Coral Reef Ecosystems in American Samoa and the Pacific Remote Island Areas Assessed during 2010 Pacific RAMP Expedition
The 2010 Pacific Reef Assessment and Monitoring Program (Pacific RAMP) expedition to American Samoa and the Pacific Remote Island Areas was conducted during January 21–April 24, 2010. Led by the PIFSC Coral Reef Ecosystem Division (CRED), the cruise by the NOAA Ship Hi'ialakai provided updated assessments of the conditions of central Pacific coral reef ecosystems.
The Hi'ialakai visited all islands of American Samoa and South Bank. At Rose Atoll, within the Rose Atoll Marine National Monument, the lasting effects of a 1993 shipwreck were seen; in the immediate area of the wreck, surveys recorded up to 50% less coralline algae than in other parts of the atoll and only 1% coral cover, despite the removal of major portions of this rusting vessel by the U.S. Fish and Wildlife Service. However, overall, no obvious changes at Rose Atoll were observed this year compared to surveys in 2008.
At the request of local agencies, South Bank, a popular fishing area 69 km south of Tutuila Island, was mapped using multibeam sonar. The results show that South Bank is a sunken coral atoll surrounded by a submerged barrier reef (minimum depth of 25 m). Using the maps, divers conducted Rapid Ecological Assessment and towed-diver surveys, recording observations of a scoured rubble surface with low coral cover.
At Ta'u Island, divers revisited some of the world’s largest ancient coral formations of Porites spp.; one measuring 7 m in height and 41 m in circumference. Along the north side of Ta’u Island, surveys showed a dramatic increase in the invasive didemnid tunicate recorded at high levels at Swains Island during the 2008 Pacific RAMP cruise. Surveys of Swains conducted during this 2010 expedition revealed that the tunicate is no longer present there at invasive levels, and the benthic cover around the island is now crustose coralline red algae. At a site on the northeast side of Ta'u Island, survey teams recorded a 1.8-m giant grouper (Epinephelus lanceolatus). On the northwest coast of Tutuila Island, divers reported a rare sighting of a 1.2-m-long white-spotted guitarfish (Rhynchobatus djiddensis).
During the surveys in American Samoa, CRED staff noted evidence of coral reef damage caused by the devastating September 29, 2009 tsunami. Towed-diver surveys at depths of 12–18 m indicated that the reefs around Tutuila Island were in good condition with little evidence of tsunami impacts. Localized areas of coral damage were noted, consisting of overturned tables of Acropora spp. and broken or scattered plating colony stands (Echinopora spp., Merulina spp., etc.). In one instance, a colony of Porites spp. about 1.5–2 m in diameter had toppled over onto its side, but was still alive. Some evidence of damage was seen on all sides of Tutuila Island; however, localized areas of damage were most often observed just offshore of villages on the southwest, west, and northwest coasts. No damage attributable to the tsunami was noted during towed-diver benthic surveys around the other islands visited.
In addition to surveys around American Samoa, the Hi'ialakai expedition visited all major remote islands and atolls included in the Pacific Remote Islands Marine National Monument. At Howland and Baker Islands, scientists observed mass coral bleaching caused by elevated water temperatures. Such warming episodes have occurred for at least the past 300 years as part of El Niño-Southern Oscillation (ENSO) dynamics, a climate pattern that occurs on average every 2–7 years throughout the tropical Pacific. During an El Niño year, warm water from the western Pacific spreads eastward, and strong thermal events, such as those predicted and now observed for 2010, can affect the health of coral reefs exposed to the elevated temperatures.
Divers recovered 4 subsurface temperature recorders (STRs) from Howland Island that had been attached to the reef at depths of 7–40 m for the past 2 years. The STR data show that surface-water temperatures (to a depth of 37 m) at Howland Island have been elevated to levels > 30°C since mid-November 2009. Coral bleaching was widespread and temperatures exceeded 30% at both Howland and Baker Islands, with a greater incidence of elevated temperature on the eastern sides of the islands than on western reefs. Overall, no substantial differences were observed between the two islands.
Branching and table corals (e.g., Acropora sp.) observed during the expedition appeared to be more affected by the bleaching than massive corals. CRED scientist Bernardo Vargas-Angel and colleagues are preparing a manuscript to formally report this event.
Before returning to Honolulu on April 24, 2010, the Hi'ialakai also visited Jarvis Island, Palmyra Atoll and Kingman Reef. Extensive analysis of data collected there will begin soon. Preliminary analysis provided the following interesting results:
- Jarvis Island was largely dominated islandwide by the hard-coral species Montipora aequituberculata.
- The west side of Jarvis, where upwelling occurs, has an extensive population of Sinularia spp. (soft corals) found nowhere else around this island, extending ~ 300 m north–south at a depth of 16 m and covering nearly 100% of the bottom.
- Fragments of live branching coral of Pocillopora spp. and Acropora spp. were found along the south-facing shore, suggesting a recent weather or wave event.
- All counts of macroinvertebrates (crown-of-thorns seastars, sea cucumbers, giant clams, urchins) were low.
- Shark (Carcharhinidae) counts, which had declined between 2006 and 2008 surveys, returned to pre-2008 levels in 2010 surveys.
- While towed-diver surveys recorded the localized proliferation of several hard coral genera, a species of Porites dominated the majority of benthic survey segments along this atoll's fore reef and western terrace.
- Low levels of bleaching were observed within numerous coral genera around Palmyra Atoll.
- Visible macroinvertebrates (crown-of-thorns seastars, sea cucumbers, giant clams, urchins) were nearly absent in surveys around this atoll.
- Percent cover of hard and soft corals varied by habitat, depth and exposure to wave energy. However, percent cover of hard corals was nearly identical at Kingman Reef and Palmyra Atoll when survey data at each place were pooled.
- The southeastern backreef of Kingman Reef continues to harbor the highest concentration of giant clams (Tridacna spp.) of any area surveyed as part of the Pacific RAMP.
- The 2010 surveys on the backreef of the east side of Kingman Reef, adjacent to a shipwreck, showed a large increase in cyanobacteria at depths of 15–18 m over the levels recorded during the 2008 surveys.