By Emily Darling
Climate change-induced bleaching can trigger sweeping mortality for reef-building corals. Recent studies have shown that La Niña-like conditions some 4,000 years ago led to an abrupt shut-down in reef growth (Toth et al. 2015), and warmer ocean temperatures linked to disease outbreaks have functionally altered the seascape of Caribbean coral reefs (Randall and van Woesik 2015).
But there are signs of hope – corals and their symbiotic zooxanthellate algae can adapt to warming oceans, and potentially slow the deadly consequences of climate change (Logan et al. 2014). Some coral reefs also have the ability to rebound from bleaching and recover to a coral-dominated state, which can be predicted from simple metrics like reef depth and habitat complexity (Graham et al. 2015).
A sign of hope was offered first-hand during a Wildlife Conservation Society (WCS) expedition last month to survey coral reefs in northwest Madagascar. Our team of scientists from Madagascar and Kenya spent a combined 80 hours underwater, counting more than 5,000 corals and invertebrates, and over 9,000 reef fish.
Underwater, we found healthy coral communities with remarkable diversity. Live coral cover makes up 55 percent of the reefs – a substantial increase from 35% cover four years ago that signals coral growth and recovery. At one site, coral cover skyrocketed to 80% with large table corals (2 meters in diameter) tiered like wedding cakes as far as I can see.
In 1998, these reefs suffered mass bleaching and mortality during a severe El Niño event that created superheated water for most of the corals in the Western Indian Ocean, and around the planet. After 17 years, reefs in northwest Madagascar have recovered to a remarkable state where coral colonies fight other coral colonies for space, with only the isolated macroalgae to be seen.
Just a short boat ride from the Madagascar coastline, these beautiful reefs support the fishing livelihoods of local Malagasy communities. We see many large and colourful parrotfish, indications of low fishing effort and the use of traditional gears, which are good signs of reef recovery. Fish biomass is calculated at 1100 kg/ha, well above a recently published target of 500 kg/ha that maintains healthy ecosystems and functioning ecosystem processes (MacNeil et al. 2015).
Working collaboratively with local communities and management agencies, WCS has developed management plans for two National Marine Parks, the first community-led Marine Protected Areas placed under legal protection by the Madagascar government. The goal is to sustainably manage fishing effort in the area and remove destructive gears that could harm corals and their recovery from climate change.
How can local climate refuges like northwest Madagascar scale up into global outcomes for coral reefs?
First, the unique oceanography of currents, gyres and eddies that swirl off the tip of Madagascar highlight the importance of understanding oceanographic processes for coral reef around the world, particularly processes that can promote climate refuges by cooling or mixing superheated waters. The reefs of northwest Madagascar provide hope that other reefs of unique diversity and beauty still exist in the aftermath of mass coral bleaching events.
Second, understanding the links between process and pattern is now possible with large datasets of coral surveys, such as a recent study published in Climatic Change that found coral communities are more resilient to climate change than expected (e.g., McClanahan et al. 2015).
Climate refuges are increasingly highlighted as a strategy for coral survival in warming oceans (Van Hooidonk et al. 2013, Chollett et al. 2014, Mumby et al. 2014, Cacciapaglia et al. 2015). Amidst expectations that we are entering a very strong El Niño year, NOAA Coral Watch states, “a full-fledged global bleaching event is likely.” Corals will need these refuges this year.
To understand and manage for this event, and others to come, scientists will continue to observe, hypothesize, test and refine our understanding of the impact of warming oceans and climate refuges on coral reefs. Only by developing these results in collaboration with people working on the ground – NGOs, local and national agencies, stakeholders and decision makers – can we hope to speed up conservation to match the pace of climate change.
Cacciapaglia, C., & Woesik, R. (2015). Reef‐coral refugia in a rapidly changing ocean. Global change biology, 21(6), 2272-2282.
Chollett, I., Enríquez, S., & Mumby, P. J. (2014). Redefining thermal regimes to design reserves for coral reefs in the face of climate change.
Graham, N. A., Jennings, S., MacNeil, M. A., Mouillot, D., & Wilson, S. K. (2015). Predicting climate-driven regime shifts versus rebound potential in coral reefs. Nature, 518(7537), 94-97.