Mai Po Research Articles

Identification of environmental plasmid-bearing Vibrio species isolated from polluted and pristine marine reserves of Hong Kong, and resistance to antibiotics and mercury

Fifty environmental isolates of Vibrio species were isolated from water samples of Mai Po Nature Reserve and the Cape d'Aguilar Marine Reserve in Hong Kong and screened for the presence of plasmid. Mai Po is a wastewater-impacted area while the Cape d'Aguilar Marine Reserve is pristine natural marine water. Plasmid was found in Vibrio isolates from both sites at similar frequencies and each site showed distinctive plasmid profiles. These plasmid-bearing Vibrio isolates were identified as different species of the Vibrio genus by both biochemical test and subsequently full-length 16S rRNA sequences. Antibiotic resistance test showed that all these plasmid-bearing Vibrio isolates showed multiple resistance to 21 antibiotics tested. In addition, selective isolates also showed tolerance to 10 microM Hg 2+ in culture medium and they generally harbored large plasmid(s) (>30 kb). Our results show that the high frequency of plasmid in Vibrio species of both polluted and pristine environments may be ecologically important to the survival of these bacteria in the environment. The specific functioning of the cryptic plasmids remains the focus of current investigations.

Risk assessment of heavy metal contamination in shrimp farming in Mai Po Nature Reserve, Hong Kong

An ecological survey was carried out to determine the sediment concentrations of nutrients and heavy metals and bioaccumulation of heavy metals in fish and shrimp including tilapia (Oreochromis mossambicus x O. nilotica), grey mullet (Mugil cephalus), gei wai shrimp (Metapenaeus ensis) and caridean shrimp (Macrobrachium nipponensis) in the traditional tidal shrimp ponds (gei wais) of Mai Po Nature Reserve, Hong Kong. The sediments collected from the landward sites contained higher nutrient contents, as well as zinc (Zn), chromium (Cr), copper (Cu), nickel (Ni) and cadmium (Cd) than those collected from the seaward sites, but vice versa for lead (Pb) and mercury (Hg). However, the concentrations of all metals were exceptionally high in the two sites located outside the reserve, suggesting that waters from Deep Bay might be the possible source of metal contamination affecting the reserve. All metals studied seemed to accumulate in the viscera of fish. Body size was the determining factor for the accumulation of heavy metals in caridean shrimp and gei wai shrimp but not fish. Concentrations of the metals studied in tissues of grey mullet and gei wai shrimp were found to be safe for human consumption. Concentrations of Cr in tilapia whole body (0.68-1.10 mg kg(-1) wet weight) were close to or over the guideline value of 1 mg kg(-1) set by the Food Adulteration (Metallic Contamination) Regulations of Hong Kong. Tilapia flesh and small caridean shrimp collected from gei wais were contaminated by Cr and Pb but still fit for human consumption. Caution is required if large caridean shrimp is to be consumed in large amounts continuously because the concentration of Pb exceeded the maximum permitted concentration (6 mg kg(-1)). The rather high Cr concentrations in tilapia whole body should not be overlooked as the fish will serve as a food source for migratory birds visiting the site.

The Herbivore Assemblage, Herbivory and Leaf Chemistry of the Mangrove Kandelia obovata in Two Contrasting Forests in Hong Kong

The herbivore assemblage, intensity of herbivory and factors determining herbivory levels on the mangrove Kandelia obovata (previously K. candel, Rhizophoraceae) were studied over a 13-month period at two forests with contrasting growing conditions in Hong Kong. Mai Po was part of an eutrophic embayment in the Pearl River estuary and generally offered more favourable conditions for mangrove growth, whereas Ting Kok had a rocky substratum and oceanic salinity. Twenty-four insect herbivore species were recorded on K. obovata, with lepidopteran larvae that consume leaf lamina being the dominant species. While leaf litter production was similar at the two forests, herbivory level at Mai Po (mean = 3.9% in terms of leaf area loss) was more severe than that at Ting Kok (mean = 2.3%). Peak herbivory levels were found in summer at both locations (6.5% for Mai Po and 3.8% for Ting Kok). Young leaves of K. obovata at both locations were generally preferred by the herbivores from the period of late spring to summer. Concentrations of most feeding deterrents (ash, crude fibre, and total soluble tannins) were significantly higher in both young and mature leaves at Ting Kok, whereas leaf nutrients (total nitrogen and water) were the same at the two sites. Young leaves at Ting Kok contained about 30% more tannins than their counterparts at Mai Po. Significant differences in leaf chemistry also existed between young and mature leaves at either site. The differences were concomitant with the observed patterns of leaf herbivory on K. obovata, and suggest a potential relationship between environmental quality and plant defence against herbivory.

