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Bleaching Thresholds

Assessment of Coral Reef Ecosystem Resilience Using Taxa-Specific Bleaching Thresholds

Ruben van Hooidonk, Ph.D.
Tom Capo
Jim Hendee, Ph.D.

Summer, 2011 to Summer, 2013

RSMAS coral culture 1Current methods to predict coral bleaching are an oversimplification and function on large temporal and spatial scales that do not account for differences in susceptibility to bleaching between species. The goal of the proposed work is to generate high quality forecasts of coral reef ecosystem resilience to bleaching. Taxa-specific bleaching thresholds based on SST and general circulation models will help project evolving species composition and ecosystem functioning of the reefs.

Corals live in conditions close (~1ÂșC) to their upper thermal limit (Hoegh-Guldberg 1999). When the thermal threshold is exceeded coral bleaching can occur. Bleaching is the whitening of reef building corals due to a reduction of their symbiotic dinoflagellate zooxanthellae and/or loss of the pigments in the symbionts (Brown 1997). Global, thermally induced bleaching is projected to occur annually on reefs in most tropical oceans within 30-50 years because of anthropogenic climate change (Hoegh-Guldberg 1999). Although bleaching is a multifactorial response to solar radiation and thermal stress (Fitt et al. 2001, Brown 1997) most of these previous predictive techniques have primarily focused on thermal stress.

Rankings of susceptibility of different taxa to thermal stress have been published (Loya et al. 2001; Okamoto et al. 2005; Marshall and Baird 2000). These previously identified rankings in combination with thresholds found experimentally will enable interpolation of thresholds to other species.

To determine thresholds we will expose seven species of coral to four different solar radiation intensities and four different thermal stress levels. Lab experiments will be conducted to measure bleaching at three different solar radiation thresholds and four different thermal exposures for seven common Caribbean species of corals. Currently, the University of Miami Coral Aquaculture Laboratory has a successful history in using controlled, closed-system, recirculating seawater systems to maintain and grow corals. Coral samples are already available and will be placed in tanks with flowing seawater, and water temperatures will be controlled by aquarium heaters. Solar radiation will be modified to levels comparable to light levels where coral samples were collected, by filtering ambient sunlight with layers of screen. Experimental levels of solar radiance and thermal stress exposure amplitude and time will be selected to replicate levels observed leading to stress. The duration of the tank experiment will be around three months and will be guided by the advice of Tom Capo. Bleaching will be monitored using color reference charts (Siebeck et al. 2006) and verified by sampling selective corals using PAM fluorometry (Manzello et al. 2009) (monitoring PAM already present). Current species composition at five different locations in the Caribbean will be determined by identifying corals along line transects (citation). This information in combination with the found and interpolated thresholds and future sea surface temperatures from general circulation models will be used to project future impacts on reefs. This will identify more resilient and more susceptible reefs.

RSMAS coral culture 2University of Miami Coral Resource for Restoration, Research and Education Laboratory ("Coral Aquaculture Lab") has successfully developed "lab rat" (i.e., one genome") culture techniques for Acropora cervicornis, A. palmata, Pocillopora damicornis, Stylpphora pistillata, Montastrea faveolata, M. cavernosa, and Porites astreoides. Few other laboratories in North American can claim such success on such a wide scale. This laboratory, in operation for over 20 years, still continues to also produce Gracilaria gracilis (red alga) for neurobiological studies, redfish, snook and snapper for stock enhancement and research. The University of Miami funds most of the operation's personnel through Joint Institute (CIMAS) arrangements.

Marine Protected Area management will be provided with more precise bleaching forecasts, which they have indicated essential in maintaining credibility with the public.

This project also has another major benefit: the identification (in outyears) cultivating of geneotypes that are resilient to bleaching (and disease, and eutrophication, etc.). Corals subjected to stressful conditions representative of not only current, but future conceptualized scenarios, can be taken to the field and transplanted there, to help realize the concept of "resilience." This is truly getting a step ahead of natural selection in the wild, by encouraging it in the laboratory.

This project is funded by NOAA's Coral Reef Conservation Program, and NOAA/AOML.

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