Kedong Yin

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

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.

Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.

Eutrophication Dynamics in Hong Kong Coastal Waters: Physical and Biological Interactions

Hong Kong is a mega-city of 6.7 million people that contributes a high nutrient load through its sewage discharge and it is one of the busiest ports in the world. Hong Kong waters are relatively unique because of its intensive utilization of marine resources and frequent occurrence of red tides. Within a small area there is a complexity and richness in eutrophication dynamics, with a sharp gradient from potential phosphorus limitation in western/southern waters to nitrogen limitation in the eastern waters in the summer (Yin at al., 2000 and 2001). Hong Kong waters are sub-tropical, with a clear wet season from May to August, accompanied by southwest monsoon winds and a November to March dry season with northeast monsoon winds, and transitional months in between. The Pearl River is Chinas third longest river and the second largest river in terms of discharge volume. It forms the Pearl River Estuary (PRE) as it flows into the northern shelf of the South China Sea, near Hong Kong (Figure 1a). Its average annual flow is approximately 10,500 m s, and 80% of the total flow occurs in the wet season due to the high rainfall during this period (annual rainfall of 2100 mm). In the summer, river water moves into the western waters of Hong Kong due to the southwest monsoon winds. During the dry season, the northeast monsoon winds cause the surface river plume to move to the western side of the estuary, away from Hong Kong. An important feature of Hong Kong waters that lie on the eastern side of PRE is that they are shallow (mainly 10 to 20 m) and interlaced with several hundred of islands and inlets (Figure 1b). This topography and bathymetry increases the complexity of the hydrodynamics, which in turn influences the occurrence of episodic events, red tides and other algal blooms.

Seasonal and spatial dynamics of nutrients and phytoplankton biomass in Victoria Harbour and its vicinity before and after sewage abatement

This study investigated the seasonal and spatial dynamics of nutrients and phytoplankton biomass at 12 stations in Hong Kong (HK) waters during a three year period from 2004 to 2006 after upgraded sewage treatment and compared these results to observations before sewage treatment. Pearl River estuary (PRE) discharge significantly increased NO(3) and SiO(4) concentrations, particularly in western and southern waters when rainfall and river discharge was maximal in summer. Continuous year round discharge of sewage effluent resulted in high NH(4) and PO(4) in Victoria Harbour (VH) and its vicinity. In winter, spring and fall, the water column at all stations was moderately mixed by winds and tidal currents, and phytoplankton biomass was relatively low compared to summer. In summer, the mean surface phytoplankton chl biomass was generally > 9 microL(-1) in most areas as a result of thermohaline stratification, and high nutrients, light, and water temperature. In summer, the potential limiting nutrient is PO(4) in the most productive southern waters and it seldom decreased to limiting levels ( approximately 0.1 microM), suggesting that phytoplankton growth may be only episodically limiting. The mean bottom dissolved oxygen (DO) remained > 3.5 mg L(-1) at most stations, indicating that the eutrophication impact in HK waters was not as severe as expected for such a eutrophic area. After the implementation of chemically enhanced primary sewage treatment in 2001, water quality in VH improved as indicated by a significant decrease in NH(4) and PO(4) and an increase in bottom DO. In contrast, there were an increase in chl a and NO(3), and a significant decrease in bottom DO in southern waters in summer, suggesting that hypoxic events are most likely to occur in this region if phytoplankton biomass and oxygen consumption keep increasing and exceed the buffering capacity of HK waters maintained by monsoon winds, tidal mixing and zooplankton grazing. Therefore, future studies on the long-term changes in nutrient loading from PRE and HK sewage discharge will be crucial for developing future strategies of sewage management in HK waters.

Environmental response to sewage treatment strategies: Hong Kong's experience in long term water quality monitoring

