Coastal Circulation and Sediment Dynamics in War-in-the-Pacific National Historical Park, Guam; measurements of waves, currents, temperature, salinity, and turbidity, June 2007–January 2008

Introduction

Flow in and around coral reefs affects a number of physical, chemical and biologic processes that influence the health and sustainability of coral reef ecosystems. These range from the residence time of sediment and contaminants to nutrient uptake and larval retention and dispersal. As currents approach a coast they diverge to flow around reef structures, causing high horizontal and vertical shear. This can result in either the rapid advection of material in localized jets, or the retention of material in eddies that form in the lee of bathymetric features. The high complexity and diversity both within and between reefs, in conjunction with past technical restrictions, has limited our understanding of the nature of flow and the resulting flux of physical, chemical, and biologic material in these fragile ecosystems.

Sediment, nutrients, and other pollutants from a variety of land-based activities adversely impact many coral reef ecosystems in the U.S. and around the world. These pollutants are transported in surface water runoff, groundwater seepage, and atmospheric fallout into coastal waters, and there is compelling evidence that the sources have increased globally as a result of human-induced changes to watersheds. In Guam, and elsewhere on U.S. high islands in the Pacific and Caribbean, significant changes in the drainage basins due to agriculture, feral grazing, fires, and urbanization have in turn altered the character and volume of land-based pollution released to coral reefs. Terrigenous sediment run-off (and the associated nutrients and contaminants often absorbed to it) and deposition on coral reefs are recognized to potentially have significant impact on coral health by blocking light and inhibiting photosynthesis, directly smothering and abrading coral, and triggering increases in macro algae. Studies that combine information on watershed, surface water- and groundwater-flow, transport and fate of sediment and other pollutants in the reef environment, and their impact on reef health and ecology are essential for effective reef management.

Two of the main anthropogenic activities along west-central Guam’s coastline that may impact the region’s coral reef ecosystems include pollution and coastal land use/development, as discussed in the review by Porter and others (2005). The pollution threats include point-sources, such as municipal wastewater (Northern District, Hagatna, Naval Station Guam, and Agat-Santa Rita Waster Water Treatment Plants), cooling water (Tanguisson Steam and Cabras Power Plants), and numerous storm water, ballast water, and tank bottom draw outfalls; nonpoint sources include septic systems, urban runoff, illegal dumping, and groundwater discharges. Poor land-use practices include development without the use of runoff management measures, increased areal extent of impervious surfaces and decreased extent of vegetative barriers, and recreational off-road vehicle use. Furthermore, feral ungulates and illegal wildfires remove protective vegetative cover and generally result in increased soil erosion. While anthropogenic point-sources have been reduced in many areas due to better management practices, nonpoint sources have either stayed constant or increased. Between 1975 and 1999, it is estimated that Guam lost more than a quarter of its tree cover, and more than 750 wildfires each year have resulted in a greater proportion of badlands and other erosion-prone land surfaces with high erosion rates (Forestry and Soil Resources Division, 1999).

Approximately 1.8 square kilometers (km²) of Asan Bay, west-central Guam, lies within the National Park Service’s (NPS) War-in-the-Pacific National Historical Park’s (WAPA) Asan Unit; the bay is the sink for material coming out of the Asan watershed. Anthropogenic modifications of the watersheds adjacent to Asan Bay, which include intentionally-set wildfires, construction, and agriculture (Minton, 2005), are believed to have increased over the past 25 years (National Resource Conservation Service, 1996). These land-use practices cause accelerated erosion by removing grasses and small trees that stabilize the soil. While even modest rains on Guam cause sediment plumes to be discharged from many rivers to coastal waters, including the Asan River (Minton and others, 2007), typhoons pass close enough to Guam every 2-3 years to cause heavy precipitation (>2 cm/hour) on the island, rapidly flushing unstabilized soil down to the coast and onto WAPA’s nearshore reefs (Porter and others, 2005).

Observations by Minton and others (2007) suggest that this terrestrial sediment discharge to the coastal waters has resulted in sedimentation, eutrophication, and pollution that has impacted WAPA’s coral reef ecosystem. These authors have shown that sediment collection rates in tube traps on the fore reef of Asan Bay’s fringing reef are very high and the trapped material is predominantly composed of fine-grained terrestrial sediment that typically has nutrients, bacteria, and pesticides adsorbed to the particles; the trap collection rates are both spatially and temporally heterogeneous. Work by Minton and others (2007) further shows that the input of terrestrial sediment to the park’s nearshore waters is greater during the wet season (July-December), which is of serious concern as this is also the time of peak coral spawning and larval settlement (Richmond and Hunter, 1990). Observations by Minton (2005) and Minton and others (2007), however, suggest that while large quantities of terrestrial sediment are being collected in traps on the fore reef and often covered their coral recruitment tiles, the reefs themselves are not being buried by mud, suggesting that oceanographic processes are sufficient to limit net sedimentation on the fore reef. Quantitative information on the deposition, residence time and advection of this fine-grained terrestrial material through the bay’s fringing reef system, along with the controls on these processes, is needed to better manage the WAPA’s marine resources.

Because of these observations, the Asan watershed is an area of concern to the NPS and was designated as one of the U.S. Coral Reef Task Force (USCRTF) Guam Local Action Strategy’s (LAS) priority watersheds. In 2007 the U.S. Geological Survey (USGS) Western Coastal and Marine Geology (WCMG) Team initiated an investigation of coastal circulation, sediment flux, and sediment residence time along west-central Guam, primarily focusing on WAPA’s Asan Unit.

This work also supports the USCRTF’s goal of investigating the impact of land-based pollution on corals reefs. Furthermore, the data collected during this experiment will provide NPS and the Guam Environmental Protection Agency (EPA) with quantitative baseline data for possible future measurements made during the planned large-scale expansion of the U.S. military installation at Apra Harbor, which is less than 2 km south of the park, and extension of the Hagatna Water treatment outfall, which is less than 4 km north of WAPA’s Asan Unit.

First posted October 13, 2009