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Enrichment of nutrients is believed to lead to coastal hypoxia which have become a seasonal phenomenon over large river estuarine areas such as the Mississippi River-Northern Gulf of Mexico and Changjiang-East China Sea. A similar nutrient enrichment process exists in the Pearl River. However, hypoxia occurs only as episodic events over a relatively small area. We hypothesize that frequent wind events play the interruptive mechanism in preventing the seasonal formation of bottom hypoxia. We used 29 years' time series data of dissolved oxygen (DO) and winds in the Hong Kong coastal waters to test the hypothesis. Our results show that bottom DO at 3 stations in southern waters of Hong Kong occasionally drops below the hypoxic level (2 mg/L), lasting only for less than one month in summer. Episodic hypoxia events appear to occur more frequently in recent years, but bottom DO does not show a significantly decreasing trend. The wind speed of 6 m/s appears to be a threshold, above which a wind event could destroy water column stratification and interrupt the formation of low-oxygen (DO <3 mg/L) water mass. The wind events above the threshold occur 14.3 times in June, 14.2 times in July and 10.0 times in August during 1990-2018. This explains why episodic events of hypoxia hardly occur in June and July, and only occasionally in August. The frequency of such the above-threshold events appears to show a decreasing trend during 1990-2018, which coincides with an increasing occurrences of episodic hypoxia events in recent years.

A typhoon produces a rapid mixing and flushing event and it can be added to the list of other factors such as shallow water depth, spring tidal mixing, the Pearl River discharge, summer upwelling that make Hong Kong waters relatively resistant to eutrophication impacts. Two typhoons passed over Hong Kong waters and provided an opportunity to document the changes in water quality in late summer 2003. Before the typhoon (Aug 19-20) and during a neap tide, a large algal bloom (>10 μg Chl-a L-1) occurred in the stratified southern waters influenced by the Pearl River estuarine waters with high NO3. However, PO4 and SiO4 were drawn down to near limiting concentrations by the large bloom. After the typhoons, Chl-a decreased to 2 μg L-1 due to vertical mixing and advection. The heavy rainfall and increased river discharge quickly re-set the water column to the usual strong summer stratification in only a few days. As a result, high nutrients in the river discharge stimulated another large algal bloom a few days after the next neap tide when tidal mixing was reduced. In the southern waters, the deeper station showed stronger stratification and lower bottom dissolved oxygen (DO) than the shallower station suggesting that the low DO in the bottom water may have come from offshore transport. (c) 2012 Elsevier Ltd. All rights reserved.

We present the elements of an algal bloom risk forecast system which aims to provide a scientific prognosis of the likelihood of an algal bloom occurrence as a function of: (a) the nutrient concentration: and (b) the forecasted wind and tide-induced vertical mixing relative to the critical value defined by the environmental and algal growth conditions. The model is validated with high frequency field observations of algal blooms in recent years and only requires the input of readily available field measurements of water column transparency, nutrient concentration, representative maximum algal growth rate, and a simple estimate of vertical mixing as a function of tidal range, wind speed, and density stratification. The forecasted region-wide risk maps successfully predicted the observed algal bloom occurrences in Hong Kong waters over the past 20 years, with a correct prognosis rate of 87\%. It is shown that algal blooms are to a large extent controlled by the interaction of physical and biological processes. This work provides a general framework to interpret the complex spatial and temporal dynamics of observed algal blooms, and paves the way for the development of real time algal bloom risk forecast systems. (C) 2008 Elsevier B.V. All rights reserved.

We present the elements of an algal bloom risk forecast system which aims to provide a scientific prognosis of the likelihood of an algal bloom occurrence as a function of: (a) the nutrient concentration: and (b) the forecasted wind and tide-induced vertical mixing relative to the critical value defined by the environmental and algal growth conditions. The model is validated with high frequency field observations of algal blooms in recent years and only requires the input of readily available field measurements of water column transparency, nutrient concentration, representative maximum algal growth rate, and a simple estimate of vertical mixing as a function of tidal range, wind speed, and density stratification. The forecasted region-wide risk maps successfully predicted the observed algal bloom occurrences in Hong Kong waters over the past 20 years, with a correct prognosis rate of 87\%. It is shown that algal blooms are to a large extent controlled by the interaction of physical and biological processes. This work provides a general framework to interpret the complex spatial and temporal dynamics of observed algal blooms, and paves the way for the development of real time algal bloom risk forecast systems. (C) 2008 Elsevier B.V. All rights reserved.

We present the elements of an algal bloom risk forecast system which aims to provide a scientific prognosis of the likelihood of an algal bloom occurrence as a function of: (a) the nutrient concentration: and (b) the forecasted wind and tide-induced vertical mixing relative to the critical value defined by the environmental and algal growth conditions. The model is validated with high frequency field observations of algal blooms in recent years and only requires the input of readily available field measurements of water column transparency, nutrient concentration, representative maximum algal growth rate, and a simple estimate of vertical mixing as a function of tidal range, wind speed, and density stratification. The forecasted region-wide risk maps successfully predicted the observed algal bloom occurrences in Hong Kong waters over the past 20 years, with a correct prognosis rate of 87\%. It is shown that algal blooms are to a large extent controlled by the interaction of physical and biological processes. This work provides a general framework to interpret the complex spatial and temporal dynamics of observed algal blooms, and paves the way for the development of real time algal bloom risk forecast systems. (C) 2008 Elsevier B.V. All rights reserved.

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 d

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 d

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Pollution Bulletin 57 (2008): 335-348, doi:10.1016/j.marpolbul.2008.01.020., 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, Financial support for this research was provided by the University Grants Council of Hong Kong AoE project (AoE/P-04/0401), and RGC project HKUST6478/05M. Support was also provided (to DMA) by U.S. National Science Foundation grants OCE-0402707 and OCE-0430724 and by NIEHS grant 1 P50-ES01274201.

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Pollution Bulletin 57 (2008): 335-348, doi:10.1016/j.marpolbul.2008.01.020., 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, Financial support for this research was provided by the University Grants Council of Hong Kong AoE project (AoE/P-04/0401), and RGC project HKUST6478/05M. Support was also provided (to DMA) by U.S. National Science Foundation grants OCE-0402707 and OCE-0430724 and by NIEHS grant 1 P50-ES01274201.

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 d

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

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

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