Hong Kong, harmful algal blooms had occurred earher

No abstract