Organochlorine Insecticides in Mudflats of Hong Kong, China

Organochlorine (OC) insecticides were measured in different environmental compartments (including seawater, sediment, and biota) at six sites within the Mai Po Marshes Nature Reserve (MPMNR) and at one site in A Chau (AC) in Hong Kong in June 2002 (wet season) and January 2003 (dry season). MPMNR is a Wetland of International Importance (Ramsar site) situated in the western part of Hong Kong, whereas AC is a relatively remote island in Starling Inlet on the eastern side of the New Territories of Hong Kong. Generally, concentrations of OC insecticides in water, sediment, and biota were greater at MPMNR than at AC. Hexachlorobenzene, dichloro-diphenyl-trichloroethane, and hexachlorocyclohexanes were detected in all samples collected from MPMNR. Preliminary risk assessments were performed by comparing the measured concentrations of OCs in different environmental compartments with the corresponding guidelines promulgated by various environmental protection authorities. Concentrations of some OCs in sediments from certain locations in MPMNR were consistently greater than their corresponding sediment guideline values. These results suggest that the sediment OC concentrations in the sediments of MPMNR might pose a risk to aquatic organisms. Despite this, concentrations of OCs in biota samples were much lower than the relevant guideline values promulgated by government agencies to protect the health of human consumers. It is postulated that the management practices adopted at MPMNR have been effective in limiting the bioaccumulation window (duration) for OCs. This situation may have been aided by a frequent exchange of water between the tidal shrimp ponds at MPMNR and the open sea, again attributable to management practices. The low-cost maintenance practices adopted at MPMNR may potentially be applicable to wetlands in other southeastern Asian countries, thus allowing sustainable fishery production and wildlife conservation.

Residues of DDTs, PAHs and Some Heavy Metals in Fish (Tilapia) Collected from Hong Kong and Mainland China

Most of the freshwater fish consumed in Hong Kong is imported from the southern part of China, although Hong Kong also produces a small amount of freshwater fish. The major aim of the present study was to conduct a health risk assessment of fish produced from the mainland (purchased from four local markets in Hong Kong) compared with those produced from Hong Kong fishponds. Thirty tilapia fish (Tilapia mossambicus) were collected from the fishponds at Tam Kon Chau, Nam Sang Wai, and Mai Po Nature Reserve (Northwest New Territories of Hong Kong), and four local markets (Tai Po, Lok Fu, Sham Shui Po and Cheung Sha Wan wholesale markets) in Hong Kong for the analyses of PAHs, DDTs and three toxic metals (cadmium, chromium, lead). Tilapia purchased from the markets (with fish supplied from the mainland) showed higher concentrations of DDTs and PAHs than those collected from Hong Kong fishponds, confirmed that the fishponds in the mainland are more polluted due to the rapid socioeconomic growth in the region during the past 20 years. The tilapia were heavily polluted by Pb, with the highest level (3519 ng g− 1 wet wt.) exceeded the EUROPA guideline (400 ng g− 1 wet wt.) 8 times, and Cr with 36% of the samples exceeded the China guideline of 500 ng g− 1 wet wt. for human consumption. The coastal Chinese populations which consume larger quantities of fish may be at risk. Tilapia collected from the Mai Po Nature Reserve contained the highest level of Cr, which may also impose adverse effects on water birds, including the endangered species that frequently visiting the site for food and shelter.

