Clipperton Atoll (eastern Pacific): oceanography,geomorphology, reef-building coral ecology and biogeography

Coral reef geomorphology and community composition were investigated in the tropical northeastern Pacific during April 1994. Three areas were surveyed in the Revillagigedo Islands (Mexico), and an intensive study was conducted on Clipperton Atoll (1,300 km SW of Acapulco), including macro-scale surface circulation, sea surface temperature (SST) climatology, geomorphology, coral community structure, zonation, and biogeography. Satellite-tracked drifter buoys from 1979–1993 demonstrated complex patterns of surface circulation with dominantly easterly flow (North Equatorial Counter Current, NECC), but also westerly currents (South Equatorial Current, SEC) that could transport propagules to Clipperton from both central and eastern Pacific regions. The northernmost latitude reached by the NECC is not influenced by El Niño-Southern Oscillation (ENSO) events, but easterly flow velocity evidently is accelerated at such times. Maximum NECC flow rates indicate that the eastern Pacific barrier can be bridged in 60 to 120 days. SST anomalies at Clipperton occur during ENSO events and were greater at Clipperton in 1987 than during 1982–1983. Shallow (15–18 m)and deep (50–58 m) terraces are present around most of Clipperton, probably representing Modern and late Pleistocene sea level stands. Although Clipperton is a well developed atoll with high coral cover, the reef-building fauna is depauperate, consisting of only 7 species of scleractinian corals belonging to the generaPocillopora, Porites, Pavona andLeptoseris, and 1 species of hydrocoral in the genusMillepora. The identities of the one Pocilpopora species and one of the twoPorites species are still unknown. Two of the remaining scleractinians (Pavona minuta, Leptoseris scabra) and the hydrocoral (Millepora exaesa), all formerly known from central and western Pacific localities, represent new eastern Pacific records. Scleractinian corals predominate (10–100% cover) over insular shelf depths of 8 to 60m, and crustose coralline algae are dominant (5–40% cover) from 0.5 to 7m. Spur and groove features, constructed of alternating frameworks ofPocillopora andPorites, and veneered with crustose coralline algae, are generally well developed around most atoll exposures. Although crustose coralline algae predominate in the breaker zone (with up to 100% cover), a prominent algal ridge is absent with only a slight buildup (ca. 10 cm) to seaward. Frequent grazing by the pufferfishArothron meleagris results in the removal of large amounts of live tissue and skeleton fromPorites lobata. Acanthaster planci is present, but rare. The grazing of large diadematid sea urchins, (2 species each ofDiadema andEchinothrix) on dead corals cause extensive erosion in some areas. Large numbers of corals on the 15–18 m terrace had recently suffered partial (P. lobata, 60–70% maximum of all colonies sampled) or total (Pocillopora sp., 80% maximum) mortality. The lengths of regenerating knobs and the rates of linear skeletal growth inP. lobata, determined by sclerochronologic analysis, indicated a period of stress during 1987. Massive skeletal growth is significantly higher at intermediate (16–17 m) than shallow (6–8 m) depths with mean extension rates of 1.5 mm yr^–1 inP. lobata and 1.4 mm yr^–1 inP. minuta at intermediate depths. Skeletal growth inP. lobata was depressed during the 1987 El Nifio event at Clipperton. The branching coralPocillopora sp. demonstrated high and similar skeletal growth rates at both shallow (25.4 mm yr^–1) and intermediate (26.5 mm yr^–1) depths. The presence of widely distributed Indo-Pacific zooxanthellate corals at Clipperton and the Revillagigedo Islands indicates that these NE Pacific Islands probably serve as a stepping stone for dispersal into the far eastern Pacific region.