Golbuu, Y 2011, 'Responses of Palau's coral reefs to disturbances at multiple scales', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright Y Golbuu 2011
The coral reefs of Palau are being increasingly exposed to local and global threats. Among these threats, two of the most serious impacts are ocean warming and sedimentation through local land-use change. This thesis examines how Palau’s coral reefs respond to, and recover from, thermal stress and land-use change. The 1998 thermal stress event caused extensive bleaching and coral mortality that reduced mean coral cover from 50-70% to 14-23% on most reefs around Palau. However, the extent of mortality depended on reef types, with nearshore fringing reefs experiencing less mortality from bleaching than patch and exposed barrier reefs. Coral cover increased rapidly after 1998 in the sheltered bays (6.6 % per year), despite a low recruitment rate, suggesting that recovery in bays was primarily a consequence of remnant regrowth. Recruitment densities were consistently high on the wave-exposed reefs, particularly the western slopes. Recovery was initially more rapid at 10 m than at 3 m on outer reefs, but in 2004 recovery rates were similar at both depths and recovery was attributed to both recruitment and regrowth of remnants. Rapid recovery was possible because Palau’s coral reefs were buffered by remnant survival plus recruitment from the less impacted habitats.
The most river-impacted coral reefs of Palau were characterized by high accumulation of mud, low-coral cover and low-coral diversity. River-sediment yield was the best indicator of land-use development. Although reef geomorphology and hydrodynamic regimes affected the flow of sediments onto the reef, the greatest contribution to reef sedimentation was from land development. Coral diversity increased with increasing distance from the heavily impacted watersheds. Yet, total coral cover was not a suitable indicator of river impact. More suitable and sensitive indicators of land-use change were the abundance of Fungiidae, Pavona spp., Acropora spp., Pachyseris spp. and Porites rus, all of which significantly increased in cover with distance from the river.
Coral cover was related to mean suspended solid concentration, while coral richness, and density and recruits richness were related to terrestrial sedimentation. Indeed, the reefs adjacent to land with little development had high coral cover and high coral richness, while reefs adjacent to developed watersheds had low-coral cover and low-coral richness. Reefs were less impacted by sedimentation at sites farther from land, but their condition was also dependent on the presence of consolidated substrate. A transplant study along the watershed gradient showed that Acropora digitifera pigmentation was clearly different among sites, yet survival rates and coral growth did not differ significantly. This is most likely due to their large initial size, which enabled the transplants to ‘escape’ severe sedimentation stress.
While nearshore reefs were less impacted by the 1998 bleaching event, they are the reefs most susceptible to sedimentation stress from land-use change. These results strongly suggest that reducing threats to coral reefs from local land-use change, increases the likelihood of recovery from thermal stress, under an increasingly warming ocean.