Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll

Maragos JE (2008) Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll. PLoS ONE 3(8): e2989. doi:10.1371/journal.pone.0002989 Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll Thierry M. Work 0 1 Greta S. Aeby 0 1 James E. Maragos 0 1 Craig R. McClain, Monterey Bay Aquarium Research Institute, United States of America 0 Funding: Funding provided by the US Geological Survey and Hawaii Institute of Marine Biology (University of Hawaii) 1 1 U. S. Geological Survey-National Wildlife Health Center, Honolulu Field Station , Honolulu , Hawaii, United States of America, 2 University of Hawaii, Hawaii Institute of Marine Biology, Kaneohe, Hawaii, United States of America, 3 U. S. Fish & Wildlife Service , Pacific Islands Refuges, Honolulu, Hawaii , United States of America Coral reefs can undergo relatively rapid changes in the dominant biota, a phenomenon referred to as phase shift. Various reasons have been proposed to explain this phenomenon including increased human disturbance, pollution, or changes in coral reef biota that serve a major ecological function such as depletion of grazers. However, pinpointing the actual factors potentially responsible can be problematic. Here we show a phase shift from coral to the corallimorpharian Rhodactis howesii associated with a long line vessel that wrecked in 1991 on an isolated atoll (Palmyra) in the central Pacific Ocean. We documented high densities of R. howesii near the ship that progressively decreased with distance from the ship whereas R. howesii were rare to absent in other parts of the atoll. We also confirmed high densities of R. howesii around several buoys recently installed on the atoll in 2001. This is the first time that a phase shift on a coral reef has been unambiguously associated with man-made structures. This association was made, in part, because of the remoteness of Palmyra and its recent history of minimal human habitation or impact. Phase shifts can have long-term negative ramification for coral reefs, and eradication of organisms responsible for phase shifts in marine ecosystems can be difficult, particularly if such organisms cover a large area. The extensive R. howesii invasion and subsequent loss of coral reef habitat at Palmyra also highlights the importance of rapid removal of shipwrecks on corals reefs to mitigate the potential of reef overgrowth by invasives. - The term phase shift as applied to coral reef ecosystems was first coined by Done [1] who used it to describe the change in reef biota from coral to macroalgae attributed to environmental degradation. Reasons cited for such phase shifts include overfishing, excessive nutrient input, predation by Acanthaster plancii, and depletion of major functional groups like fish and echinoid grazers [13]. Experimental manipulations have confirmed the important role that grazers play in keeping macroalgae from dominating reefs [4], and field observations of phase shifts from coral to algae subsequent to catastrophic disease events in echinoids confirm this [5]. Reports of overgrowth of degraded reefs by plants largely overshadow the less commonly cited phenomenon of overgrowth of reefs by other cnidaria such as anemones and corallimorphs [6 8]. Reasons for phase shifts from one type of cnidarian (coral) to another (anemones or corallimorphs) are speculative. Bleaching, damage by typhoon, overfishing, coastal development, and tourism were suspected as a cause of overgrowth of Acropora by the anemone Condylactis sp in Taiwan [6]. The corallimorpharian Rhodactis rhodostoma was most aggressively competitive in areas of Tanzanian reefs that had the highest levels of phosphate and manmade disturbance [7]. Restoring tropical reef ecosystems to a prior state subsequent to algal or cnidarian overgrowth can be a daunting task even when areas affected are small [9]. Therefore, understanding what drives such events may help prevention or aid mitigation. Palmyra is a remote and comparatively pristine atoll located in the U.S. Line Islands in the central Pacific (05u N 162u W). Palmyra was extensively modified during WWII but abandoned shortly thereafter with sparse human influence since. In 2001, Palmyra became a National Wildlife Refuge. In 1991, a long line fishing vessel wrecked on the western shelf of the reef and remains grounded. The shipwreck was first examined by one of us (JEM) in 2004 where R. howesii were observed occurring at extremely low numbers. By 2005, R. howesii populations appeared to be expanding, and in 2006 permanent monitoring transects were established on the north and south sides of the wreck. Our objectives were to quantify the extent of the corallimorph invasion around the long line wreck and to document damage to corals at the gross and cellular levels. Materials and Methods For towed diver surveys, a single boat operator (GSA) and diver on snorkel (TMW) made a straight course at constant speed from the ship toward the eight cardinal points of the compass (north, northwest, west, southwest, south, southeast, east, northeast). To ensure consistency, the same diver did all benthic surveys for corallimorphs. While transiting each course, a GPS reading was taken at 1-minute intervals and the diver visually estimated the average area of benthos covered by corallimorphs as high (.60% cover), medium (.3060% cover), light (130% cover), or no impact (no corallimorphs visible). Each one minute interval covered ca. 60 linear meters. Linear transects surveyed extended 200 m to 1400 m from the ship. To ensure that the extent of impact was mapped as completely as possible, each course heading was continued until three consecutive 1-minute (60 m) observations revealed no corallimorphs or until it became too deep to see the benthos from the surface. Estimated depth of the areas surveyed ranged from 5 to 20 m, and the benthos was visible from the surface for all but the last point of two transects (south and southeast) where depth exceeded visibility (reef slope). We quantified corallimorphs within high-density areas approximately 5 m from the wreck by haphazardly placing 10 replicate 0.5 m2 quadrats on the bottom around the ship and counting all polyps within each quadrat. The ship was measured from bow to stern with a 100 m surveyors tape. To document the mechanism of injury inflicted by corallimorphs on scleractinian corals, corals were photographed, and samples of corals and corallimorphs in contact areas (five Acropora pulchra and five Montipora capitata) were collected and fixed in Z-fix. Corals were decalcified, tissues embedded in paraffin, sectioned at 5 um, and stained with hematoxylin and eosin for microscopic examination [10]. To determine population densities of corals at risk from invasion, injury, and death by corallimorphs, benthic surveys were conducted at the ends of the eight major survey courses (N, NW, NE, E, SE, S, SW) in (...truncated)