"But a new 10-year study from Palmyra Atoll [...] shows that reefs outside the reach of local human impacts can recover from bleaching." Oh, really! And the recovery was remarkable? Who could have said that...

Decadal stability of coral reef benthic communities on Palmyra Atoll, central Pacific, through two bleaching events. Adi Khen, Maggie D. Johnson, Michael D. Fox, Samantha M. Clements, Amanda L. Carter & Jennifer E. Smith. Coral Reefs volume 41, pages 1017–1029, May 21 2022. https://link.springer.com/article/10.1007/s00338-022-02271-6

Abstract: The prevalence of coral bleaching due to thermal stress has been increasing on coral reefs worldwide. While many studies have documented how corals respond to warming, fewer have focused on benthic community responses over longer time periods or on the response of non-coral taxa (e.g., crustose coralline algae, macroalgae, or turf). Here, we quantify spatial and temporal changes in benthic community composition over a decade using image analysis of permanent photoquadrats on Palmyra Atoll in the central Pacific Ocean. Eighty permanent plots were photographed annually between 2009 and 2018 on both the wave-exposed fore reef (FR, 10 m depth, n = 4 sites) and the wave-sheltered reef terrace (RT, 5 m depth, n = 4 sites) habitats. The El Niño events of 2009–2010 and 2015–2016 resulted in acute thermal stress and coral bleaching was observed at both reef habitats during these events. Across 10 yr and two bleaching events, the benthic community structure on Palmyra shows evidence of long-term stability. Communities on the RT exhibited minimal change in percent cover of the dominant functional groups, while the FR had greater variability and minor declines in hard coral cover. There was also spatial variation in the trajectory of each site through time. Coral cover decreased at some sites 1 yr following both bleaching events and was replaced by different algal groups depending on the site, yet returned to pre-bleaching levels within 2 yr. Overall, our data reveal the resilience of calcifier-dominated coral reef communities on Palmyra Atoll that have persisted over the last decade despite two bleaching events, demonstrating the capacity for these reefs to recover from and/or withstand disturbances in the absence of local stressors.

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Popular version

CENTRAL PACIFIC CORAL REEF SHOWS REMARKABLE RECOVERY DESPITE TWO WARM-WATER EVENTS. Scripps Institution, Jul 6 2022. https://scripps.ucsd.edu/news/central-pacific-coral-reef-shows-remarkable-recovery-despite-two-warm-water-events

Excerpts, images and links removed:

The largest global coral-bleaching event ever documented struck the world’s oceans in 2014 and lasted until 2017. The onset of this abnormal whitening condition spawned widespread gloom-and-doom news reports about its calamitous effect on Australia’s Great Barrier Reef and more general predictions of coral reef extinction by 2050. 

But a new 10-year study from Palmyra Atoll in the remote central Pacific Ocean shows that reefs outside the reach of local human impacts can recover from bleaching.

“One year after each bleaching event, we did see signs of coral decline at some of the sites, but within two years this was restored,” said Adi Khen, a Scripps Oceanography PhD candidate and lead author. The research team of current and former members in marine ecologist Jennifer E. Smith’s laboratory saw only a small net change in the reef’s coral and algae populations after a decade. Khen, Smith and four co-authors published their results in the journal Coral Reefs [https://link.springer.com/article/10.1007/s00338-022-02271-6].

“This is a testament to the resilience of Palmyra’s reefs in the context of climate change and demonstrates the capacity for recovery in the absence of local stressors,” Khen noted. “Because we have this long-term time series, we’re able to see perspectives from before and after disturbances.” 

[The Palmyra Atoll is located 1,300 kilometers (about 805 miles) south of Hawaii.]

“During the warming event that occurred in 2015, we saw that up to 90% of the corals on Palmyra bleached but in the year following we saw less than 10% mortality,” Smith reported. “Measuring not just death and destruction but measuring recovery is really important. Seeing the recovery is where we can learn what might help inform future management.”

Smith first visited the Palmyra National Wildlife Refuge in 2005, shortly after it was established. The recent paper includes data that Smith or members of her team, diving in scuba gear, collected at least once annually from 2009 to 2018 during visits to the exact same 80 plots spread across eight sites around the atoll along underwater transects. 

Using simple image-analysis tools and digital tracing, her team then identifies all bottom-dwelling organisms present in the plots to see if they are increasing or decreasing in abundance. It takes from one to two hours for a team of graduate and undergraduate students to process each image. More than 1,500 images have been taken of the plots so far.

[...]

The Palmyra dataset suggests that warm-water events by themselves may cause some decline in reef-building corals and calcifying algae. But the atoll’s clean, protected waters and intact ecosystems harbor a healthy population of fishes that may contribute to the resilience of Palmyra’s reefs.

“These remote, protected reef systems  seem to be able to recover much faster than reefs that are adjacent to dense human populations where overfishing, coastal development, and fresh water runoff containing fertilizers and pesticides all may erode the ability of a reef to recover,” Smith said. 

Most coral reefs are near large human populations in locations beset with pollution, overfishing, sedimentation, warming and acidification.

“Certainly, they’re going to be more susceptible to large losses,” Smith said. “But to know that places like this are out there showing signs of resistance and resilience gives us hope and also shows us that there’s still a lot to learn about how these intact systems are functioning.”

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