Eutrophication Dynamics in Hong Kong Coastal Waters: Physical and ...

CHAPTER 13

EUTROPHICATION DYNAMICS IN HONG KONG COASTAL WATERS: PHYSICAL AND BIOLOGICAL INTERACTIONS

JOSEPH H.W. LEE, PAUL J. HARRISON, CUIPING KUANG, AND KEDONG YIN 1. INTRODUCTION 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 China's 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 m3 s-1, 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.

187 E. Wolanski (ed), The Environment in Asia Pacific Harbours, 187–206. © 2006 Springer. Printed in the Netherlands.

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Figure 1b. Bathymetry of Hong Kong waters (inside the dashed line). Inset figures are the seasonal distribution of red tides in Hong Kong during 1975-2003 (top) and predicted water level during March to April 1998 at station ELM1 in East Lamma Channel (middle inset).

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Rapid urbanization and industrialization has taken place during the past several decades in the Pearl River Delta, and in Hong Kong; the PRD region is now one of the largest and fastest growing manufacturing bases in the world. The total population in the Delta including Hong Kong and Macau is now about 35 million people, and over 100 million people live in the entire watershed of the Pearl River and this has increased the potential for eutrophication impacts. However, these impacts are much less than expected, given the relatively large nutrient and organic loads that are received by these waters. Over the last three decades, nitrate concentrations in the river have increased two to three times (up to >100 µM during high river discharge in summer), while phosphate has remained relatively constant at about 1 µM (Yin 2002). Up to 2001, most of Hong Kong’s sewage only received preliminary treatment and the sewage was discharged through a number of submarine outfalls in Victoria Harbour and surrounding waters, resulting in high nutrients and BOD loads. In 2002, as part of the Harbour Area Treatment Scheme (HATS), a central sewage tunnel collection system and chemically enhanced primary treatment (CEPT) plant came into full operation at Stonecutters Island (Figure 1a). Currently 70% of the sewage receives CEPT with about 7 tonnes of sludge removal per day and the treated sewage is discharged through a short outfall. There are two main sources of nutrients for Hong Kong waters: (i) nitrogen loads from the Pearl River and non-point sources in local catchments that fluctuate seasonally with large loads in summer; and (ii) relatively constant inputs of nitrogen from sewage discharge in Victoria Harbour (Li et al., 2003; Yin and Harrison in press). Figure 2 shows the monthly variation of total inorganic nitrogen (TIN) at a number of stations from west to east (N6, W4, V8, E1, MM8; see Figure 1b); the increase in TIN due to the Pearl River flow in the wet season is apparent at Stations N6 and W4. The nitrogen input from sewage discharge into Victoria Harbor is evident at Stations V8, V6 and V2 (Figure 2). The potential eutrophication impacts of concern in Hong Kong are excessive algal blooms leading to low dissolved oxygen in the bottom waters, beach closures, and red tides. The concern over red tides (colored waters) is mainly associated with fish kills (likely due to low oxygen stress) and there is less concern associated with toxic red tides (Lee et al., 1991b). For example, in April 1998, a devastating red tide due to the dinoflagellate Karenia digitatum resulted in the worst fish kill in Hong Kong’s history and the estimated loss was more than HK$ 312 million (Dickman, 1998; Yang et al., 2000). An analyses of long term data (1980-2002) obtained by the Hong Kong Environmental Protection Department (EPD, 2003) showed that the maximum number of red tides occur in spring (March-April; see Figure 1b) and these are mostly dinoflagellates such as Noctiluca that occur in eastern waters such as Mirs Bay (Yin, 2003). The lowest number of red tides occurred in the western waters, near the Pearl River Estuary (PRE). Therefore, most red tides occur in spring when the Pearl River discharge is relatively low (temporal disconnect from the PRE) and in the eastern waters far from the PRE (spatial disconnect). The dominant red tide species are Noctiluca scintillans, Mesodinium rubrum, Gonyaulax polygramma, Skeletomema costatum, Prorocentrum minimum, Ceratium furca,

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Prorocentrum triestinum, Thalassiosira Prorocentrum sigmoides (Yin, 2003).

spp.

Scrippsiella

trochoidea,

and

Figure 2. Annual cycle of total inorganic nitrogen (TIN) at various stations (see Figure 1b for station locations), with N6 and N2 representing western waters, V8, V6 and V2 (between the vertical lines) representing Victoria Harbor and MM8 representing more coastal/oceanic conditions in eastern waters (from Yin and Harrison in press).