A SEMIPARAMETRIC REGRESSION MODEL WITH MISSING COVARIATES IN CONTINUOUS-TIME CAPTURE-RECAPTURE STUDIES

Covariate data were missing when a semiparametric regression model was used to study bird abundance in the Mai Po Sanctuary, Hong Kong. This paper proposes an EM-type algorithm to estimate the regression parameters for that study. Analytical calculation of the expectation in the EM method is difficult, or even impossible, especially when missing covariates are continuous. A Monte Carlo method is used in the EM algorithm to ease the calculation complexity. Asymptotic variances of the parameter estimates are also derived. Properties of the proposed estimators are assessed through numerical simulations and a real example.

The residual dynamic of polycyclic aromatic hydrocarbons and organochlorine pesticides in fishponds of the Pearl River delta, South China

Hong Kong and South China are the most developed regions within China. The industrialization in these areas has resulted in severe environmental problems. Sediment and biotic samples including tilapia ( Oreochromis mossambicus), bighead carp ( Aristichthys nobilis), grass carp ( Ctenopharyngodon idellus), crucian carp ( Carassius auratus) and mandarin fish/fresh water grouper ( Siniperca chuatsi) were collected from different fishponds in the Pearl River Delta (Tanzhou, Sanjiao, Guangzhou, Shipai, Changan and Mai Po) for the analysis of polycyclic aromatic hydrocarbons (PAHs) and organochlorine (OC) pesticides. Mandarin fish, which belongs to the highest trophic level, accumulated the highest concentrations of PAHs and DDTs among all fish species. The levels of DDTs in fish samples ranged from 1.5 to 62 ng g −1 (wet wt.), with more than 30% of the fish samples exceeding the limit of 14.4 ng g −1 (wet wt.) for human consumption recommended by US EPA (2000). Levels of PAHs in fish samples ranged from 1.91 to 224.03 ng g −1 (wet wt.), but the potency-weighted total concentrations of PAHs in all muscle tissues were below the guideline value of 0.67 ng g −1 (wet wt.) for human consumption set by US EPA (2000). The guideline value calculated was based on a tissue consumption rate of 142.2 g day −1 (4–5 meals per week), which is a more protective rate for populations with a high consumption of fish, like Chinese and Asians. The effect of lipid content in PAHs and DDTs accumulation in fish tissue was not significant in general.

Estrogenic and Dioxin-like Activities and Cytotoxicity of Sediments and Biota from Hong Kong Mudflats

Persistent organic pollutants, such as organochlorine insecticides and polychlorinated biphenyls (PCBs), were measured in several environmental matrices including aerial deposition, seawater, sediment, and biota in two important coastal wetlands of Hong Kong, China. Specifically, samples were collected from within the Mai Po Marshes Nature Reserve (Mai Po), an internationally acclaimed wetland situated in the northwestern part of the New Territories of Hong Kong, and A Chau in Starling Inlet, a relatively remote island on the eastern side of Hong Kong. Hexachlorobenzene, dichlorodiphenyltrichloroethanes, and hexachlorocyclohexanes were detected in all samples collected from Mai Po. Environmental endocrine disruptors (including dioxin-like compounds and estrogenic chemicals), measured by the use of cell-based chemical activated luciferase expression assays, were found to occur at concentrations that might pose a risk to the ecologic systems in Mai Po. Dioxin-like PCBs were detected at small concentrations in some of the samples. Concentrations of 2,3,7,8 tetrachlorodibenzo-p-dioxin equivalents (TEQs) were primarily related to the relatively great concentrations (>100 ng/g dry weight) of high molecular-weight polycyclic aromatic hydrocarbons in sediments, whereas the relative proportion of TEQs contributed by nonortho-substituted PCBs was small. Polar compounds primarily contributed estrogen equivalents, which were measured in sediments. Significant concentrations of cytotoxic compounds were detected in fish samples collected from the Mai Po but not in fish collected from A Chau.

