Keywords coral reefs reef flat sea level change biogeomorphology Hainan Island
1. INTRODUCTION
Coastal (dynamic) geomorphology study focuses the three main modern processes: dynamic, sedimentary and geomorphologic, and their interaction and combination. Study for coastal biogeomorphology in biological coast (mainly with mangroves and coral reefs) should add indispensable fourth coastal biological process. The coastal biogeomorphologic process is one of the main responses of the coastal ecosystem to the global changes, and has been emphasized in Focus 2 of the Land-Ocean nteraction in the Coastal Zone (LOICZ) of the Global Changes. Coral reefs are the most typical growing landform, even though with minor dynamic modification, and become the best research objective for biogeomorphology study. This research takes the reef flat of fringing reefs in Shanya of Hainan Island as case study. We would like to get better understanding of characteristics of biogeomorphologic process of coral reefs and their response to environmental changes, like sea level change, and some ideas about management and conservation of coral reefs from this research.
2. THE STUDY AREA
Shanya fringing reefs in southern coast of Hainan Island is one of the best development fringing reefs in Hainan Island of China and became the National Coral Reefs Natural Protective Area in 1990. The study area is on the west coast (Luhuitao) and the east coast (Xiaodonghai) of Luhuitao tombolo based on raised coral reefs. Many research works about this area have done. They found the 4-5m highest sea level in 5000-6000a.BP forming raised coral reefs and that is important background for reef flat development in this area. Hermatypic coral taxa composition consists of 12 family, 24 genus, 83 species (ZOU Relin,1975) and 10 family,21 genus, 58 species (YU Dengpan,1995). Some species have been disappeared during the past 20 years. Dominant taxa are Porites lutea, Goniastrea aspera, Platygyra crosslandi in importance order. Wave climate is depends on monsoon and typhoon. The winter NE wind and waves is longer and stronger than the summer S wind and waves. Typhoon and storm waves occur in July-Nov. The tide is irregular diurnal type, the mean tidal range 0.9m and largest one 2.14m. Annual mean surface sea water temperature is 26.4* and Jan. mean temperature 22.5*. Sea water salinity is 29.9-34.2%o. Sea water transparency is about 10m.
3. THE MATERALS AND METHODS
6 transects were set up, 5 of which on the Luhuitao and 1 on Xiaodonghai. The survey items along each transect include biological community structure (corals, algae, sea grass etc); hydrodynamics (tide level, wave, etc); sedimentation (sediments, etc) and geomorphology (leveling, etc). We can exactly determine the relationship between reef flat and tidal levels using leveling survey data and tide observation data
4. RESULTS AND DSCUTON
4.1. Up limit of tidal elevation for living flat coral are 15-20cm above Theoretical Lowest Tide (Chart Datum in China and 90cm below the Mean Sea Level in Shanya waters) on Luhuitao leeward and low wave energy coast, and 37cm above Theoretical Lowest Tide on Xiaodonghai windward and high wave energy coast. These tidal elevations are controlling factor for reef flat up-growing and that is theoretical bases for reef flat as an indicator for sea level change.
4.2. On Luhuitao leeward and low wave energy coast the reef flats are with gentle sloping profile about 2‰ grade and with spreading weathered original head corals, that means it is a growing form responding to falling sea level. This accords with exist of mid-Holocene highest sea level.4.3. On Xiaodonghai windward and high wave energy coast the reef flats are with higher and horizontal profile and with wave spreading reef blocks. Two reasons for its deference from that on Luhuitao. Both storm wave spreading reef block and a litter higher up limit of living flat corals make the outer flat higher then that on Luhuitao. This is a growing form responding to falling sea level but with modification of high wave energy.
4.4. In atolls of Nansha Islands some reef flats have a lower (about 20cm above Theoretical Lowest Tide which is 100cm below the Mean Sea Level in Nansha waters) and horizontal profile with spread living corals. These reef flats are growing form responding to stable sea level.*Project No. 49776303 financed by National Natural Scientific Foundation of China.