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A comparison between algal bloom frequency in Hong Kong's eastern and western waters

Authors Dickman, Mick
Tang, Senming
Yin, Kedong
Issue Date 1998
Source Proceedings of the European-Asian Workshop on Investigation and Management of Mediterranean and South China Sea Coastal Zone, Hong Kong, 9-11 Nov., 1998, p. 125-158
Summary The Pearl River estuary is a paradox. From 1980 to 1997 there were a total of over 481 algal blooms reported for Hong Kong waters and less 6 % of these were from the Pearl River estuary. Why should so few algal blooms occur in an area so rich in nutrients? The purpose of this study was to determine why the nutrient-rich waters of the Pearl River estuary supported so few algal blooms. Sewage discharges to the Pearl River have increased quite substantially over the last thirty years, as has private industry and mariculture activities. By the year 2001 it is expected that 3,200 million tones of domestic sewage will annually be discharged to the Pearl River. Agricultural activity gives rise to pollution from fertilizers and pesticides. In only 3 years between 1986 and 1989 the reliance on chemical fertilizers in the Pearl River Delta increased by 40%. With these enormous discharges of nutrient-rich water from sewage and farmland runoff we expected to find frequent algal blooms in the Pearl River estuary. The fact that we did not gave rise to the paradox examined in this study. The presence of large amounts of dissolved and particulate organic matter makes the Pearl River estuary an ideal location for heterotrophs to multiply. During the peak of the wet season, over 200 million bacterial cells per litre were observed in plankton samples taken from near the center of the Peal River estuary. These bacteria were the basis of a food web that was dominated by ciliates and colourless microflagellates. Autotrophs were relatively rare in the center of the Pearl River estuary at this time. In 1998 during the peak of the wet season (June and July) 26 sites were sampled in the east and west of Hong Kong. About half of these were in the Pearl River estuary. It was not possible to monitor all 26 stations on a weekly basis but two sites, one in the western part of Hong Kong (Lamma Channel) and one in the eastern part of Hong Kong (Port Shelter) were chosen as representative of eastern and western Hong Kong waters. Each of these two sites was sampled at weekly intervals in 1998. Brackish water tolerant diatoms dominated the phytoplankton in the western waters while the plankton in the eastern waters was much more diverse and abundant with both diatoms and dinoflagellates reaching high densities at various times over the year. To determine why autotrophs bloom so infrequently in the Peal River Estuary we carried out an incubation study. A station located near the center of the Pearl River Estuary was chosen for study. This was the first time an incubation experiment was carried out to determine the effects of retention time, turbidity and salinity on the plankton living near the center of Pearl River Estuary. The results indicated that low salinity, poor light penetration and short water retention time in the estuary combine to limit the productivity of phytoplankton there. In summer, sewage bacteria, ciliates and colourless microflagellates characterized the plankton near the center of the Pearl River estuary. In winter (dry season) flows from the Pearl River are much smaller and the plankton at the center of the estuary was dominated by autotrophs similar to those observed in the East Lamma Channel.
Language English
Format Conference paper
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