Influence of temperature, salinity and pH on the growth of environmentalAeromonas andVibrio species isolated from Mai Po and the Inner Deep Bay Nature Reserve Ramsar Site of Hong Kong

Four environmental bacterial isolates including Aeromonas hydrophila MP-3, A. salmonicida MP-4, Vibrio vulnificus MP-2 and V. cholerae MP-1 isolated from sediment and water of Mai Po Nature Reserve of Hong Kong were examined for their responses to temperature, pH and salinity under laboratory conditions in this study. V. cholerae MP-1 was found to resist vibriostatic agent O/129 at concentration of 10 microg/ml. In addition, bacterial growth under test conditions was measured and the results were fitted into the Gompertz model to obtain important parameters related to bacterial growth, lag time (lambda), specific growth rate (mu(m)), and maximum biomass (A) for comparison. V. cholerae MP-1 did not show any apparent growth at 15 degrees C, but was adapted to a much wider environmental pH from 5.2 to 9.2 for growth while V. vulnificus MP-2 was more sensitive to pH changes yielding the highest biomass at pH 6.2. A. salmonicida MP-4 was surprisingly tolerant to salinity as high as 60.0 per thousand NaCl and grew almost equally well as under conditions of other treatments. All four bacterial isolates showed a wide spectrum of plasticity to the environmental conditions and they pose a potential threat to public health and animal health.

Atmospheric Deposition and Fluxes of Organochlorine Pesticides and Coplanar Polychlorinated Biphenyls in Aquatic Environments of Hong Kong, China

Concentrations of organochlorine (OC) pesticides and coplanar (dioxin-like) polychlorinated biphenyls (PCBs) in bulk deposition were measured at the Mai Po Marshes Nature Reserve (MPMNR) and A Chau, which are both important habitats for waterbirds in Hong Kong. OC pesticides exempted from the Stockholm Convention were present in greater concentrations than those that have been restricted for use in the region. Among the OC pesticides, HCB, sigmaDDTs, and sigmaHCHs were detected. Concentrations of HCB were greater at MPMNR than at A Chau, and this finding suggests short-range transport of this compound, which is different from the other OC pesticides. Several environmental factors including seasonal variations in temperature, particulate matter, and rainfall may influence the flux of OC pesticides. Since sources of HCB often coexist with sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxins and furans), the presence of HCB may be a useful surrogate for monitoring airborne dioxins and for understanding their deposition potential. The contribution of atmospheric deposition to the OC pesticide input to the two study sites was small. Concentrations of most OC pesticides in surface waters were greater than would be predicted based on the inputs from atmospheric deposition and sedimentation. The mass balance calculation suggests a net flux of OC pesticides from bottom sediments to the overlying water column.

Keeping the Wetland Wet: how to integrate natural and cultural Heritage preservation

Discusses the relationship between tourism strategies and heritage conservation in Hong Kong, using the example of the wetland conservation site of Mai Po. The author describes the importance of the site as a resting place for migratory birds. Nevertheless, tourism is expected to grow rapidly in the region in the next decade, due to government policies favoring development. In recent years the number of visitors to the Mai Po site has been increasing, which could put the area at risk. Visitors need to be informed of the significance of environmental conservation. The author also explains how traditional fishpond farming contributes to wetland conservation. Residential development pressures are changing the fragile equilibrium and reducing the area given over to the fishponds.

Distribution and behavior of trace metals in the sediment and porewater of a tropical coastal wetland

