Ho Alvin Y. T.

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Alvin Y. T.

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  • Preprint
    Temporal and spatial variations in nutrient stoichiometry and regulation of phytoplankton biomass in Hong Kong waters : influence of the Pearl River outflow and sewage inputs
    ( 2008-01) Xu, Jie ; Ho, Alvin Y. T. ; Yin, Kedong ; Yuan, Xiangcheng ; Anderson, Donald M. ; Lee, Joseph H. W. ; Harrison, Paul J.
    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. The relocation of the sewage discharge will possibly change the nutrient dynamics and phytoplankton biomass in this area. Therefore, there is a need to examine the factors that regulate phytoplankton growth in Hong Kong waters in order to understand future impacts. Based on a historic nutrient data set (1986-2001), a comparison of ambient nutrient ratios with the Redfield ratio (N:P:Si=16:1:16) showed clear spatial variations in the factors that regulate phytoplankton biomass along a west (estuary) to east (coastal/oceanic) transect through Hong Kong waters. Algal biomass was constrained by a combination of low light conditions, a rapid change in salinity, and strong turbulent mixing in western waters throughout the year. Potential stoichiometric Si limitation (up to 94% of the cases in winter) occurred in Victoria Harbor due to the contribution of sewage effluent with high N and P enrichment all year, except for summer when the frequency of stoichiometric Si limitation (48%) was the same as P, owing to the influence of the high Si in the Pearl River discharge. In the eastern waters, potential N limitation and N and P co-limitation occurred in autumn and winter respectively, because of the dominance of coastal/oceanic water with low nutrients and low N:P ratios. In contrast, potential Si limitation occurred in spring and a switch to potential N, P and Si limitation occurred in eastern waters in summer. In southern waters, there was a shift from P limitation (80%) in summer due to the influence of the N-rich Pearl River discharge, to N limitation (68%) in autumn, and to N and P co-limitation in winter due to the dominance of N-poor oceanic water from the oligotrophic South China Sea. Our results show clear temporal and spatial variations in the nutrient stoichiometry which indicates potential regulation of phytoplankton biomass in HK waters due to the combination of the seasonal exchange of the Pearl River discharge and oceanic water, sewage effluent inputs, and strong hydrodynamic mixing from SW monsoon winds in summer and the NE monsoon winds in winter.
  • Preprint
    A comparison of eutrophication impacts in two harbours in Hong Kong with different hydrodynamics
    ( 2009-09-03) Xu, J. ; Yin, K. ; Liu, H. ; Lee, Joseph H. W. ; Anderson, Donald M. ; Ho, Alvin Y. T. ; Harrison, Paul J.
    Eutrophication impacts may vary spatially and temporally due to different physical processes. Using a 22-year time series data set (1986-2007), a comparison of eutrophication impacts between two eutrophic harbors, Victoria and Tolo Harbours, in Hong Kong with very different hydrodynamic conditions was conducted. In the highly-flushed Victoria Harbour (Victoria), the highest Chl a (13 μg L-1) occurred due to stratification in summer as a result of the input of the eutrophic Pearl River discharge, but the high flushing rate restricted nutrient utilization and the further accumulation of algal biomass. In other seasons, vertical mixing induced light limitation and horizontal dilution led to low Chl a (< 2 μg L-1) and no spring bloom. Few hypoxic events (DO < 2 mg L-1) occurred due to strong tidal mixing. Therefore, Victoria is resilient to nutrient enrichment. In contrast, in the weakly-flushed Tolo Harbour (Tolo), year long stratification, the long residence times and weak tidal currents favored algal growth, resulting in a spring diatom bloom and high Chl a (up to 30 μg L-1) all year and frequent hypoxic events in summer. Hence, Tolo is susceptible to nutrient enrichment and it responded to nutrient reduction since sewage treatment resulted in a 32-38% decrease in algal biomass in Tolo, but not in Victoria. A significant (11-22%) reduction in bottom DO in the both harbors after sewage treatment was due to a decrease in the organic loading from sewage treatment or the diversion.