Corals vs. Macroalgae in a CO₂-Enriched and Warmer World

According to Bruno et al. (2009), one of the great concerns of marine scientists is that "coral reefs are moving toward or are locked into a seaweed-dominated state," based on observations of what occurred on several Jamaican reefs during the 1980s, which concerns are often parroted among climate alarmists. In an effort to assess the generality of these claims, however, Bruno et al. "analyzed 3,581 quantitative surveys of 1,851 reefs performed between 1996 and 2006 to determine the frequency, geographical extent, and degree of macroalgal dominance of coral reefs and of coral to macroalgal phase shifts around the world."

In doing so, the five marine researchers found that "the replacement of corals by macroalgae as the dominant benthic functional group is less common and less geographically extensive than assumed," noting that "only 4% of reefs were dominated by macroalgae (i.e., >50% cover)." In fact, across the Indo-Pacific, where regional averages of macroalgal cover were 9-12%, they found that "macroalgae only dominated 1% of the surveyed reefs." In addition, they learned that "between 1996 and 2006, phase shift severity decreased in the Caribbean, did not change in the Florida Keys and Indo-Pacific, and increased slightly on the Great Barrier Reef."

Commenting on such findings, Bruno et al. state that "coral reef ecosystems appear to be more resistant to macroalgal blooms than assumed," and that "the mismatch between descriptions of reef degradation in the literature and patterns in nature was caused by the generalization of a relatively small number of examples," concluding that their analysis suggests that "the macroalgae problem has been exaggerated," and that "overall," there has been "no general recent trend (i.e., post-1995) toward macroalgal dominance." In fact, they say that "macroalgal cover may currently be close to the historical baseline across most of the world."