Seaweeds
Giant Kelp
Macrocystis pyrifera
An iconic canopy-forming seaweed that grows through the water column, transforming cool southern reefs into complex underwater forests.
Scientific Name
Macrocystis pyrifera
Group
Brown algae
Maximum Length
More than 35 metres
Growth
Up to 50 cm per day
Australian Stronghold
Tasmania
Ecology
Fast-Growing Habitat Formers
Giant kelp is a foundation species. By growing from the reef towards the surface, it changes light, water movement and shelter across the entire water column, creating conditions that support a remarkable diversity of life.
A single kelp is anchored to rock by a holdfast. Long, flexible stipes carry blades and gas-filled floats upwards, where they form a dense surface canopy in cool, nutrient-rich water.
Structure
Built for the canopy
Giant kelp has no roots, trunk or leaves. Instead, specialised structures anchor it to the reef and lift its photosynthetic blades into the light.
01 Holdfast
A tough, root-like structure grips the rocky reef without drawing nutrients from it.
02 Stipe
Flexible stem-like tissue allows the kelp to bend with waves and currents.
03 Gas-filled floats
Pneumatocysts buoy the blades towards the surface and keep them exposed to sunlight.
04 Blade
Broad blades absorb nutrients from seawater and capture energy through photosynthesis.
Why it matters
Like trees in a forest, giant kelp creates habitat at several levels—from the holdfast and shaded seafloor to the midwater stipes and surface canopy.
These forests provide food, shelter and nursery grounds for fish, rock lobsters, abalone and many smaller animals. By slowing water movement, they can also help drifting larvae remain within reef systems long enough to settle.
An Underwater forest for reef life
A layered forest supports animals on the seafloor, through midwater and beneath the canopy.
•Three-dimensional habitat
Kelp forests underpin fisheries, local economies and memorable diving and snorkelling experiences.
•Productive coastlines
•Living coastal infrastructure
Dense forests reduce local water movement, capture carbon as they grow and soften wave energy.
“When you dive in a giant kelp forest, it’s like flying through a rainforest.”
KAREN GOWLETT HOLMES
Loss of giant kelp forests
A forest in retreat
95%
Estimated decline of giant kelp forests along Tasmania’s east coast since the 1940s.
The East Australian Current is reaching farther south and bringing warmer water into eastern Tasmania.
• Warm Current
The warmer current is nutrient-poor, leaving giant kelp with less nitrogen for growth and photosynthesis.
•Fewer Nutrients
Marine heatwaves push giant kelp beyond its temperature tolerance and can cause widespread die-off.
•Thermal Stress
Warming waters assist the spread of longspined sea urchins, which can remove kelp and create barrens.
•Urchin Grazing
Restoration case study
Restoring Tasmania’s giant kelp forests
Mick Baron and Karen Gowlett-Holmes have run the Eaglehawk Dive Centre on the Tasman Peninsula since 1991. Over those decades, they have witnessed the loss of the forests that once defined many local dive sites.
Their observations and practical knowledge now support research and rehabilitation efforts. The short film Reviving Giants follows their connection to the forest and the work underway to help bring it back.
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Kelp Forests
Kelp forests form the living foundation of the Great Southern Reef, creating food, shelter and nursery habitat for an extraordinary diversity of marine life.
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