An extensive water quality monitoring program has been in operation since 1982 and this has provided a very valuable data set for characterizing the eutrophication dynamics of the Hong Kong waters. The Environmental Protection Department has divided Hong Kong waters into 10 water quality zones (WQZ) with each zone having its own set of beneficial uses and water quality standards. Monthly sampling for a wide range of water quality parameters is routinely carried out by EPD at 86 monitoring stations. The Agriculture, Fisheries and Conservation Department (AFCD) monitors fish and shellfish culture zones and marine conservation sites for


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phytoplankton species, with an emphasis on red tides. These red tides are usually detected when the public reports coloured water and then a sample is obtained to determine the phytoplankton species involved. Therefore visual red tides such as Noctilua that are bright red (and occur as a bloom right at the surface) may be reported more frequently than high biomass diatom blooms that do not have a marked visual appearance. While the long-term data sets from EPD and AFCD have provided excellent information on seasonal and inter-annual trends of water quality, there have been very few observations on the temporal and spatial variability of algal blooms in the sub-tropical Hong Kong waters. We have a limited understanding of the importance of physical factors such as monsoon winds, tidal cycles, rainfall and stratification that affect algal bloom dynamics. In particular, we have little information about how short-lived episodic events triggered by hydro-meteorological factors such as storms and typhoons, longer-lived El Nino events and inter-annual and inter-decadal variations in river discharge (e.g. due to climate change) control the environmental assimilative capacity of Hong Kong waters. This review provides an insight into the complex temporal and spatial dynamics of physical, chemical and biological factors that influence the capacity of the Pearl River Estuary to assimilate anthropogenic inputs such as nutrients with surprisingly minor eutrophication impacts. This review will focus on Hong Kong waters in general, and specifically on Victoria Harbour and southern waters. It will exclude the infamous Tolo Harbour, which is a poorly flushed tidal inlet with a high incidence of red tides (Wear et al., 1984; Ho and Hodgkiss, 1991; Lee et al., 1991a,b; Hodgkiss and Ho, 1997), especially in the 1980s before nutrients were exported away from the Tolo catchment into Victoria Harbour. Based on long term water quality monitoring data and several intensive field and modeling studies, an integrated understanding of eutrophication dynamics in Hong Kong waters is beginning to emerge. A model is presented that shows how various factors interact to make the Hong Kong waters more robust to eutrophication impacts than one would otherwise expect, based on the high nutrient concentrations in these waters. Section 2 reviews the dry and wet season physical hydrography, including tidal characteristics, tidal currents, salinity structure, vertical mixing, dilution, horizontal net transport, and monsoon winds. Sections 3 and 4 document physical and biological interactions on two different time scales, the seasonal (dry and wet) cycle and episodic events such as the hydrodynamic tracking of the massive red tide in 1998. This review concludes by emphasizing the point that algal bloom dynamics in estuaries cannot be understood without a thorough understanding of coastal hydrodynamics and its coupling with phytoplankton ecology. This information has implications for ecosystem management. 2. PHYSICAL HYDROGRAPHY Physical processes are driving forces for biological processes. The general physical hydrography, flushing rates and algal bloom transport patterns in Hong Kong waters can be elucidated using a calibrated three-dimensional hydrodynamic model

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(Delft3D) for Hong Kong waters and the Pearl River Estuary. The Delft3D model solves the shallow water equations based on the hydrostatic approximation and a standard k-H model for turbulence closure (Delft Hydraulics, 1998); the model has been extensively validated against field data and numerical experiments (Postma et al., 1999; Lee and Qu, 2004). The computational domain (shown in part in Figure 1a) includes eight Pearl River outlets (Humen, Jiaomen, Hongqili, Hengmen, Modaomen, Hutiaomen, Aimen and Jitimen), Hong Kong waters (Deep Bay, Victoria Harbour, Lamma Channel, Mirs Bay), and part of the South China Sea. Along an open boundary, the water level is specified by a time history of water levels using nine tidal constituents (O1, P1, K1, N2, M2, S2, K2, M4, MS4) derived from long-term data; the mean sea level is calibrated against observed monsoondriven currents in the open sea. Salinity boundary conditions for the dry and wet seasons are derived from long term measurements. The seasonal mean freshwater flows of 4120 m3 s-1 and 19,422 m3 s-1 are prescribed at the inflow boundary for the dry and wet seasons respectively. A constant NE wind (5 m s-1) is assumed for the average dry season, and a SW wind (5 m s-1 ) for the average wet season. The average grid size varies from about 100 m in Victoria Harbour to 7 km in the southeast boundary. A time step of 't = 2 min is adopted. The model is run from an initial state obtained by running the model for two months from a cold-start to get reasonable initial salinity distributions in both horizontal and vertical directions. Further model implementation details can be found elsewhere (Delft Hydraulics, 1998). 2.1. Tidal Currents The hydrography of Hong Kong waters is mainly influenced by three factors: tidal currents, the Pearl River discharge, and monsoon-induced coastal currents. The general character of the tide can be expressed by the F-factor (ratio of tidal amplitudes for (K1+O1)/(M2+S2). In general, tides would be considered semidiurnal if F < 0.25 and diurnal if F > 3.0 (Bowden, 1983). In Hong Kong, F varies from 0.89 in Deep Bay (Tsim Bei Tsui) on the west to 1.27 in Mirs Bay (Kau Lau Wan) in eastern waters (Lee, unpublished; http://www.hko.gov.hk/tide/etide ). Since 0.5 < F < 1.5, tides in Hong Kong can be characterised as mixed and mainly semidiurnal. The mean tidal range is 1.7 m; corresponding values for spring and neap tides are typically 2.0 and 1.0 m respectively. A typical spring-neap tidal variation in East Lamma Channel is shown in Figure1b (inset); it can be seen that the tide can vary from semi-diurnal tide (late March) to practically diurnal tide (early April) within a spring-neap cycle. Tidal currents are determined by the interaction of ocean tides with the local topography and bathymetry; in general, the flow is from E/SE to W/NW through Victoria Harbour and East Lamma Channel which are deeper than the surrounding area, up towards the Pearl River estuary during flood, and from W/NW to E/SE during ebb. Figures 3a and 3b, and animation 1a, show the typical surface flow field during flood and ebb in the dry season. In the main tidal stream, significant peak surface velocities can be found in the northwestern waters north of Lantau Island (up to 2 m s-1 in narrow channels). During spring tide, the peak ebb velocity in Victoria