Vertical profiles (0–30 cm below surface) of four trace metals—Cadmium (Cd), Chromium (Cr), Lead (Pb) and Zinc (Zn)—in the sediment and sediment porewater of an ecologically important intertidal mudflat in the Mai Po and Inner Deep Bay Ramsar Site were thoroughly studied over a period of 10 months (from March 1999 to January 2000). Two surveys, one in summer and another in winter, involving a total of eight sampling stations were conducted to study the seasonal variation of the remobilization characteristics of these trace metals in the mudflat sediment. The range of depth averaged concentration of these trace metals in the mudflat sediment was: 0.3–0.8 μg/g (Cd); 9.8–91.0 μg/g (Cr); 7.3–69.1 μg/g (Pb); and 39.5–192.0 μg/g (Zn), while that in the sediment porewater was: 0.3–121.1 μg/l (Cd); 3.0–2704.1 μg/l (Cr); 2.6–105.6 μg/l (Pb); and 32.6–4238.3 μg/l (Zn). In general, levels of dissolved trace metals in the sediment porewater were much higher in the summer than in the winter while their concentrations in the sediment were more or less the same throughout the year. Enrichment of Cd, Pb and Zn in the sediment porewater of the upper oxic layer and that of Cr in the oxic–sub-oxic boundary was generally observed. Regions in the vicinity of the Mai Po mangroves and the river mouths of Shenzhen River and Shan Pui River were found to be hotspots of trace metal pollution. Benthic diffusive fluxes of trace metals from the mudflat sediment were also estimated. Of the four trace metals, cadmium showed the greatest tendency toward remobilization from the sediment phase to the more bio-available porewater phase.

Screening of organochlorines in freshwater fish collected from the Pearl River Delta, People's Republic of China.

HCHs, DDTs, and PCBs were analyzed in fish flesh collected from fish ponds in the Pearl River Delta and compared with those from the Mai Po Marshes (a nature reserve) in Hong Kong, which was considered an unpolluted area. The concentrations for these contaminants were <0.01-7.8 ng/g lipid for HCHs, 22.3-381 ng/g lipid for DDTs, and 60-480 ng/g lipid for PCBs. Nevertheless, levels of HCHs and DDTs in fish flesh were lower than the China National Environmental Protection Agency maximum permissible limits set for foodstuffs. As for PCBs, 16% of fish collected from the Pearl River Delta exceeded the US EPA guideline concentration.

Spatial and temporal organic and heavy metal pollution at Mai Po Marshes Nature Reserve, Hong Kong

An intensive monthly sampling of water and sediments from 12 sites over 8 months covering wet and dry seasons at Mai Po Marshes Nature Reserve was conducted during June 1997–February 1998. Major organic (C, N and P) and heavy metal pollutants (Cd, Cr, Cu, Ni, Pb, Zn) water and sediment samples were examined. The results showed that Mai Po Marshes were severely polluted by organic matter and heavy metals, and the water from Deep Bay appeared to be the source of pollution. Up to 13–55% chance that the sediments of Mai Po Marshes were classified as moderately to seriously metal contaminated materials, according to the guideline set by Hong Kong Government. Empirical models describing organic matter and heavy metal spatial and seasonal dynamics in the water and sediments were formulated, based on data analysis. During wet season (June–October), more than 58% variations of total P can be explained by ortho-P in water, while ammonia-N explained up to 90% variations of total Kjeldahl nitrogen in water. Throughout the whole sampling period (June–February), there were significant correlations ( p<0.01) between total organic C in water. pH in the sediments and salinity in water appeared to be important factors determining heavy metal mobility in sediments, while potential metal release from the sediments is a concern when any oxidizing processes such as flooding or dredging are imposed on sediments.

Copper tolerance, uptake and accumulation by Phragmites australis.

Copper (Cu) uptake and accumulation of five populations of Phragmites australis growing in two mine sites (Parys Mountain, Wales, UK and Plombières, Belgium) contaminated with Cu and three 'clean' sites (Felixstowe, UK; Wisbech, UK and Mai Po, Hong Kong) were studied under field and glasshouse conditions. Cu tolerances in these populations of seedlings raised from seeds collected from the above five sites were also studied under glasshouse conditions. Although concentrations of Cu in the plant tissues (leaves, stems, rhizomes and roots) of P. australis and the associated soils from the Cu-contaminated sites were significantly higher than those of the plant tissues and the soils collected in the clean sites, small differences were found between the two Cu-contaminated populations and the three clean populations when seedlings were grown in 0.1 and 0.5 microg ml(-1) Cu treatment solutions. In general, different populations of seedlings showed similar growth responses, metal uptake and indices of Cu tolerance when cultured in the same Cu treatment solution for 3 weeks. There was insufficient evidence to support the hypothesis that the Cu-contaminated populations have evolved to Cu-tolerant ecotypes.

