How does rising atmospheric CO2 affect marine organisms?

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A Little-Studied Way by which Corals May Survive Bleaching
Paper Reviewed
Smith, T.B., Glynn, P.W., Mate, J.L., Toth, L.T. and Gyory, J. 2014. A depth refugium from catastrophic coral bleaching prevents regional extinction. Ecology 95: 1663-1673.

Introducing their study, Smith et al. (2014) write that "species can survive a changing climate by possessing traits that allow them to tolerate new conditions, adapting to new conditions, or dispersing to ... habitats that are buffered from the changing conditions, i.e., refugia," citing Ashcroft (2010) and Keppel et al. (2012), while further noting that "environmental refugia have been observed in the fossil record (Tzedakis et al., 2002), but are not well documented or understood on ecological time scales."

In discussing the subject further, the authors note that "one mechanism by which thermally sensitive coral species may endure increasing sea temperatures is by surviving in deeper-water refugia," citing Glynn (1996) and Riegl and Piller (2003), which maneuvering "allows for recolonization of the affected habitats after the disturbance has passed," citing Bongaerts et al. (2010). But they say that to date, "the deep-water refugium concept has never been conclusively demonstrated because shallow-water populations persisted, albeit in a reduced abundance, and could have provided larvae that led to the species-level resilience."

Using a 37-year record from the eastern Pacific across the two most severe El Niño events on record (1982-1983 and 1997-1998), together with a 5000-year sub-fossil record, Smith et al. were able to demonstrate how "an exceptionally thermally sensitive reef-building hydrocoral, Millepora intricata, twice survived catastrophic bleaching in a deeper-water refuge (>11 m depth)," while "during both events, M. intricata was extirpated across its range in shallow water, but showed recovery within several years, while two other hydrocorals without deep-water populations were driven to regional extinction." In addition, they report that "evidence from the sub-fossil record in the same area showed shallow-water persistence of abundant M. intricata populations from 5000 years ago, through severe El Niño-Southern Oscillation cycles, suggesting a potential depth refugium on a millennial timescale." Thus, in the concluding sentence of their paper's abstract, the five researchers say that their data "confirm the deep refuge hypothesis for corals under thermal stress."

Ashcroft, M.B. 2010. Identifying refugia from climate change. Journal of Biogeography 37: 1407-1413.

Bongaerts, P., Ridgway, T., Sampayo, E. and Hoegh-Guldberg, O. 2010. Assessing the 'deep reef refugia' hypothesis: focus on Caribbean reefs. Coral Reefs 29: 309-327.

Glynn, P.W. 1996. Coral reef bleaching: facts, hypotheses and implications. Global Change Biology 2: 495-509.

Keppel, G., Van Niel, K.P., Wardell-Johnson, G.W., Yates, C.J., Byrne, M., Mucina, L., Schut, A.G.T., Hopper, S.D. and Franklin, S.E. 2012. Refugia: identifying and understanding safe havens for biodiversity under climate change. Global Ecology and Biogeography 21: 393-404.

Riegl, B. and Piller, W.E. 2003. Possible refugia for reefs in times of environmental stress. International Journal of Earth Sciences 92: 520-531.

Tzedakis, P.C., Lawson, I.T., Frogley, M.R., Hewitt, G.M. and Preece, R.C. 2002. Buffered tree population changes in a quaternary refugium: evolutionary implications. Science 297: 2044-2047.

Reviewed 17 September 2014