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Coral Reefs Along West-Central Guam—Historical Impact of Watershed Change and Sedimentation
Erosion on land and associated sedimentation along the coast are significant threats to Guam’s terrestrial and marine resources, including its coral reefs. Understanding land-based sources of pollution is a high priority for the U.S. Coral Reef Task Force and resource managers in War in the Pacific National Historical Park on Guam’s west-central coast. Until recently, however, there have been no systematic studies of historical sedimentation and its effects on Guam’s coral reefs.
To fill this gap, the U.S. Geological Survey (USGS), in conjunction with the U.S. National Park Service and the University of Guam, set out to augment instrumental records of rainfall, climate, and land use in the northwestern tropical Pacific with “proxy” records from geochemical analysis of coral cores from the west-central coast of Guam. Climate proxies are natural recorders of climate variability, such as tree rings, fossil pollen, and, in this case, chemical characteristics preserved in coral skeletons.
“The goal of this study was to provide a historical perspective on sediment input to coral reefs adjacent to a watershed with a history of land-use change and determine the implications for future management policies,” explains lead scientist Nancy Prouty, a research oceanographer at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California.
By measuring certain elements in the coral cores collected from west-central Guam, Prouty and her team demonstrated that the ratio of terrestrially derived barium to calcium (Ba/Ca) can be used as a historical indicator of sedimentation. In general, the higher the ratio Ba/Ca, the greater the input of terrestrial sediment. The results, published in May 2014 in the journal Coral Reefs (“Historic impact of watershed change and sedimentation to reefs along west-central Guam”), show that the fore reef at Asan Bay is within the zone of impact from sediment transported from the Piti-Asan watershed. (The fore reef, which slopes down from the reef crest toward the open ocean, is typically home to the largest corals and highest coral diversity on the reef.)
“Our proxy record illustrates elevated terrestrial sedimentation since the 1940s,” Prouty says. “In the mid-1940s, we see an abrupt shift in sedimentation recorded in coral from Asan Bay in War in the Pacific National Historical Park.” This shift occurred at the same time as the sudden denudation of the landscape by military ordnance during pre-invasion bombardment by the U.S. Navy in July 1944; landings at Asan Beach by the U.S. 3rd Marine Division on July 21, 1944; the ensuing battle to retake Guam; and the development of the Asan area for military operations through the end of the war in 1945.
The abrupt shift in Ba/Ca values in the Asan Bay coral record, centered in the mid-1940s, was accompanied by a decrease in coral growth and calcification rates and an increase in density, suggesting a response to increased sedimentation and concomitant reduction in light levels. These patterns were not seen in coral cores collected from nearby reefs associated with watersheds that have not undergone the same degree of landscape denudation.
“The high susceptibility of Guam’s soil to erosion, coupled with increasing human activities in the watershed, illustrates how compounding natural factors and watershed land-cover change can lead to increased sediment loading in nearshore environments,” notes Prouty.
The reefs of Guam provide an estimated value of US$127 million a year in tourism and fishing revenue. Because Guam’s population is expected to keep increasing over the next few decades, the current threat to this valuable ecosystem will likely continue or grow unless measures are taken to mitigate erosion and surface runoff.
“There is mounting evidence that the vulnerability of coral reefs to the effects of climate change is increased by chronic, local stressors,” explains Prouty. “Therefore, managing and mitigating local stressors, particularly sedimentation, will play a critical role in enhancing reef resilience and the possibility for acclimation under future climate-change scenarios.”
This study of historical sedimentation along west-central Guam complements measurements of present-day sedimentation patterns, such as a 2007 study described in “USGS Researchers Collaborate with National Park Service Scientists to Understand the Impact of Watershed Erosion on Coral Reefs in War in the Pacific National Historical Park, Guam,” Sound Waves, October 2007. To read about USGS sedimentation studies in other Pacific coral reef areas, please visit “Circulation and Sediment, Nutrient, Contaminant, and Larval Dynamics on Reefs.”
The full citation for the recent article is:Prouty, N.G., Storlazzi, C.D., McCutcheon, A.L., and Jenson, J.W., 2014, Historic impact of watershed change and sedimentation to reefs along west-central Guam: Coral Reefs, v. 33, no. 3, p. 733–749, [http://dx.doi.org/10.1007/s00338-014-1166-x].
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