REPORT A. S. Ogston Æ C. D. Storlazzi Æ M. E. Field M. K. Presto Sediment resuspension and transport patterns on a fringing reef flat, Molokai, Hawaii Received: 20 May 2002 / Accepted: 25 September 2003 / Published online: 30 July 2004 Ó Springer-Verlag 2004 Abstract Corals are known to flourish in various turbid environments around the world. The quantitative dis- tinction between clear and turbid water in coral habitats is not well defined nor are the amount of sediment in suspension and rates of sedimentation used to evaluate the condition of reef environments well established. This study of sediment resuspension, transport, and resulting deposition on a fringing reef flat off Molokai, Hawaii, uses a year of time-series data from a small, instru- mented tripod. It shows the importance of trade winds and ocean wave heights in controlling the movement of sediment. Sediment is typically resuspended daily and the dominant controls on the magnitude of events (10–25 mg/l) are the trade-wind-generated waves and currents and tidal elevation on the reef flat. The net flux of sediment on this reef is primarily along the reef flat in the direction of the prevailing trade winds (to the west), with a secondary direction of slightly offshore, towards a zone of low coral abundance. These results have application to reef studies and reef management in other areas in several ways. First, the observed resuspension and turbidity results from fine- grained terrigenous sediment that appears to be trapped and recycled on the reef flat. Thus corals are subjected to light attenuation by the same particles repeatedly, however small the amount. Secondly, the measurements show high temporal variability (from daily to seasonal scales) of sediment resuspension, indicating that single measurements are inadequate to accurately describe conditions on a reef flat. Keywords Suspended sediment Æ Turbidity Æ Coral reefs Æ Sedimentation Æ Reef flat Introduction Sedimentation and suspended sediment are generally recognized as important factors that can negatively affect coral health (Buddemeier and Hopley 1988; Acevedo et al. 1989; Rogers 1990; Fortes 2000). Conversely, some workers (e.g., Roy and Smith 1971; Tudhoe and Scoffin 1994; Anthony 2000; Larcombe et al. 2001) have shown that coral communities can flourish in turbid environ- ments. Terrestrial sediment delivered to reef waters at minimal levels can provide nutrients to corals and en- hances growth (Anthony 2000), while higher levels of suspended sediment may attenuate light, and sediment accumulation can eliminate recruitment sites, physiolog- ically stress individual corals, and even bury coral colonies (e.g., Rogers 1990; Brown 1997). Although there is evi- dence that increased sediment supply can adversely affect coral reefs (e.g., Cortes et al. 1994; Field et al. 2000; Szmant et al. 2000), the precise meaning of ‘‘sediment supply,’’ as Woolfe and Larcombe (1998) point out, is open to a number of interpretations. A rule-of-thumb in reef environments has been that suspended sediment concentration (SSC) of greater than 10 mg/l may attenuate enough light to adversely affect coral health (Dodge et al. 1974; Dodge and Vaisnys 1977), and that accumulation rates of greater than 10 mg/cm 2 /d are associated with fewer coral species, less live coral, and decreased net productivity (Rogers 1990; Te 1992). There is a range of turbid environments in which active coral growth exists. Issues such as the fre- quency and duration of suspension events and sediment type must also be evaluated in determining impact on reef health. Sedimentation due to increased sediment runoff from up-slope practices has been identified as a chronic problem affecting reefs throughout the Hawaiian Islands (Gulko et al. 2000), and specifically for areas on Molokai Communicated by: P.K. Swart A. S. Ogston (&) Æ M. K. Presto School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195, USA E-mail: ogston@ocean.washington.edu Tel.: +1-206-5430768 Fax: +1-206-5436073 C. D. Storlazzi Æ M. E. Field U.S. Geological Survey, Pacific Science Center, 1156 High St., Santa Cruz, CA 95064, USA Coral Reefs (2004) 23: 559–569 DOI 10.1007/s00338-004-0415-9