This study investigates the role of present day crustacean bioerosion and bioturbation in the development and taphonomy of a muddy coral reef environment (Phuket, South Thailand). At the main locality, a 200 m wide, 400 m long, intertidal fringing reef flat drops 3 m on to a muddy sea bed where fine surface sediments comprise 80% terrigenous clays. Adjacent intertidal sand waves and tidal pools are composed of gravel - to sand-size sediments. <I>In situ</I> experimentation involved transect and quadrat surveys, resin casting and airlift excavation of burrows, coring, measurement of sediment turnover rates and direct observation of sediment mixing using tracer sediment. Grain size analysis, carbonate content determination and microscopy studies of sediments, borers and coral growth patterns were carried out in the laboratory. Crustacean bioerosion, especially 'passive' boring by hapalocarcinid crabs, pyrgomatid barnacles and alpheid shrimps, alters coral colony morphology which in turn affects reef development. Reef progradation relies on the continued growth of coral boulders that have fractured along large upogebiid and alpheid boreholes and then toppled from the reef front on to the fore-reef slope. Bioerosion also results in the production of calcareous sediment. Active erosion by upogebiid and alpheid shrimps directly produces grains in the gravel to silt size range. Coral colonies weakened by boring are prone to physical disintegration into boulder- to silt-size fragments. Crustacean bioturbation, mainly by callianassid and alpheid shrimps and <I>Dotilla</I> crabs, is abundant in both intertidal and subtidal reef sediments. The subtidal sediment slope shows an offshore zonation of crustacean burrowers. Simple, sloping burrows of near-reef alpheids give way to deep, spiral, off-reef alpheid burrows and then to complex, Thalassinoides-type burrow networks of callianassids offshore. Burrow distribution seems to be related to the grain size and sediment thickness of the areas they inhabit. Tiered burrows, in densities of up to 350/m<SUP>2</SUP>, were measured subtidally. Depths of turnover of up to 25cm (subtidal callianassid and alpheid shrimps) and turnover rates of up to 0.4 m<SUP>3</SUP>/m<SUP>2</SUP>/year (550kg/m<SUP>2</SUP>/year (550kg/m<SUP>2</SUP>/year; <I>Dotilla</I> crabs) were calculated. Subtidal bioturbation rates are three times the rate of sediment accumulation on the fore-reef slope. Biogenic sorting of sediment occurs, with subtidal callianassids burying coarse grains in shelly pockets at depths of >25cm, maintaining a relatively gravel-free surface layer. In contrast, intertidal and near-reef alpheids sort grains to produce a heterogeneous distribution of gravel patches exposed at the sediment surface.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641877 |
| Date | January 1997 |
| Creators | Bradshaw, Clare |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15236 |
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