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Harboring fish and mysteries

Speed read
  • Study investigates influence El Niño has on coral larvae dispersal.
  • 14 years of ocean movement modeled on BlueCyrstal supercomputer. 
  • Models indicate eastern Pacific coral are isolated from central Pacific replenishment sources.

Eastern Pacific corals are lucky to be alive.

With more than 5,000 km of open ocean between them and their nearest neighbors in the central Pacific, the survival of eastern Pacific coral in an adverse and isolated place might hold some important information about how other reefs respond to future climate change.

A common theory is that coral larvae may, on rare occasions, migrate from west to east with the help of El Niño, a climatic phenomenon that speeds up eastward surface currents along the equator.

Drifters. Monthly larval trajectories from the Northern Line Islands, January 1997 to August 1998. Density of larval paths by month of release from the Northern Line Islands. Courtesy Sally Wood.

But evidence to support this theory is based largely on ocean current speeds recorded below the surface, whereas coral larvae mostly travel near the surface, where currents can be slightly different.

Until now, it has been impossible to directly test this theory. Millimeters in size, coral larvae are impossible to track directly over long distances in the open ocean. While coral larvae have been observed in eastward-flowing currents, there’s no guarantee they will ever reach a reef in the eastern Pacific or, if they do, that they’ll survive long enough to form a new coral colony.

“The vast majority of coral larvae, which are released in the billions, settle close to home or never survive to find a new home, ” says Sally Wood, a researcher with the University of Bristol who collaborated with an international team of scientists for this study. “But the infinitesimally small number that do make it to travel further afield are so important. They can repopulate a damaged reef and spread genetic diversity that will help coral populations adapt to changing climate."

Three recent events made it possible to test this theory for the first time in a simulated environment:

  1. High-resolution ocean current reanalysis data from the Hybrid Coordinate Ocean Model (HYCOM) became available, crucially including one of the two strongest El Niño events in the last 100 years, the El Niño of 1997-98.
  2. The Connectivity Modeling System (CMS) was developed by Claire Paris at the Rosenstiel School of Marine and Atmospheric Science. CMS was specifically designed to model the very large numbers of individual particles, as required to test the theory.
  3. Improvements to the BlueCrystal supercomputer at the University of Bristol made it possible to meet the large computational demands of modeling so many individual larvae.

<strong> Charting course. </strong> Top: Central and eastern Pacific model reef cells (black squares), observed sea surface temperatures and schematic of large-scale surface currents (black arrows). Below: Regional reef delimitations. Courtesy Sally Wood.

Wood and her colleagues modeled the release of 1,600 coral larvae a day from each of 636 reef locations, grouped into 10 eastern and 10 central Pacific regions. This amounted to 5,054 release events and more than 5.1 billion larvae over 14.5 years of simulations.

The study revealed that there were no eastward connections at all between the central and eastern Pacific reefs for any of the billions of larval paths modeled over the entire study period. This finding implies that eastern Pacific coral populations have been isolated from central Pacific sources of population replenishment since at least 1997.

This has implications for the ability of eastern Pacific coral populations to recover from current and future disturbances. Without sufficiently regular influxes of coral larvae from external sources, reefs in the eastern Pacific are in peril of never being able to recover from increasingly frequent damaging events, like coral bleaching.

“One of our biggest challenges is the sheer amount of variability in all the factors influencing dispersal, including the biology of the larvae and the currents transporting them,” Wood said. “This is where modeling comes into its own, allowing us to explore and visualize this variability and test which factors have the greatest influence on dispersal patterns. This will then help to determine what research should be prioritized — What is it about larvae or currents that we need to really tie down? Where should we sample to look for genetic evidence of population connections?” 

In order to delve deeper into the mysteries of coral reef migration and survival, Wood and her colleagues need to run many more simulated larvae over a range of conditions. Thanks to high performance computing resources like BlueCrystal and the Connectivity Modeling System, they’ll be able to do just that.

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