To dive in a kelp forest is to enter a place of other worldly beauty. Great golden fronds rise toward the sunlight, swaying gently with the surge. Fish weave through them. Sea stars cling to the rocks below. Sea otters float at the surface, sometimes sleeping wrapped in ribbons of kelp. The forest seems ancient and permanent, but appearances can be deceiving. Unlike forests on land, kelp forests are constantly on the move. They grow, break apart, drift away, disappear, and return. Entire underwater landscapes can change within a few seasons; the kelp forest is never still.
Kelp is not a plant, but an alga
Surprisingly, kelp is not a plant. It belongs to a group of large brown algae that evolved separately from the flowering plants and trees we know on land. Yet kelp has evolved structures that resemble roots, trunks, and leaves. Holdfasts anchor it to the rock, while flexible stalks lift it towards the light and broad, waving blades capture the sun’s rays near the surface.
It is a remarkable example of evolution finding different solutions to the same challenge.
Following the cold water
Unlike trees on land, kelp forests do not spread simply by dropping seeds nearby. Each kelp plant releases millions of microscopic spores that drift with ocean currents. If they settle on hard rock in cool, nutrient-rich water, they can grow into new forests. But if they land on soft sand or in water that is too warm, they perish.
As ocean conditions change, kelp forests slowly shift their boundaries. They may retreat from warming coastlines while expanding into cooler regions. In this way, the forest appears to migrate, generation by generation, following the waters in which it can live.
Kelp forests spread like a slow tide of life, advancing wherever conditions become favourable and disappearing where they no longer are.
Forests of the cool seas
Kelp forests flourish along rocky coastlines where cool, nutrient-rich water rises from the deep. They are found on every continent except Antarctica, although the richest forests occur along the Pacific coasts of North and South America, around South Africa, southern Australia, New Zealand, and parts of northern Europe.
Most giant kelps grow in water between about 5°C and 20°C, although the exact range varies among species. They flourish in shallows of about two metres, where sunlight is abundant, to depths of about thirty metres, sometimes deeper when the water is exceptionally clear. Their upper fronds often reach the surface, forming floating canopies that resemble the crowns of terrestrial forests.
From above, some kelp forests are so vast that they can be seen from space.
The fastest-growing forests on Earth
Some giant kelps can grow more than half a metre in a single day. Within months, a bare rocky shoreline may become a dense underwater forest alive with fish, crabs, sea lions, and countless smaller creatures. For these magnificent marine forests create homes for hundreds of species. Further, they slow ocean currents, help protect the shore by softening the force of waves, and produce enormous amounts of oxygen while capturing carbon from the atmosphere.
Kelp forests do not simply provide habitat; they help create entire ecosystems.
Kelp forests can move
Storms can tear giant kelp from the seabed, but instead of dying immediately, floating rafts may drift for hundreds—even thousands—of kilometres, carried by ocean currents. As they travel, they become floating islands. Fish shelter beneath them, crabs climb on board, and young animals use them as nurseries. Some drifting kelp eventually reaches distant shores, where it can establish a new forest. Thus, these rafts of kelp act as a means of dispersal for a myriad of other species that are not buoyant themselves.
Kelp forests are constantly changing. Warm ocean temperatures may cause them to shrink, while cool, nutrient-rich waters help them flourish. Sea urchins sometimes multiply dramatically, grazing entire forests down to bare rock and creating underwater "urchin barrens." Then, months or years later, predators arrive, the urchins decline, and the kelp slowly grows back. To a diver visiting the same place over decades, the transformation can be astonishing.
Scientists continue to study why some kelp forests recover rapidly after storms or marine heatwaves while others do not. Some populations appear surprisingly resilient while others vanish for decades. The reasons for these differences is still not known.
A forest unlike any other
On land, forests move over a period of time spanning thousands of years as trees slowly spread into new landscapes. But kelp forests spring up where conditions are right, drift across oceans, disappear after storms, return with changing seasons, and reshape themselves again and again.
So, when gazing through a waving kelp forest, remember that those towering fronds are more than just seaweed. They are the architects of ecosystems and builders of underwater cities. You are momentarily part of a forest that grows with astonishing speed, journeys across the seas, and begins again after every storm. Perhaps that is its greatest mystery.
Not that it travels, but that it never stops changing.
Ethologist Ila France Porcher, author of Yes, Fish Feel Pain, The True Nature of Sharks and six other books on wildlife behaviour, spent 15 years closely observing wild fish and sharks in Tahiti, resulting in several scientific papers. Her writings are based on decades of first-hand observations of wildlife and focus on individuality and intelligence, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates and international media for its unique blend of field observation, art and science.
