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Recent Flood-Derived Sediment Collected on Moloka'i's Coral Reef
One of the primary goals of this multidisciplinary study is to understand the sediment dynamics and to assess the impact of sediment on the health of this coral reef. Land use on Moloka'i has changed in the recent past, and the increased discharge of eroded sediment during infrequent but intense rainfall is a potential threat to the health of the reef. No rain fell during the first 10 months of the instrument deployments; however, kona storms (Hawai'ian storms characterized by strong southerly or southwesterly winds and heavy rain) in November 2001 and January 2002 brought wind and heavy rain from the south-southwest to the normally dry side of Moloka'i. The rain flooded intermittent stream channels, deposited boulders on the coastal highway, and turned the nearshore waters red with land-derived sediment. These pulses of new sediment provide an excellent opportunity to test our hypotheses about sediment-transport processes in this setting. Two time-series sediment traps were in place for these flood events. Modified for application in this wave-dominated, shallow reef environment, each trap consisted of a 20-cm (inner diameter) x 75-cm cylinder with an internal funnel that directed particles into one of 21 sample bottles. The bottles were on a carousel that rotated a new bottle under the funnel at programmed intervals (usually about 5 days) during a typical 100-day deployment. To prevent fish and other organisms from inhabiting the trap, a honeycomb-like baffle with approximately 1-cm-diameter cells was installed in the trap opening. The value of the baffle was demonstrated upon recovery of the support framework that contained an unbaffled pipe. To our surprise, when the framework landed on deck, a small but aggressive moray eel squirmed out of the pipe and nipped one of the scientists as he kindly tried to return it to the ocean. A similar organism living in a trap could alter the sediment composition considerably. The modular instrument tripod supporting the time-series sediment trap was designed by Ray Davis (USGS, Woods Hole). This design allowed scuba divers to recover only the sampling package for periodic refurbishment. The heavy framework and 300 lb of stainless-steel leg weights remained on the bottom until the final recovery last February. The tripod also accommodated a wave gauge and sensors for turbidity, temperature, and salinity.
The sediment-trap bottles recovered from a study site on the south shore of Moloka'i (10-m water depth) contain drastically different volumes of sediment collected during successive 5-day intervals. The largest volumes, in bottles 3, 12, and 17, accumulated after periods of significant rainfall and large waves, which are known to resuspend sediment at this water depth. Analyses of temporal differences in the chemical and mineralogic composition of these trap samples, now underway, will help us understand the rates at which new sediment is diluted and removed from this reef system. The Alyce C., a 28-foot sport-fishing boat owned and operated by Joe Reich of Moloka'i, was chartered by the USGS to conduct this fieldwork. Captain Joe's extensive knowledge of the reef and his outstanding boat-handling abilities were key factors in four successful deployments and recoveries during the 1-year experiment.
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in this issue:
Moloka'i Coral Reef Sediment
Marine Environmental Careers Symposium Congressional Briefing on Wetlands Talks—DOE and College of William and Mary
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U. S. Department of the Interior | U.S. Geological Survey
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