The 2016 mass bleaching event across the Great Barrier Reef and Coral Sea resulted in rapid, regional scale changes in marine communities that are dependent on reefs, according to a study published online in Nature this week. The study shows that fish community structures on southern reefs have become more similar to those in the north and invertebrate communities have also changed considerably.

Rick Stuart-Smith and colleagues investigated the ecological changes in corals, algae, fish and mobile invertebrates (such as sea urchins) following the 2016 mass bleaching event. The authors surveyed 186 reef sites along the Great Barrier Reef and in the Coral Sea before the bleaching event (data from 2010 to 2015) and afterwards (data from 8 to 12 months after the event). They found that 44 of the surveyed sites experienced declines in live coral cover of more than 10%, with the northern Coral Sea reefs suffering the most consistent losses. In addition, declines in coral-eating fish were evident at the most heavily affected reefs.

However, the authors suggest that region-wide ecological changes occurred largely independent of coral loss and seemed to be directly linked to sea temperature. They found evidence of community-wide trophic restructuring, with weakening of pre-existing latitudinal gradients in the diversity of fish, invertebrates and their functional groups. For example, northern reefs saw a decrease in the number of local species of fish whereas there was an increase in the number of small cryptic fish species on southern reefs.

On the basis of their observations, the authors conclude that the recovery process and the scale of the impact remain uncertain. The trajectories of bleached reefs will be greatly influenced by the new community structures that were observed, which are linked to warming-related reshuffling of reef communities.