Representing the world’s last remaining large wilderness area for coral reefs, Palmyra has been described as “a crown jewel of the Central Pacific.” Its diverse marine habitats include steep coral reef walls, extensive and shallow perimeter coral reef shelves, reef pools, sand flats, and protected lagoons. Although altered and impacted by the military during World War II, and recent bleaching events in all habitats except the lagoon, a lack of recent commercial fishing results in a naturally functioning marine ecosystem.
The atoll supports more than twice as many coral as found in Hawai‘i, and nearly four times as many stony coral species as in the Florida Keys and nearly three times the stony species reported for the entire Caribbean. Surveys have identified 175 stony coral species and 47 genera and 15 other cnidarian species at Palmyra.
The coral fauna at Palmyra is more diverse compared to other Line Islands, except for nearby Kingman Reef NWR. One reason suggested for this diversity is its location within the North Equatorial Countercurrent, which may be transporting species of coral larvae from the more diverse western Pacific region. As a result, Palmyra may be functioning as a coral larvae source, dispersing species to neighboring atolls and reef islands in the central Pacific.
The condition and species composition of coral reefs at Palmyra vary depending on location and have shown drastic changes over time. Coral reefs within the lagoon were lost by military dredging and filling activities during World War II and have not recovered. Additionally, a mass coral bleaching occurred in 1997 or 1998 and another in 2009. It is speculated that the bleaching was caused by warm ocean waters passing over the reef as a result of an El Nino Southern Oscillation (ENSO) event and further exacerbated by heated waters discharging from the degraded lagoon. Attributing to the resiliency of Palmyra’s functional coral reefs, the most affected areas were showing signs of recovery by 2002.
In 2002, the southeastern coral gardens remained exceedingly diverse and healthy. Over 25 species of Acropora were thriving in the pools including rare species and species absent elsewhere at the atoll. But even the most pristine of areas may be unable to escape the impacts of humans, such as altered hydrologic flows, changes in ocean chemistry and temperature, increased visitation, and other as yet unidentified factors. By 2008, a measurable and substantial decline in coral composition, numbers and cover has occurred in the coral gardens.
Palmyra’s lagoons formed when the original volcanic islands sank beneath the sea. Originally, all 3 lagoons experienced water exchange with the ocean at high and moderate tides, and may have once provided a lush fertile ecosystem supporting a diversity of terrestrial and sea life.
In its pristine state, the ebb and flow of the tides at Palmyra may have circulated considerable volumes of oceanic water through the inner lagoons. Military activities during World War II included constructing a north-south causeway between East and Center Lagoons, constructing smaller causeways and two airfields in Center Lagoon, and dredging of a deep entrance channel through the perimeter reef southwest of West Lagoon. There were several immediate negative effects of these alterations. Reduced and altered water circulation in West and Center Lagoons and circulation in East Lagoon was completely blocked, which in turn eliminated most coral reef life and increased temperature regimes in all three lagoons. Subsequently, recent coral bleaching and lagoon shoreline erosion have occurred in the Palmyra lagoons, continuing to prevent recolonization by corals.
Roving schools of convict tang, manta rays, peacock groupers, and green turtles have been seen just outside the lagoons and in shallow areas of East Lagoon. Lagoon filter feeders include black-lipped pearl oysters and pen shells. The lagoons flats at Palmyra support a large bonefish population and are prized habitats for foraging eagle rays and black-tip reef sharks, and an unidentified beaked whale.
Several marine mammals have been observed in the pelagic waters surrounding Palmyra, including pilot whales (Globicephala scammoni) and bottle nosed dolphins (Tursiops truncates). Palmyra also supports a population of melon headed whales (Peponocephala electra).
Threatened and Endangered Marine Species
Listed as endangered in November 1976, the Hawaiian monk seal (Monachus schauinslandi) has been reported outside Palmyra Atoll on two separate occasions. A single individual was observed on the northeast side of Sand Island in 1990 and two monk seals were sighted the following day on the west side of Eastern Island.
Palmyra Atoll NWR has been noted as a significant habitat for the threatened green turtle (Chelonia mydas). This sea turtle was listed as threatened throughout the Pacific range in 1978. Populations of the Pacific green turtle have seriously declined due primarily to direct take of turtles and eggs. Military construction eroded many beaches at Palmyra that may have served as nesting habitat for green turtles before World War II era.
The hawksbill sea turtle (Eretmochelys imbricata), was listed as endangered in June 1970. Historically, the meat, eggs, shell of this turtle were harvested for trade and food. More than one million hawksbill sea turtles have been killed for their shells since 1970 and illegal international trade continues to be the primary threat to the species.
Nesting green turtles have primarily been observed on the wide sandy beach of the northwestern portion of Cooper Island. The outer islets of Palmyra Atoll NWR are less conducive to marine turtle nesting activity due to the narrow area, lack of sand during high tide, and continuing beach erosion exacerbated by World War II era dredging and filling.