Coral growth and erosion in Hong Kong

  • Yang James Xie

Student thesis: Doctoral Thesis


Coral ecosystems are highly diverse and productive ecosystems in tropical and subtropical oceans, playing a significant role in marine ecosystems. They have many important functions: a carbon sink in the global carbon cycle via calcification, habitats for many economically important species, acting as shoreline buffers, and a potential source of natural chemical substances of medical importance (Moberg et al. 1999). Growth and erosion are the two driving forces that determine the fate of a coral reef. Coral growth is achieved by calcification - the deposition of calcium carbonate skeleton by living coral polyps, and erosion refers to the removal of calcium carbonate by physical or biological factors. When calcification exceeds erosion, a reef is considered to be growing and vice versa. Hence, the study of this growth-erosion balance is the key to evaluating the health status of a reef. Hong Kong, as a marginal environment for coral survival has a remarkable diversity of coral communities in its waters. However, little is known about the calcium carbonate budget of these communities. My study thus aims to fill in this gap of knowledge in order to better understand and conserve these valuable communities. This study is timely given that many global and regional stressors are expected to affect coral calcium budget. The results of my study can contribute to a better understanding of how corals respond to environmental changes. This study aims to 1) explore any correlation between environmental factors and abundance of internal borers on corals; 2) study the growth rate of corals across different environmental gradients in Hong Kong; and 3) study the rate of erosion of corals across different environmental gradients across nine sites in Hong Kong. Field surveys were carried out at 33 sites from October 2012 to December 2012 covering two environmental gradients - from estuarine to oceanic and from sheltered to exposed. Two 50-meter transects were laid at each site and coral coverage and abundance of eroders per colony was determined using photo quadrants. Three sediment traps were also deployed at each site and collected after a month to determine sedimentation and nutrition deposition rate. Correlation analyses were conducted to explore any underlying relationships between borehole densities on corals and environmental factors. It was found that polychaete boreholes were significantly positively related to the amount of sedimentation. Also, the bioerosion of corals in Hong Kong was found out to be much more serious than that in other regions. From the 33 sites surveyed to determine borehole densities, 10 sites chosen to cover two environmental gradients were selected for more detailed studies of coral growth. Three colonies of Porties lutea of around 20 cm x 20 cm x 20 cm were collected from each site, and were cut into 1cm slabs parallel to the direction of maximum growth. X-ray radiography was done for each slab to analyze the growth rate. The growth of Porties lutea across the 10 sites were compared against other regions and underlying relationships with environmental factors were explored. It was found that the growth of corals was negatively correlated with sedimentation rate, and the calcification rates of corals in Hong Kong were much lower than those reported from many study conducted in tropical regions. To understand the rate of bioerosion of corals in Hong Kong, a study was conducted by deploying experimental coral skeleton blocks at nine chosen sites. Three blocks were deployed at each site. Blocks were retrieved after one year and scanned with MicroCT to examine the contribution on internal bioerosion by different taxa as well as the total amount of bioerosion at each site. The data were analyzed to understand internal how bioerosion is determined by environmental factors. It was found that bioerosion contributed by polychaetes had positive correlation with the sedimentation rate, which was consistent with the results found in the forth-mentioned study of coral slabs. The internal bioerosion rates of corals in Hong Kong were within the range of the corresponding data reported from overseas.

Date of Award11 Jan 2017
Original languageEnglish
SupervisorJianwen QIU (Supervisor)

User-Defined Keywords

  • China
  • Coral bleaching
  • Hong Kong.

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