Coral reefs retreating from equatorial waters

Coral Reefs in Indonesia. A diversity of corals, echinoderms, sponges, and other life compete for space and plankton on the reefs surrounding Bangka Island, North Sulawesi, Indonesia, Pacific Ocean.  © Ethan Daniels
 
 
According to a new research study? in the journal of Marine Ecology Progress Series, coral reefs are shifting away from the equator and are finding new reefs in more temperate regions. Researchers found that the number of young corals on tropical reefs has declined by nearly 85% and have doubled on subtropical reefs during the last four decades.

Nichole Price, a senior research scientist at Bigelow Laboratory for Ocean Sciences, said that climate change seems to be redistributing coral reefs the same way as it is shifting other marine species-and the trend is quite obvious. However, the uncertainty lies in whether the new reefs can support the incredible diversity of tropical systems. As the ocean gets warmer, subtropical environments are becoming more favourable for corals than the equatorial waters where they normally thrived. This is allowing 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 fledgling ecosystems.

Researchers believe that only certain types of coral are able to reach these new locations which are based on how far the microscopic larvae can swim and drift on currents before they run out of their limited fat stores. The exact composition of most new reefs is currently unknown, due to the expense of collecting genetic and species diversity data. "We are seeing ecosystems transition to new blends of species that have never coexisted, and it's not yet clear how long it takes for these systems to reach equilibrium," said Satoshi Mitarai, an associate professor at Okinawa Institute of Science and Technology Graduate University and an author of the study. "The lines are really starting to blur about what a native species is, and when ecosystems are functioning or falling apart."
 
New coral reefs grow when larvae settle on suitable seafloor away from the reef where they originated. The research team examined 35 degrees north and south of the equator and found that the shift of coral reefs is perfectly distributed on either side. They also looked at the location where “refugee corals” could settle in the future, potentially bringing new resources and opportunities such as fishing and tourism. The researchers, an international group from 17 institutions in six countries, compiled a global database of studies dating back to 1974, when record-keeping began. They hope that other scientists will add to the database, making it increasingly comprehensive and useful to other research questions. "The results of this paper highlight the importance of truly long-term studies documenting change in coral reef communities," said Peter Edmunds, a professor at the University of California Northridge and author of the paper. "The trends we identified in this analysis are exceptionally difficult to detect, yet of the greatest importance in understanding how reefs will change in the coming decades. As the coral reef crisis deepens, the international community will need to intensify efforts to combine and synthesize results as we have been able to accomplish with this study."
 
Coral reefs are intricate interconnected systems, and it is the interplay between species that enables their healthy functioning. It is unclear which other species, such as coralline 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 new reefs to understand the dynamics of these evolving ecosystems.
 
Some of the research that informed this study was conducted at the National Science Foundation's Moorea Coral Reef Long-Term Ecological Research site near French Polynesia, one of 28 such long-term research sites across the country and around the globe. "This report addresses the important question of whether warming waters have resulted in increases in coral populations," says David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research. "Whether this offers hope for the sustainability of coral reefs requires more research and monitoring."
 
Journal Reference:
1.    NN Price, S Muko, L Legendre, R Steneck, MJH van Oppen, R Albright, P Ang Jr, RC Carpenter, APY Chui, TY Fan, RD Gates, S Harii, H Kitano, H Kurihara, S Mitarai, JL Padilla-Gamiño, K Sakai, G Suzuki, PJ Edmunds. Global biogeography of coral recruitment: tropical decline and subtropical increase. Marine Ecology Progress Series, 2019; 621: 1 DOI: 10.3354/meps12980

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© 2019 Oceanographer Daneeja Mawren