4 minute read
"Warming Water, and a Higher Tolerance''
In 2015 the Pacific Ocean experienced a massive, year long heat wave. These types of massive ocean warming events are called “El
Niño events, ” and they can be devastating, leading to drought, severe rain, food insecurity, temperature rise, and coral bleaching.
In the Phoenix Island Protected
Area (PIPA) Dr. Michael Fox and his team have been studying and collecting data over an eighteenyear period. PIPA is a World Heritage Site that encompasses 408,250 km² of ocean and eight small islands, and has roughly 34 km² of reef total. Its islands are relatively uninhabited and experience very few cyclones and rough weather. Fox and his team were specifically analyzing the effects of these El Niño events on the shallow water coral reefs of the islands. They were monitoring temperature based changes across four of the eight islands. Their data
by Lucy Gaffneyboro
New York, USA
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dates back to 2000 and their collection ended in 2018. Their study period encompasses three El Niño events. Fox 's team used three different metrics to compare the intensity of stress the corals felt during each El Niño event. They used a daily satellite collected data to learn what the surface sea temperature was on any given day, and they used weekly satellite to collect data sets to determine the maximum temperature during each bleaching event. They also determined the total heat stress corals experienced over the past twelve weeks by adding up all temperatures that spiked above the bleaching threshold.
The three El Niño events recorded over this research period give way to fascinating results and interesting conclusions, as the heatwaves became progressively hotter, corals became increasingly more resilient to this temperature rising.
The first El Niño event took place from 2002 to 2003. There was a total of 76.4% loss in coral cover. By 2005 coral cover in PIPA had dipped to 10.4% from to 44.9%, a devastating loss to this reef system. But there was some hope. Fox 's team believed that the corals that survived this El Niño event would lead to more thermally-tolerant corals in the future, saying: “These thermally tolerant survivors were likely the most important source of new recruits that bolstered the
by Lucy Gaffneyboro
New York, USA subsequent recovery.
” Post-2005 coral communities were luckily able to improve themselves over the next two years; coral cover doubled from 10.4% to 24.4%.
In 2009 the next El Niño event began only six months later; this heat wave was not quite as intense or long lasting as the previous one, but miraculously two years after this El Niño event coral cover had increased 5.7%. Fox 's team was surprised by this shocking recovery. They had been expecting devastation of the PIPA reefs. This El Niño event had been slightly smaller than the 2002-2003 event, but nevertheless this recovery was not expected.
In 2015 a super El Niño event occurred in the Pacific. There were thirty-nine weeks that the bleaching temperatures were reached: more than double the amount of weeks where bleaching temperatures were reached in the 2002-2003 El Niño event. Scientists from around the globe, including Fox 's team expected catastrophic loss in the PIPA coral
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reefs, but this was not the case. In May of 2018, two years after the super El Niño event coral cover had diminished by 40%, roughly half of coral lost in the 2002-2003 El Niño event, despite double the thermal stress.
Scientists determine bleaching and mortality potential of coral communities by measuring the magnitude and length of a thermal stress event. But there are other potential factors that affect bleaching, like environmental factors or adaptations to thermal stress over time. Eleven different potential environmental factors were examined, but differences in these factors did not explain the more than 6% of difference in coral cover loss between the first and third El Niño events. After considering what this could mean, Fox said, “By ruling out environmental factors, we hypothesize that the diminishing effect of thermal stress on coral mortality over three successive heat waves is at least partly associated with an increase in the baseline thermal tolerance of PIPA’s coral communities between 2002 and 2018. ”
Dr. Fox and his team believe that the 2002-2003 El Niño event led to weaker, less temperature resistant corals being weeded out. That first thermal event was extremely hot for its time, and the corals that were able to survive had a genetic makeup that made them more resistant to
by Lucy Gaffneyboro
New York, USA temperature rise. The massive difference in mortality between the 2002-2003 and the 2015 2016 El Niño events is likely due to an increase in the populations of thermally tolerant corals in PIPA reefs.
Dr. Fox and his team believe that this kind of adaptation is most likely to happen in extremely isolated coral reef systems. This kind of adaptation is most limited by corals long life span limiting their ability to adapt quickly. Additionally an increase in one kind of coral decreases population diversity. This hurts reef systems in the long term. What would be best for reefs is if multiple different species of corals where to adopt these genetic phenotypes.
Fox 's team believe there is real change happening in this area between the relationship to coral mortality and an increase in thermal stress.
Works Cited/Image Credits
Fox, Michael D., et al. 2021.
“Increasing Coral Reef Resilience Through Successive Marine Heatwaves. ” Geophysical Research Letters 48 (17): 1-11. doi.org/10.1029/2021GL094128.