In many coastal cities around the world, marine outfalls are used for disposal of partially treated wastewater effluent. The combined use of land-based treatment and marine discharge can be a cost-effective and environmentally acceptable sewage strategy. Before 2001, screened sewage was discharged into Victoria Harbour through many small outfalls. After 2001, the Hong Kong Harbour Area Treatment Scheme (HATS) was implemented to improve the water quality in Victoria Harbour and surrounding waters. Stage I of HATS involved the construction of a 24 km long deep tunnel sewerage system to collect sewage from the densely populated urban areas of Hong Kong to a centralized sewage treatment plant at Stonecutters Island. A sewage flow of 1.4 million m3 d(-1) receives Chemically Enhanced Primary Treatment (CEPT) followed by discharge via a 1.2 km long outfall 2 km west of the harbor. The ecosystem recovery in Victoria Harbour and the environmental response to sewage abatement after the implementation of HATS was studied using a 21-year data set from long term monthly water quality monitoring. Overall, the pollution control scheme has achieved the intended objectives. The sewage abatement has resulted in improved water quality in terms of a significant reduction in nutrients and an increase in bottom DO levels. Furthermore, due to the efficient tidal mixing and flushing, the impact of the HATS discharge on water quality in the vicinity of the outfall location is relatively limited. However, Chl a concentrations have not been reduced in Victoria Harbour where algal growth is limited by hydrodynamic mixing and water clarity rather than nutrient concentrations. Phosphorus removal in the summer is suggested to reduce the risk of algal blooms in the more weakly-flushed and stratified southern waters, while nutrient removal is less important in other seasons due to the pronounced role played by hydrodynamic mixing. The need for disinfection of the effluent to reduce bacterial (E. coli) concentrations to acceptable levels is also confirmed and has recently been implemented.

Physical–biological coupling induced aggregation mechanism for the formation of high biomass red tides in low nutrient waters

Port Shelter is a semi-enclosed bay in northeast Hong Kong where high biomass red tides are observed to occur frequently in narrow bands along the local bathymetric isobars. Previous study showed that nutrients in the Bay are not high enough to support high biomass red tides. The hypothesis is that physical aggregation and vertical migration of dinoflagellates appear to be the driving mechanism to promote the formation of red tides in this area. To test this hypothesis, we used a high-resolution estuarine circulation model to simulate the near-shore water dynamics based on in situ measured temperature/salinity profiles, winds and tidal constitutes taken from a well-validated regional tidal model. The model results demonstrated that water convergence occurs in a narrow band along the west shore of Port Shelter under a combined effect of stratified tidal current and easterly or northeasterly wind. Using particles as dinoflagellate cells and giving diel vertical migration, the model results showed that the particles aggregate along the convergent zone. By tracking particles in the model predicted current field, we estimated that the physical-biological coupled processes induced aggregation of the particles could cause 20-45 times enhanced cell density in the convergent zone. This indicated that a high cell density red tide under these processes could be initialized without very high nutrients concentrations. This may explain why Port Shelter, a nutrient-poor Bay, is the hot spot for high biomass red tides in Hong Kong in the past 25 years. Our study explains why red tide occurrences are episodic events and shows the importance of taking the physical-biological aggregation mechanism into consideration in the projection of red tides for coastal management.

Spatial and seasonal variations of nutrients in sediment profiles and their sediment-water fluxes in the Pearl River Estuary, Southern China

Three cruises were launched in the Pearl River Estuary (PRE) in 2005 to investigate the biogeochemical cycling of nutrients associated with early diagenesis related to degradation of organic matter. Seasonal and spatial variations of pore water nutrient concentrations and profile patterns in sediments were studied. Nutrient fluxes at the sediment-water interface (SWI) were measured by incubation experiments, and we here discussed the accumulation and transformation processes of nutrients at the SWI. The nutrients generally decreased from the Pearl River outlets downstream, indicating anthropogenic influences on the nutrient inputs in the estuary. NO3-N concentration was the highest of the three forms of DIN (dissolved inorganic nitrogen, the sum of NH4-N, NO3-N and NO2-N) in the overlying water, and NH4-N was the main component of DIN in pore water. The gradual increase of NH4-N and the rapid decrease of NO3-N with sediment depth provided the evidence for anaerobic conditions below the SWI. Negative fluxes of NO3-N and positive fluxes of NH4-N were commonly observed, suggesting the denitrification of NO3-N at the SWI. The DIN flux direction suggested that the sediment was the sink of DIN in spring, however, the sediment was generally the source of DIN in summer and winter. PO4-P distribution patterns were distinct while SiO4-Si inconspicuously varied in sediment profiles in different seasons. The flux results indicated that PO4-P mainly diffused from the water column to the sediment while SiO4-Si mainly diffused from the sediment to the water column. Generally, the incubated fluxes were the coupling of diffusion, bioturbation and biochemical reactions, and were relatively accurate in this study.

