The well-being of hundreds of millions of people worldwide depends upon healthy coastal ecosystems. Access to fishing, ability to attract tourism, and harvesting of resources like wood are just some of the services provided by such habitat in tropical regions. But what will happen to these ecosystems and the services they provide as water levels start to rise is not yet clear.

Back in 2007 the western Solomon Islands were shaken by a magnitude 8.1 earthquake, resulting in localized uplift to the islands of Ranongga and subsidence to the Roviana and Vonavona lagoons.

Soon after the earthquake, local communities noticed major changes in the coastal ecosystem, including dieback of mangrove forests and fresh coral growth in previously unpopulated regions of ocean. They alerted marine scientist Simon Albert from the University of Queensland in Australia, who has been interacting with communities in the Solomon Islands for more than 15 years.

"Their deep connectivity and understanding of the natural environment allows them to rapidly detect when changes occur," said Albert. As well as finding out what had caused the sudden changes to the ecosystem, Albert saw an opportunity to monitor the impact of sea-level rise.

Using satellite imagery as well as measurements from the ground, Albert and his colleagues mapped the spatial extent and composition of mangrove, seagrass and shallow coral areas in the Roviana Lagoon for 2006, 2009, 2012 and 2014.

The researchers found that the earthquake-induced increase in sea-level rise – between 30 and 70 cm – resulted in a steady lateral growth of coral reef habitats, with a 157% increase in areal coverage over the seven-year period. Meanwhile, mangrove ecosystems suffered a rapid dieback, losing around 35% coverage by 2009 then slowly recovering, albeit with a different community structure. Seagrass ecosystems showed less of an obvious trend, but experienced both losses and gains over small-scale areas.

Although climate change is unlikely to bring such a sudden change in sea level, the earthquake-induced sea-level changes demonstrate that what can be bad news for one habitat can turn out to be good news for another.

"In this case, sea-level rise allowed the coral more vertical and lateral room to grow," said Albert, whose findings are published in Environmental Research Letters (ERL) . But for the mangroves rising water levels were too much to cope with. "Mangroves use complex root structures that allow them to breathe when submerged at high tide," Albert added. "When these water levels suddenly rise and they remain submerged for too long, they can literally suffocate."

By understanding these differing responses to sea level, Albert and his colleagues believe that we can better support the resilience of all coastal species to sea-level change. Indeed, they hope that their findings from the Solomon Islands will feed into the mitigation strategies that are being planned to buffer us from sea-level rise over the coming decades.

"It is imperative that we leave room in the coastal zone for coastal ecosystems to migrate as sea-levels rise," added Megan Saunders, also at the University of Queensland. "Hardening the shoreline, for instance, with sea-walls or levees to defend infrastructure, will prevent coastal ecosystems from moving inland as water levels rise. Early planning for the impacts of sea-level rise will have much better outcomes than if we wait until it is too late."

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