Risk to breeding success of fish-eating Ardeids due to persistent organic contaminants in Hong Kong: evidence from organochlorine compounds in eggs

Eggs of two Ardeid species, the Little Egret ( Egretta garzetta) and the Black-crowned Night Heron ( Nycticorax nycticorax), were collected from two egretries located in the New Territories of Hong Kong with one located near the internationally acclaimed wetland reserve, the Mai Po Marshes, and the other in a remote site (A Chau). The eggs were analysed for organochlorine (OC) compounds including the DDTs, PCBs, hexachlorocyclohexanes (HCHs) and the chlordanes (CHLs). All of the OCs under investigation were detected in the eggs of both species with significantly higher levels in the Little Egret (DDTs, 560–2200; PCBs, 270–1700; CHLs, 81–470 ng g −1 wet weight) than the Night Heron (DDTs, 210–1200; PCBs, 85–600; CHLs 59–75 ng g −1 wet weight). The DDTs consisted mainly of DDE with levels ranging from 85% to 95% of the total. The HCHs were at about the same levels in both species (8.4–30 ng g −1 wet weight). All of the OCs had linear concentration probability distributions on a log-normal basis which were used to evaluate exposure associated with these compounds as part of a probabilistic risk analysis. A linear dose/response relationship for the percentage reduction in the survival of young associated with DDE in eggs was developed. This probabilistic relationship was used to establish the threshold level (1000 ng g −1 wet weight) at which there was a significant level of reduction in the survival of young above zero and the variability in DDE concentrations at this effect level. Using a threshold level of 1000 ng g −1, the calculated Risk Quotient (RQ) had a 12.4% probability of RQ exceeding unity with the Night Heron, and 40.9% with the Little Egret. These results indicate that the DDTs in eggs would be expected to be associated with adverse effects on the survival of young of both species, particularly the Little Egret.

Distribution and sources of polycyclic aromatic hydrocarbons in the sediment of a sub-tropical coastal wetland

Sediment cores (0–35cm below surface) from twelve sampling stations in the Mai Po and Inner Deep Bay Ramsar Site of Hong Kong were retrieved in the period March–December 1999. Vertical profiles of 15 priority polycyclic aromatic hydrocarbons (PAHs) in each sediment core were determined. Ranges of total PAH concentration, [ΣPAH], in the wetland sediment were 0.18–0.83 (x̄=0.36)μg/g dried sediment (mudflats) and 0.63–0.96 (x̄=0.77)μg/g dried sediment (mangroves). A decreasing trend in depth averaged [ΣPAH] was observed from the landward end towards the seaward end of the Marshes. On the mudflats, vertical profiles of the PAHs were quite uniform. At the fringe of the Mai Po mangroves, significantly higher concentration of all PAHs was observed at the upper 0 to −8cm layer. No significant difference in the distribution patterns of the 15 priority PAHs in summer and winter was observed. This indicates that distribution of PAHs in the sediment of the Mai Po Marshes is not very sensitive to sub-tropical climatic changes of the region. Two PAH isomer ratios, [Phen]/([Phen]+[Anthra]) and [Pyrene]/([Pyrene]+[Fluoran]), were used to identify potential sources of PAH contamination in the wetland. Results revealed that local deposition is a more important source than long-range atmospheric transportation.

Polychlorinated dibenzo- p-dioxins and polychlorinated dibenzofurans in sediments from Hong Kong

Concentrations of 2,3,7,8-substituted polychlorinated dibenzo- p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were determined in 14 sediment samples collected from four sites in the Mai Po Marshes Nature Reserve (within a RAMSAR Site) and from another six sites in Victoria Harbour and along the Hong Kong coastline. Elevated levels of PCDDs, and particularly OCDD, were detectable in all samples collected from the Mai Po Marshes and five of the six sites. In contrast to PCDDs, PCDFs were mainly found in sediment samples collected from industrial areas (Kwun Tong and To Kwa Wan) in Victoria Harbour. PCDD/F levels and congener profiles in the samples from the Mai Po Marshes Nature Reserve in particular show strong similarities to those reported in studies which have attributed similar elevated PCDD concentrations to nonanthropogenic PCDD sources.

