Assessing evidence of phase shifts from coral to macroalgal dominance on coral reefs

TLDR

Many marine scientists have concluded that coral reefs are moving toward or are locked into a seaweed‑dominated state, yet the magnitude of such phase shifts was unknown due to a lack of regional‑ or global‑scale analyses. The study aimed to quantify the frequency, geographic extent, and severity of macroalgal dominance and coral‑to‑macroalgal phase shifts worldwide using 3,581 quantitative reef surveys. We analyzed 3,581 quantitative surveys of 1,851 reefs collected between 1996 and 2006 to determine these patterns. The analysis revealed that macroalgal dominance (>50 % cover) occurred on only 4 % of reefs globally, with the Caribbean showing the highest local increases but overall reefs are more resistant to macroalgal blooms than previously assumed, and phase‑shift severity varied regionally, decreasing in the Caribbean, remaining stable in the Florida Keys and Indo‑Pacific, and increasing slightly on the Great Barrier Reef.

Abstract

Many marine scientists have concluded that coral reefs are moving toward or are locked into a seaweed‐dominated state. However, because there have been no regional‐ or global‐scale analyses of such coral reef “phase shifts,” the magnitude of this phenomenon was unknown. We analyzed 3581 quantitative surveys of 1851 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. Our results indicate that the replacement of corals by macroalgae as the dominant benthic functional group is less common and less geographically extensive than assumed. Although we found evidence of moderate local increases in macroalgal cover, particularly in the Caribbean, only 4% of reefs were dominated by macroalgae (i.e., >50% cover). Across the Indo‐Pacific, where regional averages of macroalgal cover were 9–12%, macroalgae only dominated 1% of the surveyed reefs. 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 due to moderate coral loss. Coral reef ecosystems appear to be more resistant to macroalgal blooms than assumed, which has important implications for reef management.