Monsoonal influence on seasonal variations in nutrients and phytoplankton biomass in coastal waters of Hong Kong in the vicinity of the Pearl River estuary

Some estuarine ecosystems appear to be more resistant to eutrophication than others, but the mechanisms are not well understood. The Pearl River is the second largest river in China and little is known about the long-term nutrient enrichment processes in the Pearl River estuary and adjacent coastal waters of Hong Kong. Seasonal monsoons are a climatic feature of tropical and subtropical regions. However, monsoonal influences on seasonal dynamics of nutrients and phytoplankton biomass have been little studied. This is the first report of such a study using time series of water quality data at stations around Hong Kong from 1991 to 2000. There was a strong seasonality and spatial gradient in salinity, nutrients and phytoplankton biomass around Hong Kong waters. In winter, when the NE monsoon prevailed (winds were stronger), coastal waters and the eastern part of the Pearl River estuary were dominated by high salinities (> 30) low nutrients and low phytoplankton biomass. In summer, the coastal plume from the Pearl River estuary on the west moved across the southern waters of Hong Kong to the eastern waters of Hong Kong, as the SW monsoon (winds were weaker) dominated and the Pearl River discharge was maximal. Concentrations of nitrate were high (> 60 muM) during summer in the Pearl River estuary, decreased (<30 muM) in the coastal plume south of Hong Kong and were low (< 10 muM) in the eastern waters. Chlorophyll a (chl a) concentrations were low, high and low, respectively, in the corresponding waters. Because PO4 concentrations were low (< 1 muM) in all waters, nutrient ratios of N:P and N:Si (N = total inorganic nitrogen, P = PO4) displayed seasonal and spatial changes. In winter, N:P and N:Si were low (near or < 16:1 and 1:1, respectively). In summer, however, the N:P ratio increased markedly (>64:1) in the southern waters of Hong Kong and was even higher than in the estuary. Low chl a relative to high N concentrations suggests that the eutrophication effect is not as severe as one would expect from N over enrichment. These results suggest an important role of P limitation in controlling phytoplankton biomass production. It appears that seasonal monsoons serve as a flushing mechanism in 2 ways: (1) monsoons reduce seasonal eutrophication effects by nutrient enrichment during summer; and (2) they prevent long-term (yr) accumulation of organic matter in the sediments due to nutrient enrichment in the region. In summer, when coastal upwelling resulting from the SW monsoon causes offshore movement of the coastal plume in the surface layer, the continental shelf oceanic waters moving shoreward at the bottom flush the coastal waters. In winter, coastal waters with low nutrients replace the entire coastal water of Hong Kong. Due to the monsoon-influenced processes and low phosphorus in the Pearl River estuary, the Pearl River estuary and adjacent coastal waters of Hong Kong appear to be more resilient to enrichment of N.