MEPS 194:295-305 (2000)  -  doi:10.3354/meps194295

ABSTRACT: 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 freshwater/seawater 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^-1) in freshwater and the highest (2.41 d^-1) in 100% seawater (salinity = 29). PO₄ was lower in freshwater (0.3 µM) than in seawater (1.2 µM), whereas concentrations of NO₃+NH₄+urea (~80 µM) and SiO₄ (150 µM) were higher in freshwater than those in seawater (25 µM for nitrogen and 26 µM for SiO₄). During the incubation PO₄ 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 diagrams of NO₃ and SiO₄ showed conservative behaviour with salinity 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 PO₄ during July. On the estuarine (west) side of the regional 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 limitation of nutrients to the phytoplankton community. In the same eastern waters, both PO₄ and SiO₄ were very low. However, NO₃ and NH₄ 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 PO₄ and SiO₄ before nitrogen.

KEY WORDS: Nutrient dynamics · Salinity · Rain · P limitation · Phytoplankton blooms · Pearl River estuary · Hong Kong