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In tropical islands with high elevations, terrestrial runoff can pose a severe threat to the health of surrounding coral reefs. Fine terrigenous sediment entering the nearshore ocean during runoff events affects corals in two ways: (1) suspended in seawater, the sediment drastically reduces the amount of light reaching coral reefs and other shallow benthic systems; and (2) as the sediment settles, it can bury corals or cause them to expend a large amount of energy keeping their surfaces clean. Although a clear link exists between high sediment loads and coral-reef degradation, the mechanisms responsible for coral decline are not well quantified.
July 2004 marked the end of a year-long field study of the growth of Pocillopora damicornis coral transplants at five sites along the west coast of Maui, Hawai'i. In addition to two relatively undeveloped sites, at Sugar Beach and Olowalu Beach, three sites were established in the highly developed Ma'alaea Small Boat Harbor, which has a sediment gradient ranging from fine, terrigenous mud at the west end of the harbor to coarse marine sand near the harbor mouth. An added benefit of working in the harbor is that the coral-growth data collected there can provide a baseline for evaluating proposed projects to redesign the breakwater and expand the harbor.
All coral transplants were collected from a donor site in the center of the harbor, where the water is calm and the sediment impact moderate. Growth rate was measured seasonally, using the buoyant-weight technique; sediment traps were deployed approximately monthly.
During the study period, coral-transplant growth in the harbor negatively correlated with sediment load and positively correlated with light levels. At the muddy site in the west end of the harbor, corals had high survivorship but did not grow. Transplants there photo-adapted to lower light levels caused by suspended sediment, and they bleached when low wave energy allowed excess mud to settle on top of the corals and smother them. The coral transplants at the donor site in the harbor had high survivorship and high growth rates, with rapid increases in colony volume. At the east end of the harbor, growth of the coral transplants was affected not only by sediment load and light levels, but also by mortality due to fish grazing and periodic algal blooms. The coral transplants at Sugar Beach grew as well as at the donor site during the summer, but with a different morphology: corals in the sheltered harbor site grew larger, lighter skeletons than the corals at Sugar Beach, which had smaller, denser skeletons in response to the beach's higher wave action. For most of the year, however, coral growth at Sugar Beach was removed because of mortality from storm waves or extended periods of south swell. Growth at the Olowalu Beach site was also affected by wave damage and by fish grazing.
To complement the field study, a week-long laboratory experiment was carried out in July 2004 to determine the short-term effects of sediment exposure on coral health. Fragments of two coral species, Montipora capitata and Porites lobata, were exposed to either carbonate sand or terrigenous mud for intervals ranging from 6 to 45 hours. A pulse-amplitude-modulated (PAM) fluorometer was used to measure stress effects, as determined by the fluorescence yield of the symbiotic zooxanthellae.
The two coral species in the experiment differed in their response to sediment stress, which was influenced by the length of the exposure and the type of sediment used. After 6 hours of sediment coverage, corals were slightly damaged but recovered quickly. After 30 hours, however, corals were severely damaged, and even after the sediment was removed, coral health continued to decline for another 24 hours before recovery began. The effects of terrigenous mud were more severe than those of carbonate sand. The species P. lobata, though vulnerable to long-term mud exposure, was able to minimize sediment stress because its mounding morphology helped shed sediment from the colony surface. In contrast, flat M. capitata fragments were not efficient at sediment removal and were more severely damaged.
This research was conducted by Greg Piniak (U.S. Geological Survey [USGS] Pacific Science Center, Santa Cruz, CA) as part of the USGS Mendenhall Postdoctoral Research Fellowship Program. Collaborators included Eric Brown (University of Hawai'i) and the staff of the Maui Ocean Center.
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