An experimental investigation into the skeletal signatures of coral stress
Projected climate change during the coming century is expected to impose deleterious effects on marine calcifying organisms. Reef-building corals are often considered to be among the organisms most sensitive to the effects of climate change. Given projections of degradation of coral reefs within this century1, there is a great need to conserve those reefs most likely to persist through this century. However, identifying coral resilience to climate change has proven difficult, prompting a need for more tools to quantify coral resilience. Analyzing the frequency of coral bleaching events, the loss of symbiotic photosynthetic algae that reef-building corals need to survive, and coral recovery following bleaching events is one way that coral resilience can be quantified.
Porites spp. corals form thin (30-50 µm) sheets of skeleton, called dissepiments, at the base of the tissue layer to separate the living tissue from seawater in the skeletal pore spaces below. Initial measurements of spacing between consecutive dissepiments from photomosaic images of skeleton slabs suggest that dissepiment spacing is related to tissue layer thickness. Especially encouraging is that dissepiment spacing in skeleton formed during a time of known bleaching showed a sharp decline followed by rapid recovery, consistent with depletion of biomass during bleaching observed in Porites2. Thus, dissepiments are a potentially valuable tool for reconstructing coral resiliency to past bleaching events.
Geochemical signals preserved in coral aragonite skeleton reveal clues about the coral response to past environmental variability. Corals use energy to modify seawater carbonate chemistry and induce calcium carbonate nucleation3. We conducted abiogenic aragonite precipitation experiments to identify that aragonite U/Ca is a proxy for calcifying fluid carbonate chemistry. Therefore, coral skeletal U/Ca should preserve information about the coral calcification response to past environmental variability.
The goal of this project is to apply novel techniques, dissepiment spacing and skeletal U/Ca, to assess the coral response to periods of intense bleaching stress. Groundtruthing these techniques will provide new tools to test hypotheses of the environmental or genetic settings under which corals show resiliency to environmental stress.
This project will take advantage of our existing collaboration with the Palau International Coral Reef Center (PICRC) and the experimental setup that our group has already established in Palau. I traveled to Palau in April 2013 and prepared coral “plugs”, which are 3 cm diameter coral skeletal cores drilled from wild Porites spp. colonies and left on the reef for recovery. These plugs can be readily used in aquaria experiments.
Corals will be cultured in aquaria under anomalously warm temperature in order to induce coral bleaching. Filtered seawater will be heated in reservoirs that continuously supply seawater to aquaria tanks. Bleaching will be induced at two different temperatures, each of which will include short-(several days) and long-(one to two weeks) exposures. Separate aquaria will be used for control experiments at ambient local seawater temperature.
We will track the symbiont (pigmentation, symbiont density) and coral (tissue thickness) responses through bleaching and subsequent recovery. These directly measureable responses will be compared to our potential proxies for tissue thickness (dissepiment spacing) and energy available for calcification (skeletal U/Ca). Our experiments will provide a test of whether our novel techniques record (1) the severity of coral bleaching and (2) the ability of corals to recover from bleaching stress.
Modern climate change poses serious threats to the future of coral reef ecosystems through the effects of rising sea surface temperature and ocean acidification. Coral bleaching events may become annual phenomena within this century1, and may be exacerbated by ocean acidification4. Given these bleak future projections for coral reefs, a critical question in coral reef conservation is how to select reefs for marine protected areas (MPAs) that are most likely to persist into the future. ‘Coral resilience to climate change’ has recently begun to be incorporated into MPA design5, but unfortunately is limited by a lack of data to quantify resilience. This project seeks to close this gap in our knowledge by experimentally proving the applicability of novel tools to probe coral resiliency to environmental variability. These tools will then be available to test hypotheses of what conditions convey resiliency to coral communities.
Description of how funds will be used
As part of my General’s research, I conducted laboratory experiments to evaluate aragonite U/Ca as a proxy of calcifying fluid carbonate chemistry. In addition, I have made preliminary measurements of dissepiment spacing on previously collected coral samples. I received a National Science Foundation graduate fellowship to apply these techniques to search for coral resiliency on Taiwanese reefs.
Proving the applicability of these techniques in an experimental setting would be a valuable connection between the experiments I have already conducted and the application of these techniques in Taiwan. Palau is an ideal location to conduct these experiments because we have an ongoing collaboration with the PICRC, where we have previously conducted coral culture experiments. My proposed experiments have partial funding available from an NSF grant awarded to my advisor and $5,000 that I received from the WHOI Ocean Ventures Fund. Funding from the Coastal Ocean Institute would supplement these funding sources in covering expenses of travel and lodging in Palau.
1. van Hooidonk, R., Maynard, J. & Planes, S. Temporary refugia for coral reefs in a warming world. Nature Climate Change (2013).
2. Grottoli, A., Rodrigues, L. & Juarez, C. Lipids and stable carbon isotopes in two species of Hawaiian corals, Porites compressa and Montipora verrucosa, following a bleaching event. Marine Biology 145, 621-631 (2004).
3. Cohen, A. L. & Holcomb, M. Why corals care about ocean acidification: uncovering the mechanism. (2009).
4. Anthony, K., Kline, D., Diaz-Pulido, G., Dove, S. & Hoegh-Guldberg, O. Ocean acidification causes bleaching and productivity loss in coral reef builders. Proceedings of the National Academy of Sciences 105, 17442-17446 (2008).
5. Green, A. et al. Designing a resilient network of marine protected areas for Kimbe Bay, Papua New Guinea. Oryx 43, 488-498 (2009).