Scientists have found macroalgae (seaweed) can help protect some corals from bleaching, and the removal of the canopy-forming seaweed should be carefully timed to help coral reefs thrive.
James Cook University research officer Hillary Smith was part of a team that examined what happened when they removed macroalgae at a site near Townsville before a severe bleaching event in early 2020.
“When algae become too abundant it can be bad for corals, as algae compete for space and can directly harm both juvenile and adult corals.
“However, algae is not all bad, as the canopy provided by upright macroalgae might reduce direct sunlight and provide refuge for corals during heat events. We wanted to see if this was true,” said Ms Smith.
She said the scientists found there was no overall difference in bleaching prevalence in plots where macroalgae had been removed and control plots where it had not been removed.
“However, bleaching prevalence varied with the different morphology (shape and form) of corals. Mounding corals (such as brain corals) were more likely to experience bleaching in removal plots compared to controls, indicating that these corals do get some degree of protection from the macroalgae canopy,” said Ms Smith.
She said a follow-up survey months after the bleaching event had ended found that corals in plots where the macroalgae had been removed had recovered to a greater extent than those where macroalgae had been left to grow.
“The greater recovery of coral communities in removal plots might result from reduced competition with macroalgae,” said Ms Smith.
Associate Professor Scott Heron, a co-author of the study, said such findings were becoming increasingly important.
“With the escalating frequency and intensity of anomalously-high temperature events, climate-driven coral bleaching and mortality is now occurring somewhere in the world every year. This is the biggest risk to reef ecosystems over the coming decades.
“It’s in no way a theoretical problem – broad-scale coral bleaching events have already had major effects on the Great Barrier Reef in 2016, 2017, 2020, and again this year in 2022,” he said.
“When corals die, macroalgae can proliferate quickly, so it’s important to understand how these two groups interact on reefs,” Ms Smith said.
She said environmental managers were looking at more and more novel options to boost coral reef resilience and the team’s work pointed to a nuanced approach to macroalgae removal being helpful.
“Our study suggests that the timing of removing macroalgae on the Great Barrier Reef may maximise benefits. Algae removal before the annual mass spawning period in September/October will reduce competition for space between algae and baby corals.
“Conversely, it seems that allowing some regrowth of the algae canopy prior to the maximum summer heat in February and March will give some protection to mounding corals.”
