Review: A need for weed: why seaweeds are important for healthy tropical seascapes

Christopher J. Fulton, Rene A. Abesamis , Charlotte Berkström, Martial Depczynski, Nicholas A. J. Graham, Thomas H. Holmes, Michel Kulbicki , Mae M. Noble, Ben T. Radford, Stina Tano , Paul Tinkler, Thomas Wernberg , Shaun K. Wilson

Macroalgal reef. Photo credit: C. Fulton.
Macroalgal reef. Photo credit: C. Fulton.

Meadows of fleshy seaweeds are often a prominent part of our tropical coastlines, yet they are often overlooked in favour of coral reefs. Our study explains how seaweeds that form extensive meadows with tall and complex canopies can provide important food and habitat to help sustain healthy tropical seascapes.

Tropical seaweed meadows can grow huge canopies each summer, which break off in winter and raft along our coastlines to fertilise a range of other important habitats like coral reefs, seagrass beds, and mangroves. This seasonal growth of tropical seaweed can be substantial, rivalling that of other important organisms like corals. The habitat that seaweed meadows provide is especially important in summer, when baby fish hide and feed amongst the fronds. In this way, seaweed meadows foster the next generation of fish that replenish adult populations, which helps maintain food security for many tropical nations. These effects can extend beyond the meadows, because many of the fishes that use seaweed nurseries as juveniles later migrate to nearby coral reefs as adults. Other fishes regularly move between coral reefs and seaweed meadows to forage. This demonstrates the important connections between habitats, and shows us why healthy seascapes need a mosaic of habitat types that include patches of seaweed alongside coral reef, seagrass and mangroves.

Worryingly, our study found that tropical canopy-forming seaweeds are under threat from climate change. This is because the seasonal growth of the seaweed canopy is closely linked to sea temperature. In places that get hot during the summer we see less seaweed canopy growth. In the Red Sea, tropical seaweeds have actually “flipped” to grow during the cooler winter months to avoid the scorching summer water temperatures. If this sort of seasonal flip in seaweed growth were to happen across the world with global warming, then the fishes who use these meadows as nurseries will need to flip their breeding season to match. If they do not, then the next batch of baby fish will not have a nursery to call home and we may see fewer adult fish along our tropical coastlines in the future.

Read the paper in full here. This paper is part of an upcoming Special Feature: Coral Reef Functional Ecology in the Anthropocene.

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