The world has seen the effects of climate change: intensifying wildfires, more record-breaking floods–devastation after devastation of ecosystems that threaten all species all over the planet.

One ecosystem that’s concerning is coral reefs. As we know, coral reefs are important for our planet; they sustain marine biodiversity in all oceans.

However, they’re facing more challenging conditions as waters heat up. When temperatures of waters increase, that leads to a phenomenon known as marine heat waves.

Since about 10 years ago, Katie Barott at the University of Pennsylvania has led a team of researchers. Together, they’ve studied two dominant coral species in Hawaii to have a better grasp at knowing their adaptability to climate change effects.

Their report reveals how corals are adapting to the rapidly changing environment in complex and varied ways. The researchers also show how some corals are struggling to adapt to it as well.

According to Barrot, the team tracked more than 40 large coral colonies. They found that certain species have better ability to endure and recover from heat waves.

“It’s a bit like working out; the more often you exercise, the easier it is to go through the same exercise stress,” Barott highlighted the similarities.

As mentioned, the researchers studied 2 coral species that are dominant in Kaneohe Bay in Oahu, Hawaii. The species are rice coral (Montipora capitata) and finger coral (Porites compressa). 

Both corals experienced significant marine heatwaves in 2014, 2015, and 2019. Therefore, they gave the researchers a chance to identify ones that are bleach resistant or bleach susceptible. 

After identifying coral individuals, the researchers observe their responses to continuous heat stress. 

The researchers found that there are some resilient individuals who have adapted to the change of environment. However, there is also an increased vulnerability of others.

Acclimatization and its role for coral resilience

First author of the paper Kristen Brown said that acclimatization plays a role in the corals’ resilience.

In the study, acclimatization refers to the “ability of some corals to adjust to higher temperatures, thereby reducing their susceptibility to bleaching”

And as we all know, coral bleaching is where coral expel the algae living within their tissues. When that happens, the corals turn white, increasing their risk of death. 

According to the researchers, individuals that are resistant to bleaching doesn’t lose their pigment throughout the study period. This suggests that the individuals show a persistent form of heat tolerance.

Pigmentation alone, however, didn’t correctly indicate each coral’s health or resilience.

There are also contrasting recovery patterns between corals that are susceptible to bleaching after the heat waves.

Montipora capitata, for instance, showed evidence of acclimatization. However, the species experience bleaching frequently. It also showed significant mortality for about 3 years after the last heat wave.

On the other hand, species that showed initial sensitivity like Porites compressa showed a notable ability to recover and acclimatize. 

This species didn’t experience bleaching or mortality during the third heat wave, and its conditions returned to normal within a year.

Such a difference highlights an important aspect of coral resilience. Resilient corals show the ability to survive heat stress and also recover from it effectively. 

According to the researchers, responses to climate change from the corals are complex and diverse. There are a lot of factors that may influence them, including past exposure to stressors and species-specific characteristics.

Moving forward, the team wants to continue monitoring and exploring other aspects. 

More specifically, they want to find out more about coral growth,  calcification, and the impacts of recurring marine heatwaves.

Other than developing resilience, the corals may also get help from other species to face the rapidly warming planet.

The fair-feathered friends

Researchers at Lancaster University have led a new study about the unlikely connection between corals and seabirds.

Per the study, the presence of seabirds on islands near tropical coral reefs can increase coral growth rates on the reefs by more than double. 

Since the corals grow faster, any coral reefs near seabird colonies can recover quicker from bleaching events.

The study focused on Acropora. It’s an important type of coral that supports fish populations and reef growth.

As we know, reef growth plays a vital role for protecting coastal areas from waves and storms. 

Acropora around islands with seabirds, the researchers found, recovered from bleaching events faster.

Reefs located away from seabird colonies recovered in about four years and six months. Those that are near seabird colonies recovered by around 10 months faster.

So what made it happen? 

It’s simple: seabird droppings. 

Seabirds feed on fish in the open ocean, and they return to islands to rest. There, the birds deposit nitrogen and phosphorus-rich nutrients through their droppings.

Some of the droppings get washed off the islands by rain into the surrounding seas. 

And just like how cattle manure fertilizes soil and plants, the guano does the same to corals and other marine species. 

The study’s lead author Dr Casey Benkwitt said, “This faster recovery may be critical as the average time between successive bleaching events was 5.9 years in 2016 — a reduction from 27 years in the 1980s.

“Even small reductions in recovery times during this window may be key to maintaining coral cover over the short-term.”

In terms of location, the study focused on a remote archipelago in the Indian Ocean. 

Here, the researchers studied reefs near populations of searbids like the red-footed boobies, sooty terns and lesser noddies.

As mentioned, they compared it to  islands with few seabirds. These islands have instead thriving populations of rats–a destructive invasive species that negatively impacts birds.

 

 

Rats plaguing coral reef health

Aside of surveying and observing the two differently located reefs, the researchers also did and experimental study.

They wanted to make sure if the faster growth was directly caused by nutrients and not other factors. So, they transplanted some Acropora corals between islands with and without rats. 

The researchers confirmed that the presence of seabirds leads to nutrient enrichment.

Moreover, coral colonies transplanted to seabird islands grew twice as fast as those transplanted to rat-infested islands.

Dr Benkwitt expressed excitement as the team showed a link between the seabirds’ presence and faster coral growth. The findings signified that there is a natural solution to boost coral resilience as the climate is changing.

“By restoring seabird populations, corals can quickly take-up and benefit from the supply of new nutrients, and our three-year experiment shows that these benefits are not just a short boost–they can be sustained over the long-term,” Dr Benkwitt said.

Furthermore, the researchers added that their findings showed ecological damage from invasive rats both on land and sea. 

Principal investigator of the study Professor Nick Graham stated that the results suggested getting rid of rats. 

In addition, restoring seabird populations are important to re-establish the natural flows of seabird nutrients to the marine environment. It’s important because we expect to see more frequent climate disturbances in the future.

Also, the study suggests that the benefits of seabird nutrients aren’t limited to coral recovery.

Co-author of the study Dr Shaun Wilson added, “Growth rates of fish on reefs adjacent to islands with large seabird colonies is also faster and overall biomass of fish is 50% greater than on reefs next to islands with rats.

“Consequently, rates of grazing and bioerosion by fishes is three times faster on islands with seabirds, which are key processes helping to maintain a healthy reef.”

 

Sources:

https://www.sciencedaily.com/releases/2024/01/240110114600.htm

https://www.sciencedaily.com/releases/2023/12/231206150525.htm