The unprecedented back-to-back global bleaching events of 2014–17 spread a “mountain of warm water” to coral reefs worldwide, according to Tom Oliver, the ocean acidification program manager at the Pacific Islands Fisheries Science Center at the National Oceanic and Atmospheric Administration (NOAA).
When ocean temperatures spike, corals expel the algae that supply them with nutrition and their brilliant colors. That leaves corals a ghostly white and they can die unless the water cools.
The agency recently declared the global bleaching event over – for now.
In March, NOAA researchers embarked on an 88-day expedition to investigate how Pacific reefs have fared since 2015. The expedition was part of NOAA’s annual National Pacific Reef Assessment and Monitoring Program, and had originally planned to visit just the Marianas Islands and Guam. But in the wake of the global coral bleaching event the expedition made extra stops at the United States territories of Jarvis, Howland, Baker and Wake Islands.
Thomas Oliver, Brett Schumacher and Kelvin Gorospe oversaw different aspects of the journey. Like Oliver, Schumacher and Gorospe work at the Pacific Islands Fisheries Science Center, as a coral reef researcher and a reef fish researcher, respectively. While the health of the reefs they saw varied from island to island, one of the biggest contrasts was among their first stops: Jarvis, Howland and Baker.
Oceans Deeply spoke with the scientists about the recent expedition, the contrast between Jarvis, Howland and Baker and the future of coral reefs in the Pacific.
Oceans Deeply: Why is NOAA researching these reefs?
Thomas Oliver: Over the past 20 or 30 years, on average in the Pacific, we have been seeing fewer fish due to a number of issues on coral reefs. Right now, the most pressing is the warming of the ocean, leading to a phenomenon called coral bleaching. Generally speaking, reefs are in a recovery phase in these places in the Pacific. Why that recovery is important is because lots and lots of people, and lots and lots of biodiversity depend on those reefs. The people in the region depend on the reefs for food, for fish, for protection from surf. Reefs are also the most biodiverse habitat in the ocean – literally a quarter of known marine species occur on reefs, and when reefs die, due to bleaching or other causes, that biodiversity is threatened.
Oceans Deeply: What did you find on Jarvis?
Brett Schumacher: Jarvis was our first stop after leaving Honolulu. What we saw there was pretty utter devastation of what the reefs were. I had never been there before this trip, so I had only seen it in pictures and heard my coworkers talk about it. But going down there this time, 98 percent of the coral cover was lost at Jarvis. Entire groups of corals were lost.
There were a few survivors and a few glimmers of hope there. Some of the really large Porites colonies – these massive [coral] colonies that can be several meters across. Some of them died, but some of them did survive.
And then, some of the areas on the reef were overgrown by these patches of a red macro algae. Besides the fact that that is a pretty big change from what used to be a vibrant coral reef, the algae also take up space and prevent coral from settling in those areas. That is something that can limit reef return and recovery.
Oliver: One thing that I think I should say about Jarvis: it literally broke our metrics for how hot reefs get. The main way that we measure heat stress is not just how hot it got, but how hot it got over a certain threshold for a certain amount of time. We call those “degree heating weeks.” The scale that we used to use ran from 0 to 16 [degrees C]. At 4 we expect bleaching, at 8 we expect bleaching and mortality, and above 8 life is bad. In Jarvis in 2015, we hit 38-degree heating weeks. It is one of the highest that we have ever recorded and a whole new level of heat stress for the area.
Oceans Deeply: Kelvin, you had seen Jarvis before the heating in 2015. Can you describe what it was like?
Kelvin Gorospe: The one time that I made it to Jarvis, I dove for one day. That one day of diving there was the most epic diving that I had done as part of NOAA. On the first dive, we jumped in the water and before we even finished getting set up, we turned and just saw a parade of sharks coming in toward us. No exaggeration, 30–40 sharks just coming straight for us to come check us out.
It’s a really special place. You just don’t see that amount of fish biomass in any of the other places that we go to. There is no comparison.
Oceans Deeply: How long could it take this ecosystem to recover to what Kelvin saw?
Oliver: You just asked a really hard, and potentially speculative question. We’re really concerned right now about the dynamics of coral reef recovery. Largely because we expect disturbances on reefs to continue. Reefs are going to be in this repeated state of recovery, so figuring out the dynamics and the timing of recovery is a critical and active scientific question.
Jarvis is a really interesting case for this question, because El Niños are not new. The most recent, biggest bleaching event was 1998. There were conditions that were almost as bad as this time, at Jarvis. Looking at the reef in 2000, two years later, there was some dead coral, but the system was still pretty intact. So, the potential for recovery at Jarvis is really huge, and even the potential for rapid recovery is pretty huge.
But, that’s because [Jarvis] is limited from fishing and limited from nutrient impact because it is so remote. Its remoteness does not protect it from climate change.
Oceans Deeply: And what did you find at other central Pacific Islands, like Howland and Baker?
Oliver: Howland and Baker actually did quite well during this last event. We don’t have records of their bleaching, no one was there, but they got less exposure [to temperature change] and saw potentially less mortality than Jarvis. In fact, the change in coral cover is within the margin of error.
It was a huge weight off our shoulders. Rolling up to Howland and Baker, we were expecting Jarvis all over again.
Schumacher: As we had our four-day transit to Howland and Baker, and having just spent several days at Jarvis not seeing coral and seeing algae and thinking that there is no beauty left in the world, getting to Baker and jumping in and seeing that it was still a beautiful reef, that there was still magnificent areas and diverse species of fish and corals, was huge.
Oliver: For me, not having been there, the difference between Jarvis and Howland and Baker doesn’t make me hopeful or happy. Because the difference between those two places is not in the corals that are there, it is not in the ability of those corals to resist what is going on. The difference is: Jarvis got really hot, and Howland and Baker didn’t get quite so hot. There is some randomness in that. Jarvis speaks to the scale of the problem, and Howland speaks to [the fact that] it’s not horrible everywhere yet.
Oceans Deeply: What were your biggest takeaways from this expedition?
Gorospe: I had the pleasure of doing three presentations. I gave one public talk, one talk to the politicians and one talk to scientists. Surprisingly, the people who were least [aware] of the whole coral bleaching phenomenon were the politicians. That was a surprise, and it highlighted how much more work we have to do to educate the people who can potentially effect change.
Schumacher: Being at those places that are thousands of miles from any center of human population, [and] are still feeling the effects of these El Niños. The global consequences of some of our actions and the reach of our effect across the planet is one of the things that I came away from the cruise with.
Oliver: The takeaway from this whole expedition, for me, was that the disturbances are coming. We can limit how bad and how long they last by focusing, as a global community, on what we’re putting in the atmosphere. But really, on a local scale, what we need is to set up reefs for recovery. They’re very adaptable creatures – there’s a lot of diversity in their ability to deal with temperature stress, and so I have great faith that corals can, theoretically, deal with new levels of temperature stress. The question is, can they make that transition fast enough?