Multi-scale variation in coral reef metrics on four Philippine reef systems

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Multi-scale studies were conducted to examine variations in hard coral cover and generic richness in four reef systems in the Philippines in relation to different stressors. The study sites included Bolinao, where sedimentation and eutrophication due to intensive fish farming are causing seawater acidification, and reefs inside and outside embayments in Lian, which experience different levels of sedimentation. The other two reef systems are those of Mabini and Tubbataha, which are popular sites for dive tourism and are thus better managed. Mabini experiences periodic upwelling while the offshore Tubbataha sites are generally well-flushed by currents which might mitigate thermal stress in these areas. We tested differences in hard coral cover and generic richness between locations, sites in a location, and stations between sites to capture scales at which differences attributable to such factors might be observed. Statistically significant variation was detected at all scales (station, site, and location) examined. Hard coral cover was found to have the largest variance at the site level (scales of hundreds to thousands of meters), while generic richness varied most at the location level (scales of tens to hundreds of kilometers). The coral metrics were shown to be related to factors such as thermal stress, nutrient enrichment and eutrophication, and water motion relative to the degree of embayment. Understanding the nature of the dynamics between coral metrics and environmental factors can aid managers in maintaining reef quality and projecting future trajectories of reef states. This work provides specific hypotheses on the factors affecting coral reefs (coral bleaching, eutrophication and acidification from fish farming, embayment, and associated processes), particularly coral cover and generic richness at the location and biogeographic region (sub-national) scales. It also provides a framework by which the relative importance of these factors may be recognized at different spatial scales. This framework is potentially valuable in developing countries with significant reef areas, especially where a more ecosystem-based approach to the management of coastal waters is applicable.