How Best to Help Corals Cope with Heat-Induced Bleaching

It has been hypothesized that "any increases in coral bleaching that may have occurred in response to periods of elevated water temperature over the past two decades have only occurred because of a long-term weakening of coral resistance to thermal stress caused by the gradual intensification of a multitude of local anthropogenic assaults upon the watery environments in which corals live (Idso et al., 2000)," including -- among a large number of other things -- "chemical insults to once-pristine reef environments," such as "the rising levels of nutrients and toxins in coastal waters caused by runoff from agricultural activities on land and associated increases in sediment delivery." And now, the study of Carilli et al. (2009) provides significant evidence for the validity of that contention.

The five researchers collected 92 cores from Montastraea faveolata corals growing at four different sites on the Mesoamerican Reef off the coast of Belize in the Caribbean Sea that had been exposed to either relatively high (Sapodilla Cayes, Utila) or low (Turneffe Atoll, Cayos Cochinos) chronic local stress levels induced by "(1) sedimentation, (2) nutrient input, (3) local human population size adjacent to [the] sites, and (4) a relative measure of fishing pressure based on fish abundance surveys." Then, based on measurements made on these cores, they determined yearly coral extension rates from 1955 to 2006, from which they assessed the degree of rapidity with which the corals at each site recovered from the huge growth-retarding bleaching event of 1998.

Results indicated that following the 1998 bleaching event "coral growth rates at sites with higher local anthropogenic stressors remained suppressed for at least eight years, while coral growth rates at sites with lower stress recovered in two to three years," leading the authors to state that "it is clear that coral colonies experiencing higher local stress before 1998 were more severely affected by bleaching and recovered more slowly than those exposed to lower chronic stress," adding that "local stressors such as increased sedimentation may depress a coral's energy reserves (Rogers, 1990), making it less likely to survive or recover from a bleaching event (Rodrigues and Grottoli, 2007)." Therefore, they confidently conclude that "local conservation efforts that reduce stress, such as reducing runoff by replanting mangroves at the coast or protecting an area from overfishing, could have significant impacts on the ability of corals to withstand the effects of climate change," or as they state more succinctly in the final sentence of their abstract, "reducing chronic stress through local coral reef management efforts may increase coral resilience to global climate change."

Additional References
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO₂, global warming and coral reefs: Prospects for the future. Technology 75S: 71-93.

Rogers, C. 1990. Responses of coral reefs and reef organisms to sedimentation. Marine Ecology Progress Series 62: 185-202.

Rodrigues, I. and Grottoli, A.G. 2007. Energy reserves and metabolism as indicators of coral recovery from bleaching. Limnology and Oceanography 52: 1874-1882.