Risk to breeding success of Ardeids by contaminants in Hong Kong: evidence from trace metals in feathers.

The feathers of two Ardeid species, the Little Egret (Egretta garzetta) and the Black-crowned Night Heron (Nycticorax nycticorax) were collected from six egretries and two egretries respectively, located in different areas in the New Territories of Hong Kong, including the Mai Po Marshes (within a Ramsar site). These feathers were digested and concentrations (microg/g dry weight) of copper (4.6-19.4), iron (8.1-641.3), manganese (0.4-19.4), zinc (51.3-183.5), lead (0.1-5.1), cadmium (0.01-0.15), chromium (0.06-1.7) and mercury (0.0-7.1) were determined by ICP-AES, ICP-MS and CVAAS. The levels of manganese, mercury and lead found were equal to or less than the concentrations found in previous investigations, reflecting a slight downward trend most apparent with lead. As a general rule, the levels of lead and mercury were higher in the egretries close to the polluted Deep Bay. A probabilistic risk assessment of the possible adverse effects on the breeding success of the Little Egret was carried out with respect to mercury, lead and cadmium. It was concluded that mercury (0.5-7.1 microg/g dry weight feathers) probably has had adverse effects at the Au Tau egretry of the Little Egrets, but there was no evidence of adverse effects at other egretries. The probabilistic analysis also indicated a low likelihood of adverse effects of mercury on the breeding of the Black-crowned Night Herons at A Chau (0.3-1.2 microg/g) and Mai Po Village (0.0-1.4 microg/g). The evidence for the effects of lead and cadmium was limited but suggested there may possibly be adverse effects with lead but not cadmium.

Macrobenthos of a tidal impoundment at the Mai Po Marshes Nature Reserve, Hong Kong

The composition and spatial distribution patterns of the macrobenthic faunal assemblages of an 8-ha tidal impoundment operated as a traditional shrimp pond at the Mai Po Marshes Nature Reserve, Hong Kong, were studied in relation to temporal changes in local environmental conditions. Species richness, abundance and biomass of macrobenthos across 5 different sub-habitats (seaward, middle, and landward parts of open water unvegetated areas, and Phragmites- and Kandelia-dominated, vegetated areas) within the pond were examined bimonthly between January 1997 and January 1998. Grab samples were collected randomly within the sub-habitats. Key physical environmental parameters of the sampling sites were also measured. A total of 46 species of macrobenthos was recorded: 11 polychaetes, 11 molluscs, 13 crustaceans and 11 insects. Mean species density in the five sub-habitats ranged from 0 to 3907 indċm−2, with mean biomass ranging from 0 to 96.9 gċm−2. The macrobenthos showed spatial and temporal differences among the sub-habitats and across sampling times. Species abundances of Polychaeta, Mollusca and Crustacea were significantly higher in the three open water areas than in the two vegetated (Phragmites- and Kandelia-dominated) areas, with an inverse pattern for Insecta. There were no clear temporal patterns although abundance and biomass generally increased in the cooler months. Results of a canonical correspondence analysis suggest that macrobenthic species richness, abundance and biomass in the open areas were positively correlated with salinity, while water depth, dissolved oxygen and sediment organic matter content had little relationship with the macrobenthic assemblage parameters. Ordination by multi-dimensional scaling suggests that different habitats supported distinct macrobenthic assemblages. The macrobenthic assemblage in the tidal pond was less species rich but denser than those in the neighboring tidal mangrove and mudflat, suggesting that conversion of these areas into extensively managed tidal aquaculture ponds results in reduced species richness in tropical mangrove habitats.