CLIPPERTON ATOLL (EASTERN PACIFIC) - OCEANOGRAPHY, GEOMORPHOLOGY, REEF-BUILDING CORAL ECOLOGY AND BIOGEOGRAPHY

Coral reef geomorphology and community composition were investigated i n the tropical northeastern Pacific during April 1994. Three areas wer e surveyed in the Revillagigedo Islands (Mexico), and an intensive stu dy was conducted on Clipperton Atoll (1,300 km SW of Acapulco), includ ing macro-scale surface circulation, sea surface temperature (SST) cli matology, geomorphology, coral community structure, zonation, and biog eography. 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 Clip perton from both central and eastern Pacific regions. The northernmost latitude reached by the NECC is not influenced by El Nino-Southern Os cillation (ENSO) events, but easterly flow velocity evidently is accel erated at such times. Maximum NECC flow rates indicate that the easter n Pacific barrier can be bridged in 60 to 120 days. SST anomalies at C lipperton occur during ENSO events and were greater at Clipperton in 1 987 than during 1982-1983. Shallow (15-18 m) and deep (50-58 m) terrac es 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 depa uperate, consisting of only 7 species of scleractinian corals belongin g to the genera Pocillopora, Porites, Pavona and Leptoseris, and 1 spe cies of hydrocoral in the genus Millepora. The identities of the one P ocillopora species and one of the two Porites species are still unknow n. Two of the remaining scleractinians (Pavona minuta, Leptoseris scab ra) and the hydrocoral (Millepora exaesa), all formerly known from cen tral and western Pacific localities, represent new eastern Pacific rec ords. Scleractinian corals predominate (10-100% cover) over insular sh elf depths of 8 to 60 m, and crustose coralline algae are dominant (5- 40% cover) from 0.5 to 7 m. Spur and groove features, constructed of a lternating frameworks coralline algae, are generally well developed ar ound most atoll exposures. Although crustose coralline algae predomina te in the breaker zone (with up to 100% cover), a prominent algal ridg e is absent with only a slight buildup (ca. 10 cm) to seaward. Frequen t grazing by the pufferfish Arothron meleagris results in the removal of large amounts of live tissue and skeleton from Porites lobata. Acan thaster planci is present, but rare. The grazing of large diadematid s ea urchins, (2 species each of Diadema and Echinothrix) 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% maxi mum of all colonies sampled) or total (Pocillopora sp., 80% maximum) m ortality. The lengths of regenerating knobs and the rates of linear sk eletal growth in P. lobata, determined by sclerochronologic analysis, indicated a period of stress during 1987. Massive skeletal growth is s ignificantly higher at intermediate (16-17 m) than shallow (6-8 m) dep ths with mean extension rates of 1.5 mm yr(-1) in P. lobata and 1.4 mm yr(-1) in P. minuta at intermediate depths. Skeletal growth in P. lob ata was depressed during the 1987 El Nino event at Clipperton. The bra nching coral Pocillopora sp. demonstrated high and similar skeletal gr owth rates at both shallow (25.4 mm yr(-1)) and intermediate (26.5 mm yr(-1)) depths. The presence of widely distributed Indo-Pacific zooxan thellate corals at Clipperton and the Revillagigedo Islands indicates that these NE Pacific Islands probably serve as a stepping stone for d ispersal into the far eastern Pacific region.