IMPACTS ON SPATIAL SEDIMENT TRANSPORT PATTERNS ON A FRINGING REEF: MOLOKA'I, HAWAI'I

Coral reefs can be greatly affected by the input of terrigenous sediment due to the decrease in light available for photosynthesis, abrasion, loss of hard substrate, and direct burial. The fringing reef on the south coast of Moloka'i, Hawai'i is the focus of a large-scale project conducted by the U.S. Geological Survey to examine the processes and consequences of land-derived sediment on Hawaiian reefs. Molokai has one of the most extensive fringing reefs in the Hawaiian Islands, more than 50km long, with generally very high coral coverage on the forereef. Grazing and agricultural land use practices as well as the construction of Kaunakakai wharf in the 1930's resulted in an unstable watershed and modified circulation on the reef flat, creating the potential for large quantities of sediment to be delivered to and retained on the reef flat. Spatial surveys of sediment transport processes aimed at studying the effect of terrestrial sedimentation on the health of coral reefs were conducted to complement a 2-year, single point temporal study in the area of Kaunakakai. Spatial data were collected with a portable sediment dynamics "backpack" containing instruments to measure near-bed current velocities, suspended sediment concentration (SSC), wave orbital velocities, temperature, salinity, and mean water elevation at point locations on the reef flat.

Our measurements reveal a horizontal near-bed current and SSC structure that is primarily controlled by water depth, trade wind conditions, coastline orientation, and man-made structures (i.e. an impermeable wharf and native Hawaiian fishponds). The highest SSC were measured on the inner and central reef flat during high tide when there is the potential for increased propagation of offshore wave energy and generation of trade-wind waves to resuspend sediment. Most of the suspended sediment transport on the reef flat is in a westward alongshore direction, as a response to prevailing trade winds. The impermeable wharf acts as a barrier for sediment transport, halting the alongshore flow, trapping sediment, and creating an offshore flux component. Although a definitive cause for the decrease in coral coverage on the forereef has yet to be determined, the spatial patterns of sediment transport occurring on the reef flat may be linked to the processes on the adjacent forereef.