Effect of Wind Speed and Direction on Summer Tidal Circulation and Vertical Mixing in Hong Kong Waters

Abstract In subtropical coastal waters around Hong Kong, a well-mixed water body is usually observed after typhoons or strong easterly wind events in summer. A calibrated three-dimensional (3-D) hydrodynamic model for the Pearl River Estuary (Delft3D) was applied to study the physical hydrography of Hong Kong waters and its relationship with wind events in the summer wet season. The general 3-D hydrodynamic circulation and salinity structure in the partially mixed estuary are presented here. The effect of wind on vertical mixing was studied for two representative wind directions (NE and SW) and three wind speeds (5, 7.5, and 10 m/s). The computations show that: (i) in general, in the summer wet season, the river plume moves into the western waters of Hong Kong due to the SW monsoon winds, and the current flow is mainly from W/SW to E/NE in the southern Hong Kong waters; the salinity vertical profile indicates that the water is strongly stratified; (ii) a strong SW wind pushes the river plume into a narrow band and decreases the salinity of the surface water in the estuary and its neighboring region; it may also enhance the mixing in the upper layer of water column, but the whole water body is still stratified; and (iii) a strong NE wind pushes the river plume westward away from Hong Kong waters, and more saline coastal waters enter Hong Kong waters; the water only becomes vertically well mixed after a 10 m/s NE wind blows for 5 d, but wind speeds of 5 and 7.5 m/s do not result in the same extent of mixing. We also examined the role of wind in an episodic storm event in August 2003. The strong SE wind from 23 to 26 August strongly mixed the water column. The moderate to weak NE wind during 16–20 August and the spring tide also contributed to the vertical mixing.

Spatial-temporal variability of total and size-fractionated phytoplankton biomass in the Yangtze River Estuary and adjacent East China Sea coastal waters, China

The spatial and seasonal dynamics of total and size-fractionated phytoplankton biomass (chlorophyll- a) as well as physical and chemical factors in the Yangtze River Estuary and adjacent East China Sea coastal waters were investigated from April 2002 to February 2003. Average surface total and water column integrated chlorophyll a biomass showed a clear seasonal variation in response to the Yangtze River discharge, with the highest in summer (∼4 mg m−3 and >60 mg m−2), intermediate in spring and autumn (∼1 mg m−3 and 26–28 mg m−2), and the lowest in winter (0.5 mg m−3 and <20 mg m−2). Summer maximum chlorophyll a concentrations (>10 mg m−3) occurred at intermediate salinities (∼20–30) region beyond the front zone between 112.5°E and 123°E with sufficient nutrients replenishment for phytoplankton growth. Generally, spatial distribution of size-fractionated phytoplankton showed that phytoplankton biomass was dominated by the large size fraction (>20 μm) in the turbid eutrophic estuarine and near-shore waters, while the small-sized phytoplankton (<5 μm) were dominant in the offshore stations. Phosphate was the main limiting nutrient of phytoplankton biomass in river diluted water and most near-shore stations, while dissolved inorganic nitrogen became the potential limiting nutrient in some offshore stations, except for summer when phosphate limited almost all in the whole investigation region. Controlling the inputs of phosphate loading from the Yangtze River is one of the most effective strategies for reducing the increasing eutrophication and occurrences of harmful algal blooms in Yangtze River Estuary and adjacent East China Sea coastal waters.

Chemical–chemical interaction between cyanogenic toxicants and aldehydes: a mechanism-based QSAR approach to assess toxicological joint effects

A QSAR approach was proposed to assess toxicological joint effects based on the mechanism of chemical–chemical interactions between cyanogenic toxicants and aldehydes. It has been observed that the chemical–chemical interaction between cyanogenic toxicants and aldehydes resulted in the formation of carbanion intermediates, and therefore this interaction led to different toxicological joint effects between cyanogenic toxicants and aldehydes. Analysis of this chemical–chemical interaction showed that the formation of carbanion intermediate highly depended on the charge of the carbon atom in the –CHO of aldehydes and this of the carbon atom (C*) in the carbochain of cyanogenic toxicant. By using the Hammett Constant (sgr p) to measure the charge of carbon atom in the –CHO of aldehydes, a mechanism-based QSAR approach ( with M=sum of toxic units) was proposed to assess the toxicological joint effects between α-hydroxy-isobutyronitrile and individual aliphatic aldehydes. Another one ( with ) was also proposed to assess the toxicological joint effects between α-hydroxy-isobutyronitrile and individual aromatic aldehydes. Lastly, by using the charge of carbon atom (C*) in the carbochain of cyanogenic toxicant, a mechanism-based QSAR model ( with ) was derived to assess toxicological joint effects between p-nitrobenzaldehyde and cyanogenic toxicants.