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Nutrient limitation in the pearl river estuary, Hong Kong waters and adjacent South China Sea waters

Authors Xu, Jie
Issue Date 2007
Summary In 2001, the Hong Kong government implemented the Harbor Area Treatment Scheme (HATS) under which 70% of the sewage that had been formerly discharged into Victoria Harbor is now collected and sent to Stonecutters Island Sewage Works where it receives chemically enhanced primary treatment (CEPT), and is then discharged into waters west of the Harbor. To examine the effects of the relocation of the sewage discharge on the nutrient dynamics and phytoplankton biomass in this area, an analysis of the 15 year (1986-2001) monitoring dataset from the Environmental Protection Department of Hong Kong was conducted, with a focus on the spatial and seasonal variations in nutrients, nutrient ratios and phytoplankton biomass in Hong Kong waters. After HATS was implemented, ambient nutrient ratios were determined and nutrient enrichment bioassays were conducted in western and southern waters, and Victoria Harbor in 2006, and in Port Shelter waters in 2005-2006, along with 33P uptake and turnover times. There was a change in the limiting nutrient in Victoria Harbor and its adjacent areas after HATS. Before HATS, Si was the stoichiometrically limiting nutrient. In contrast, after HATS, there was considerable temporal variability with a shift from potential N limitation in spring and winter, to P limitation in summer and Si limitation in the fall due to the decrease in sewage inputs. However, the relocation of the sewage discharge had little influence on the phytoplankton biomass in Hong Kong waters due to the influence of hydrodynamic mixing. During autumn to spring, phytoplankton growth was limited by physical factors such as light availability and the strong vertical mixing induced by strong tidal currents and the northeast monsoon winds. By comparison, in summer, the limiting factors for phytoplankton growth exhibited clear spatial variability. In the areas with high hydrodynamic mixing such as the western waters and Victoria Harbor, phytoplankton growth was primarily constrained by strong hydrodynamic mixing and high flushing rates, and as a result, the phytoplankton biomass was low. In contrast, in the more stable areas such as the southern waters and eastern waters, the limiting nutrient (PO4) was often exhausted, and phytoplankton growth was regulated by PO4 availability due to the input of freshwater (Pearl River discharge and rainfall) with a very high N:P ratio and an increase in light availability. In southern waters, the eutrophication effect was more severe relative to other areas due to high ambient phytoplankton biomass (41 μg Chl a L-1). However, in eastern waters, eutrophication potential was low due to the greatly reduced influence of the Pearl River discharge and sewage effluent. The temporal and spatial variations in the limiting factor for phytoplankton growth were complex and were associated with the bottom topography, hydrodynamics, year round sewage effluent inputs and seasonal exchange of the Pearl River discharge and oceanic water derived from the alteration of SW monsoon in summer and NE monsoon in winter. Therefore, in summer, the removal of P should be the primary consideration in sewage treatment. In contrast, the Hong Kong coastal ecosystem is more resistant to the eutrophication impacts in winter due to the strong hydrodynamic mixing, and hence, nutrient removal is less important. In summer, the excess N in the Pearl River outflow plume is not utilized and ultimately the excess N flows onto the inner continental shelf of the South China Sea and as a consequence, in river-impacted areas of the shelf, phytoplankton and bacterial growth was P-limited in late summer. In contrast, on the outer shelf which is dominated by the oceanic water, N may be the primary limiting nutrient. In summer, the extent of P limitation on the shelf was closely associated with the size of the Pearl River plume which is related to the amount of rainfall. In contrast, in the basin, N was the potential primary limiting nutrient in late summer.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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