Dynamics of nutrients and phytoplankton biomass in the Pearl River estuary and adjacent waters of Hong Kong during summer : preliminary evidence for phosphorus and silicon limitation

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). PO 4 was lower in freshwater (0.3 μM) than in seawater (1.2 μM), whereas concentrations of NO 3 +NH 4 +urea (-80 pM) and SiO 4 (150 pM) were higher in freshwater than those in seawater (25 pM for nitrogen and 26 pM for SiO 4 ). During the incubation PO 4 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 NO 3 and SiO 4 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 PO 4 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 PO 4 and SiO 4 were very low. However, NO 3 and NH 4 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 4 and SiO 4 before nitrogen.

[1]  T. Malone,et al.  Interactions between suspended organic matter and copepod grazing in the plume of the Hudson River , 1981 .

[2]  G. Lang,et al.  Variations in primary production of northern Gulf of Mexico continental shelf waters linked to nutrient inputs from the Mississippi River , 1997 .

[3]  P. Harrison,et al.  Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. II. Effects of Springvs.Neap Tides and River Discharge , 1995 .

[4]  P. Tréguer,et al.  Relationships between different forms of inorganic nitrogen in rainwater of a coastal area and ground-level gaseous nitrogen oxides , 1989 .

[5]  J. Cloern.  PHYTOPLANKTON BLOOM DYNAMICS IN COASTAL ECOSYSTEMS' A REVIEW WITH SOME GENERAL LESSONS FROM SUSTAINED INVESTIGATION OF SAN FRANCISCO , 1996 .

[6]  J. Kirshtein,et al.  A refined dilution technique for measuring the community grazing impact of microzooplankton, with experimental tests in the central equatorial Pacific , 1995 .

[7]  Q. Dortch,et al.  Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River spatial and seasonal variations , 1996 .

[8]  G. Slawyk,et al.  Comparison of two automated ammonium methods in a region of coastal upwelling , 1972 .

[9]  L. Albright.  Heterotrophic Bacterial Biomasses, Activities, and Productivities Within The Fraser River Plume , 1983 .

[10]  Louis Legendre,et al.  Hierarchical control of phytoplankton succession by physical factors , 1984 .

[11]  Lawrence W. Harding,et al.  Phytoplankton production in two east coast estuaries: Photosynthesis-light functions and patterns of carbon assimilation in Chesapeake and Delaware Bays , 1986 .

[12]  F. A. Richards,et al.  Determination of nitrate in sea water by cadmium-copper reduction to nitrite , 1967, Journal of the Marine Biological Association of the United Kingdom.

[13]  J. Strickland,et al.  The measurement of upwelling and subsequent biological process by means of the Technicon Autoanalyzer® and associated equipment , 1967 .

[14]  B. Morton,et al.  The hydrology of the coastal waters of Hong Kong. , 1975, Environmental research.

[15]  P. Harrison,et al.  Effects of a fluctuation in Fraser River discharge of primary production in the central Strait of Georgia, British Columbia, Canada , 1997 .

[16]  H. Paerl,et al.  Regulation of estuarine primary production by watershed rainfall and river flow , 1993 .

[17]  J. Baross,et al.  Particle-attached bacteria and heterotrophic plankton associated with the Columbia River estuarine turbidity maxima , 1996 .

[18]  T. Malone.  Environmental regulation of phytoplankton productivity in the lower Hudson Estuary , 1977 .

[19]  T. Malone,et al.  The production and fate of phytoplankton size fractions in the plume of the Hudson River, New York Bight1 , 1979 .

[20]  P. Hills,et al.  Transboundary Pollution between Guangdong Province and Hong Kong: Threats to Water Quality in the Pearl River Estuary and Their Implications for Environmental Policy and Planning , 1998 .

[21]  T. Malone,et al.  Influences of river flow on the dynamics of phytoplankton production in a partially stratified estuary , 1988 .

[22]  D. Mackas,et al.  Aggregation of the copepod Neocalanus plumchrus at the margin of the fraser river plume in the strait Georgia , 1988 .

[23]  M. Sinclair.  Summer Phytoplankton Variability in the Lower St. Lawrence Estuary , 1978 .

[24]  D. Vaulot,et al.  Standing stock and production of phytoplankton in the estuary of the Chang-jiang (Yangste River) and the adjacent East China Sea , 1988 .

[25]  M. Bianchi,et al.  Seasonal Evolution of Microplanktonic Communities in the Estuarine Front Ecosystem of the Rhône River Plume (North-western Mediterranean Sea) , 1993 .

[26]  L. Brand,et al.  The salinity tolerance of forty-six marine phytoplankton isolates , 1984 .

[27]  Shen Zhiliang.  A study on the relationships of the nutrients near the Changjiang River estuary with the flow of the Changjiang River water , 1993 .

[28]  J. Rijstenbil.  Phytoplankton composition of stagnant and tidal ecosystems in relation to salinity, nutrients, light and turbulence , 1987 .

[29]  J. Therriault,et al.  Relationships between bacteria, phytoplankton and particulate organic carbon in the upper St. Lawrence Estuary , 1989 .

[30]  P. Harrison,et al.  Phosphate limitation of phytoplankton in the Changjiang estuary , 1990 .

[31]  J. Rijstenbil.  Selection of phytoplankton species in culture by gradual salinity changes , 1988 .

[32]  P. Harrison,et al.  Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. III. Effects of Winds , 1995 .

[33]  S. Wofsy,et al.  A chemical survey of the Mississippi estuary , 1987 .

[34]  Paul J. Harrison,et al.  Importance of wind and river discharge in influencing nutrient dynamics and phytoplankton production in summer in the central Strait of Georgia , 1997 .

[35]  P. Harrison,et al.  Spring bloom in the central Strait of Georgia: interactions of river discharge, winds and grazing , 1996 .

[36]  Paul J. Harrison,et al.  Phosphate limitation in estuarine and coastal waters of China , 1990 .

[37]  R. P. Hassett,et al.  Estimating the grazing impact of marine micro-zooplankton , 1982 .

[38]  H. Paerl,et al.  Stimulation of phytoplankton production in coastal waters by natural rainfall inputs: Nutritional and trophic implications , 1990 .

[39]  L. Schlüter.  The influence of nutrient addition on growth rates of phytoplankton groups, and microzooplankton grazing rates in a mesocosm experiment , 1998 .