Man-made mudflat is not common in Hong Kong but it can be created to compensate the loss of similar natural habitats owing to urban developments. A case in point is the development of the Tin Shui Wai New Town, in which about 1 ha of mudflat was re-established as an ecological mitigation area within the Hong Kong Wetland Park for attraction of shorebirds and waterfowl. However, it is important to assess whether or not this man-made mudflat can function as expected with settlement and colonization of animals, which, in turn, form the food source for the shorebirds/waterfowl. A preliminary study was conducted to determine the existing situation of the man-made mudflat. This entailed a spatial sampling of sediments at 26 grid points (20 m × 30 m apart) on the mudflat using plastic cores (H: 10 cm, D: 8 cm). At each point, one core sample was collected for analysis of macrobenthic infauna (size >0.5 mm) and sediment physio-chemical characteristics, including particle size, moisture content, total Kjeldahl nitrogen (TKN), total phosphorus (TP), and total organic content (TOC). From all these cores, a total of 8 macrobenthic species were recorded, comprising Amphipoda, Oligochaeta, Polychaeta and Gastropoda. Particle size (median diameter), moisture content, TKN, TP and TOC of the mudflat sediments ranged from 0.05 mm - 0.125 mm, 27.4% - 40.8 %, 0.47-1.38 mg/g dw, 0.22-1.15 mg/g dw and 0.11-2.42%, respectively. Based on the results of the preliminary study, 5 locations representative of the mudflat were selected for further monthly monitoring. At each location, 4 core samples were collected, 3 for macrobenthos analysis and 1 for physio-chemical analysis over a period of one year. A total of 13 macrobenthic species and 8,703 individuals were recorded, including Amphipoda, Oligochaeta, Polychaeta and Gastropoda. Results of Shannon-Wiener diversity (H’) from the pooled core data per location showed an increasing trend during the monitoring period. However, the community diversity has not yet reached to its maximum at the end of the study. Results of non-metric multidimensional scaling analysis also revealed seasonal variations within macrobenthic community in the mudflat. The above field study showed that H’was significant positively correlated with TOC and TP (p< 0.01). An experiment was thus conducted to examine the response of macrobenthic community to addition of nutrients using slow-release gypsum matrix fertilizer stakes containing nitrogen (N) and phosphorus (P). Three treatment groups: 0× (control), 2× (15.86 g N m-2, 12.34 g P m-2) and 8× (68.76 g N m-2, 51.13 g P m-2) of background nutrient concentrations were set up, in which calculated amount of fertilizer stakes was applied to 108 plastic trays (H: 7.4 cm, L: 27.4 cm, W: 20.7 cm) containing defaunated sediment. These trays were randomly deployed on the mudflat over a period of one year. Two series of experiments were conducted: (1) three trays from each treatment were retrieved at monthly intervals from the deployed trays, and (2) three trays from each treatment were retrieved at bimonthly intervals and were replaced by new defaunated sediment trays at each sampling. Sediment trays were brought back to the laboratory for analysis of macrobenthic infauna and physio-chemical parameters, including in situ measurements of redox and pH, and laboratory analysis of sediment particle size, chlorophyll a, TKN, TP and TOC. Results of analysis of variance on the physico-chemical data in the series 1 experiment showed that there was significant difference (p<0.05) among the 3 treatment groups. Higher chlorophyll a concentration and lower redox potential were noted in the 8× treatment than in the 2× and control groups. The increase in chlorophyll a content in the 2× and 8× treatments implied that additional nutrients enhanced primary production. Species number and H’ from 3 replicates in the 2× treatment group was higher than that in the control, which was, in turn, higher than the 8× treatment. The low redox potential in the 8× treatment suggested that a reducing state with lower oxygen content was developed, result of which might cause stress on the colonization of macrobenthos in the experimental trays. Results of the series 2 experiment showed that there were no significant difference among the 3 treatments in terms of S, N, H’ and J and suggested that the availability of species in settlement would depend on the time when larvae are abundant. Results of non-metric multidimensional scaling analysis also revealed seasonal variations of the initial settlement. The present findings suggested that the timing of initial colonization can play a key role in determining the subsequent succession of the macrobenthic community. To conclude, mudflats are critical component of coastal ecosystems, providing a foraging ground for shorebirds and waterfowl. Results of present experiments showed that colonization of macrobenthos can successfully occur in a man-made mudflat over time, and even be enhanced by addition of proper amount of nutrients. The present findings thus have implications on the re-establishment of ecologically important habitats.
| Date of Award | 14 Jul 2006 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Kam Shing Paul SHIN (Supervisor) |
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