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.

Climate Crisis Ahead - Sticker by MLSSA Member Dan Monceaux
Climate Crisis Ahead – Sticker by MLSSA Member, Dan Monceaux

By Steve Reynolds

Steve Reynolds is the current President of MLSSA and is a long-standing member of the Society. Steve was a keen diver, underwater explorer & photographer before illness struck. He is chief author of the Society's extensive back catalogue of newsletters and journals.

One thought on “Seaweeds found to be also sensitive to ocean warming”
  1. Good interesting article.One point I must flag – I don’t see the ‘big picture ‘ finding recognition in the otherwise seemingly logical and perhaps evidence based statement that once seaweeds are established on dead coral reefs it’s difficult for coral to reestablish . Firstly, marine algae and seagrasses each produce more oxygen and fix more carbon than the equivalent square meter of healthy coral .Secondly and more importantly (because coral reefs support a very different biota to that associated with algal gardens, irrespective of the latitude and water temperature ,so we don’t want our coral reefs to vanish even though the new algal canopies will be nature’s way of maximizing productivity under altered conditions of pH, temperature, UV exposure ,etc ), coral WILL outcompete seaweeds quite quickly UNDER THE RIGHT CIRCUMSTANCES .

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