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Harbour is typically about 0.35 m s-1 near Stonecutters Island, 0.85 m s-1 in central Victoria Harbour, and increases to about 1.05 m s-1 at the eastern harbour entrance; the peak flood velocity is generally smaller at about 70-90% of ebb velocities.

Figure 3. General surface flow during: a) flood (HHW-3 hr); and b) ebb flow (HHW + 4 hr) in the dry season; and c) flood (HHW-3 hr); and d) ebb flow (HHW + 4 hr) in the wet season.

During neap tide, the peak flood and ebb surface velocities are about 77-86% of the spring tide values. The tidal velocity decreases towards the eastern waters; peak ebb velocity in Mirs Bay is about 0.2-0.3 m s-1 near the entrance, and 10-15 Pg l -1) and only very episodic events of hypoxia occur in late summer (Yin et al., 2004a). Recent field observations and modelling studies have facilitated an integrated understanding of eutrophication impacts. In this paper, we have given a general overview of temporal and spatial patterns of algal blooms in Hong Kong waters. Key features of the estuarine hydrography are summarized, and algal dynamics have been shown to depend strongly on physical and biological interactions. Physical processes drive biological processes. Despite the fact that nutrient concentrations are generally above the threshold required to trigger a bloom, large-scale and sustained blooms are not a common occurrence. First, tidal flushing in Victoria Harbour and most of Hong Kong waters is efficient in both the dry and wet season. Any algal species introduced into a given water body (e.g. from ship ballast water) will not have sufficient time to develop into a bloom before one of the growth limiting factors reduces growth (e.g. favourable hydro-meteorological conditions may not prevail). Second, wind and tidal mixing (barotropic and baroclinic) can mix phytoplankton cells downwards out of the photic zone, and discourage the formation of blooms. High light extinction due to high sediment concentrations from the river in summer is another impediment to bloom formation. Eutrophication impacts can also be traced to a number of causes ranging from a chance combination of wind and tidal conditions, to a possible El Nino influence (Yin et al., 1999) and variability of nutrient limitation across the PRE. However, potential impacts in summer may be reduced by upwelling since the nitrogen-rich surface water from the PRE mixes with and is diluted by the upwelled water with relatively lower nitrogen concentration. This effect of upwelling is quite the opposite in other areas where upwelling increases the nitrogen concentration in the surface layers. In general, intense algal blooms can develop in semi-enclosed and poorlyflushed tidal inlets with good water transparency and long residence times, and under low wind conditions. Other factors that require further study may include the effects of temperature and turbulence on algal growth, algal sedimentation, and zooplankton grazing. 6. ACKNOWLEDGEMENT The work reported herein was supported by a Hong Kong Research Grants Council (RGC) group research project (1/02C), and partially by a grant from the University Grants Committee of the Hong Kong Special Administrative Region, China (Project No. AoE/P-04/04 and P-04/02) to the Area of Excellence in Marine Environment Research and Innovative Technology (MERIT) and a RGC grant (Project 6296/03M). 7. REFERENCES Bowden, K.F., 1983. Physical Oceanography of Coastal Waters, Ellis Horwood, 302 pp. Choi, K.W., Lee, J.H.W., 2004. Numerical determination of flushing time for stratified waterbodies. Journal of Marine Systems 50, 263-281.


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