Large populations of fish help coral reefs to grow — but can’t protect against heatwaves.Credit: Barry Fackler/Getty
Minimizing land-based and sea-based human impacts at the same time might help reefs to recover from marine heatwaves, a study has found. But researchers warn that, in the absence of aggressive action to limit global warming, addressing impacts such as pollution and overfishing are insufficient to counter the growing threat. The findings — published this week in Nature1 — come as record ocean temperatures scorch reefs off the coast of Florida and scientists fear for the effects of El Niño on Australia’s Great Barrier Reef.
Researchers analysed 20 years of reef data from a 200-kilometre stretch of coastline on Hawai’i Island. The data included an unprecedented marine heatwave in 2015, providing the researchers with a unique opportunity to explore how factors such as wastewater run-off and fishing intensity shaped the coral reef’s response. The heatwave was the strongest in 120 years of record-keeping, and saw ocean temperatures rise to 2.2 °C above normal, with a peak at 29.4 °C.
A year after the heatwave, nearly one-quarter of the reefs had lost more than 20% of their coral cover. The hardest-hit reef lost close to half of its cover. But for 18% of reefs, coral cover was unaffected or even increased.
Local impacts
The team used modelling to determine which factors best explained the differences in coral-cover changes.
In the 12 years leading up to the heatwave, reefs with more fish — particularly herbivorous scrapers — and lower levels of wastewater pollution experienced coral growth.
Scrapers “literally scrape the reef”, says Jamison Gove, an oceanographer at the Pacific Islands Fisheries Science Center at the US National Oceanic and Atmospheric Administration in Honolulu, Hawaii. “They not only remove fast-growing algae, but they clear space that allows for the settlement of crustose coralline algae, which is a precursor for coral growth,” he says.
On the flipside, wastewater pollution from sewage disposal systems and run-off from urban environments damage coral health, says Gove. “It’s not only that you have this soup of virulent bacteria and other things that can cause coral disease,” he says. “You also have whatever everybody’s flushing down or putting down their sink. You have household chemicals, you have pharmaceuticals, so you have all sorts of toxins.”
Modest gains
Overall, the researchers estimate that addressing land-based and sea-based human impacts on reefs simultaneously improves their chances of having higher levels of coral cover in the four years after a heatwave by three to six times, compared with addressing them independently. But lower pollution levels and more scrapers — factors that drove increases in coral cover before the heatwave — offered little protection against temperature spikes. Reefs with the lowest levels of urban run-off lost slightly less cover, but the presence of more fish — particularly scrapers — did not have a substantial effect on coral loss or survival.
Gove says that the study reveals how human activity, both on land and in the sea, affects coral reefs, but stresses that these local impacts must be addressed in the context of action on climate change.
“We can do all the local management you want, but in the decades to come, if we’re not aggressively curbing greenhouse-gas emissions, then climate change and marine heatwaves will most likely overwhelm any of the effects that we’ve identified within our paper,” he said.
Terry Hughes, a marine biologist at James Cook University in Townsville, Australia says the study has confirmed that there are three main stressors on coral reefs: overfishing, pollution and climate change. “If you clean up the water quality and rebuild depleted populations of herbivores it does help the reef to recover. But the big, big caveat is provided they have enough time to recover,” he says. “It’s no use improving the recovery rate by managing water quality and fishing if the frequency of these bleaching events continues to increase.”
Hughes gives the example of back-to-back bleaching events that struck the Great Barrier Reef in 2016 and 2017, and says that climate modelling suggests that most coral reefs will experience bleaching events every year by 2050 under a business-as-usual emissions scenario.
“They don’t discriminate between well-managed reefs, or a polluted and non-polluted reef,” he says. “They all fry if the temperature is high enough.”
Source: Ecology - nature.com
