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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)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(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 HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript 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.

There is a need to determine the spatial and temporal dynamics of nutrient limitation to decide which nutrients should be removed during sewage treatment in Hong Kong. We compared 3 methods to assess potential or actual nutrient limitation. Ambient nutrient ratios were calculated, and nutrient enrichment bioassays were conducted, along with 33 P turnover times. Comparison of nutrient ratios and bioassays demonstrated that the ambient inorganic nutrient ratios, based on the Redfield Si:N:P ratio of 16:16:1, were a rapid and effective method that could be used to predict the potentially limiting nutrient of phytoplankton biomass, except in eastern waters in summer, since the DIN:PO 4 uptake ratio was occasionally below the Redfield ratio. The agreement between nutrient limitation indices of growth rate and biomass yield suggested that phytoplankton biomass and growth rate were P-limited in southern waters, with more stable conditions during summer. In contrast, a lack of agreement between these indicators showed that phytoplankton growth in potentially P-limited cases in western waters and Victoria Harbour was controlled by physical processes (e.g. strong hydrodynamic mixing and dilution). The limiting factor for phytoplankton growth varied spatially and temporally. In summer, there was a change from physical processes (e.g. the rapid dilution and possible light limitation due to strong turbulent mixing) in hydrodynamically active western waters and Victoria Harbour to P limitation, or N + P co-limitation, in southern and eastern waters with more stable conditions. In winter, phytoplankton growth was regulated by strong wind-induced vertical mixing. Hence, different seasonal sewage treatment strategies should be considered for nutrient removal.KEY WORDS: Nutrient ratios · Nutrient enrichment bioassays · 33 P turnover times · Pearl River discharge · Sewage effluent · Hong Kong · Nutrient limitation · Light availability Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 388: [81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97] 2009 nutrient loading from the Mississippi River watershed (Malakoff 1998).The concept of nutrient limitation has been the keystone to understanding eutrophication impacts (Smith et al. 1999), and determining the limiting nutrient for phytoplankton growth was the central theme of eutrophication research in the 1970s (Jong 2006). Nutrient enrichment causes a change in phytoplankton biomass yield, as well as growth rate. Hence, the concept of nutrient limitation has 2 meanings: the limitation of biomass and/or of growth rate (Paasche & Erga 1988).Nutrient availability in coastal waters can be strongly influenced by both freshwater inputs and oceanic and tidal exchange, with the latter typically diluting nutrient concentrations (Gobler et al. 2005). It has been debated which nutrient, N or P, limits primary production in coastal waters. Nitrogen has traditionally been viewed as the nutrient limiting productivity in coastal ...

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27Eutrophication impacts may vary spatially and temporally due to different 28 physical processes. Using a 22-year time series data set , a comparison of 29 eutrophication impacts between two eutrophic harbors, Victoria and Tolo Harbours, in 30 Hong Kong with very different hydrodynamic conditions was conducted. In the 31 highly-flushed Victoria Harbour (Victoria), the highest Chl a (13 μg L -1 ) occurred due 32 to stratification in summer as a result of the input of the eutrophic Pearl River 33 discharge, but the high flushing rate restricted nutrient utilization and the further 34 accumulation of algal biomass. In other seasons, vertical mixing induced light 35 limitation and horizontal dilution led to low Chl a (< 2 μg L -1 ) and no spring bloom. 36 Few hypoxic events (DO < 2 mg L -1 ) occurred due to strong tidal mixing. Therefore, 37 Victoria is resilient to nutrient enrichment. In contrast, in the weakly-flushed Tolo 38 Harbour (Tolo), year long stratification, the long residence times and weak tidal 39 currents favored algal growth, resulting in a spring diatom bloom and high Chl a (up 40 to 30 μg L -1 ) all year and frequent hypoxic events in summer. Hence, Tolo is 41 susceptible to nutrient enrichment and it responded to nutrient reduction since sewage 42 treatment resulted in a 32-38% decrease in algal biomass in Tolo, but not in Victoria. 43 A significant (11-22%) reduction in bottom DO in the both harbors after sewage 44 treatment was due to a decrease in the organic loading from sewage treatment or the 45 diversion.46

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