Seaweeds found to be also sensitive to ocean warming
by Steve Reynolds
Alexia Graba-Landry says that seaweeds are just as sensitive to ocean warming as corals. Alexia is a 2017-2019 PhD candidate at the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland. She was a Lizard Island Reef Research Foundation Doctoral Fellow in 2017.
The Lizard Island Reef Research Foundation Doctoral Fellowship funded Alexia’s study. She says that it also funded two additional chapters of her PhD further investigating the effect of temperature to seaweed-fish interactions and how they may shift as oceans continue to warm. She says that it is an important question for both ecology and management to help predict the future seascape of coral reefs.
On the Lizard Island Reef Research Foundation’s blog, Alexia describes ocean warming as “one of the greatest threats to coral reefs.” She says that “increasing temperatures have already caused mass coral bleaching events, which, if severe and frequent enough, lead to widespread coral mortality. Overgrowth of seaweeds is a threat to future coral reefs, as dead coral skeletons are rapidly colonised by canopy forming seaweeds and turf algae. Once seaweeds are established it is hard for new coral recruits to compete for space to grow. However, little is known about the sensitivity of the seaweeds themselves to temperature and how this may influence seaweed overgrowth on coral reefs.”
In describing her work Alexia writes that “Sargassum spp. is a leafy brown seaweed common on degraded reefs. We investigated the effect of increasing temperature to three species of adult and one species of juvenile Sargassum at three different temperatures. These temperatures were reflective of October-December ambient temperatures, +2°C and +4°C, which approximated summer mean and summer maximum temperatures.”
“We measured growth, physical toughness, nutritional quality, and survival. We also measured susceptibility to herbivory by deploying the cultured seaweeds onto the reef, and filming them for 3 hours and recording bites by the wild herbivore assemblage.”
“We found that increasing temperature decreased the growth and survival for all species at both adult and juvenile stages. Nutritional quality and physical toughness was also reduced in two of the three adult species. Susceptibility to herbivores also decreased in two of the species, with the wild herbivore assemblage preferring seaweeds cultured at lower temperatures.”
“Our results indicate that canopy forming seaweeds like Sargassum may be just as sensitive to increasing temperature as their coral counterparts. Therefore seaweed overgrowth following mass coral bleaching events may be less likely as previously assumed. Instead we might see a rise in novel ecosystems low in both coral and seaweed cover as ocean temperatures continue to increase.”
Sargassum spp. are commonly found on South Australian reefs, including those of many popular recreational dive sites in Gulf St. Vincent.