Coral Reefs Shifting away from Equator

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An experiment in the Palmyra Atoll National Wildlife Refuge collects coral larvaeAn experiment in the Palmyra Atoll nature reserve collects coral larvae, allowing researchers to enumerate the number of small corals that settle on a coral reef. Recent research shows that corals are creating new barriers in temperate regions as they withdraw from the ever warmer waters of the equator.Credit: Nichole Price / Bigelow Laboratory for Ocean Sciences

Coral reefs are retreating from equatorial waters and are creating new barriers in more temperate regions, according to new research in the magazine Marine ecology progress series. The researchers found that the number of young corals on tropical coral reefs decreased by 85% – and doubled in the subtropical reefs – during the last four decades.

"Climate change appears to redistribute coral reefs, much as many other marine species are moving," said Nichole Price, senior researcher at the Bigelow Laboratory for Ocean Sciences and lead author of the paper. "The clarity of this trend is surprising, but we do not yet know if the new coral reefs can support the incredible diversity of tropical systems".

While climate change warms the ocean, subtropical environments are becoming more favorable for corals than equatorial waters where they traditionally prospered. This allows the drifting coral larvae to settle and grow in new regions. These subtropical reefs could provide refuge for other species challenged by climate change and new opportunities to protect these nascent ecosystems.

Researchers believe that only certain types of coral are able to reach these new places, based on how far the microscopic larvae can swim and drift on the currents before the limited fat reserves end. The exact composition of most new coral reefs is currently unknown, due to the cost of collecting data on genetic diversity and species.

"We are witnessing the transition of ecosystems towards new mixtures of species that have never coexisted, and it is not yet clear how long it takes for these systems to reach equilibrium," said Satoshi Mitarai, associate professor at Okinawa Institute of Science and Technology Graduate University and an author of the study. "The lines are really starting to get confused about what is a native species, and when ecosystems are working or falling apart."

New coral reefs grow when the larvae settle on a suitable seabed far from the coral reef from where they originated. The research group examined the latitudes up to 35 degrees north and south of the equator, and found that the displacement of coral reefs is perfectly mirrored on both sides. The document assesses where and when "refugee corals" could settle in the future, potentially bringing new resources and opportunities such as fishing and tourism.

Researchers, an international group of 17 institutions in six countries, have compiled a comprehensive database of studies dating back to 1974, when registration began. They hope that other scientists will add to the database, making it ever more complete and useful for other research questions.

"The results of this work underscore the importance of truly long-term studies documenting change in coral reef communities," said Peter Edmunds, professor at the University of California Northridge and author of the article . "The trends we have identified in this analysis are exceptionally difficult to detect, but of the utmost importance in understanding how the barriers will change in the coming decades. With the worsening of the coral reef crisis, the international community will have to step up efforts to combine and summarize the results as we were able to do with this study ".

Coral reefs are complicated interconnected systems, and it is the interaction between species that enables their healthy functioning. It is unclear which other species, such as coral algae that facilitate the survival of vulnerable coral larvae, are also expanding into new areas, or how successful young corals can be without them. Price wants to investigate the relationships and diversity of species in the new barriers to understand the dynamics of these evolving ecosystems.

"There are still many questions about which species they are and are not doing to these new places, and we do not yet know the fate of these young corals in longer time periods," said Price. "The changes we are seeing in coral reef ecosystems are unsettling, and we must work hard to document how these systems work and learn what we can do to save them before it's too late."

Some of the research that informed this study was conducted at the long-term ecological research site Moorea Coral Reef of the National Science Foundation near French Polynesia, one of 28 long-term research sites across the country and around the world .

"This report addresses the important issue of whether warming waters have led to an increase in coral populations," says David Garrison, program director of the Ocean Sciences Division of the National Science Foundation, which funded the research. "If this offers hope for the sustainability of coral reefs it requires more research and monitoring."

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