Kelp grows along cool, rocky coastlines throughout the world’s temperate zones, with the largest and most well-known forests concentrated along the Pacific coast of North America from Alaska down to Baja California, Mexico. These underwater forests also thrive along the coasts of South America, southern Australia, New Zealand, South Africa, and northwestern Europe. While kelp is typically a cold-water organism, researchers have even found it growing in tropical latitudes, hidden in deep water where temperatures stay cool enough to support it.
Major Kelp Regions Around the World
The Pacific coast of North America hosts the densest kelp forests on the planet. Giant kelp dominates from central California southward into Baja California, while bull kelp takes over in the colder waters from northern California up through Alaska. These two species often overlap in central California, creating particularly rich underwater habitat.
In the Southern Hemisphere, giant kelp stretches along the coast of South America from Peru all the way around Cape Horn and into Argentina. This range spans a wide temperature gradient, from about 3°C to 20°C. Southern Australia and New Zealand support their own extensive kelp forests, as do the coasts of South Africa and Namibia. In the Northern Hemisphere outside North America, kelp forests grow along the shores of the United Kingdom, Norway, and other parts of northwestern Europe, as well as around Japan and Korea.
What Kelp Needs to Survive
Kelp is picky about its growing conditions. Four factors determine whether a stretch of coastline can support a kelp forest: water temperature, a rocky seafloor, clear water with enough sunlight, and the right salinity.
Temperature is the biggest constraint. Kelp forests are generally limited to water below 22 to 24°C, and successful reproduction narrows that window further, requiring temperatures below about 18°C. The sweet spot for reproductive health in giant kelp sits around 12°C. This is why kelp thrives in cold currents and upwelling zones, where deep, nutrient-rich water rises to the surface and keeps temperatures down.
Unlike land plants, kelp doesn’t have roots that dig into soil. Instead, it anchors itself using a structure called a holdfast, which grips onto submerged rocks. This means kelp can only grow where the seafloor is rocky. Sandy or muddy bottoms won’t support it, no matter how perfect the temperature and light conditions are.
Salinity matters too. Bull kelp and other species can tolerate some dilution from freshwater runoff, growing well in salinities down to about 20 parts per thousand. But below that threshold, their early life stages struggle. At 13 parts per thousand (roughly the lower limit for Arctic kelp species), neither bull kelp nor ribbon kelp can complete their reproductive cycle. This is why you won’t find kelp forests near large river mouths where freshwater dramatically lowers the ocean’s salt concentration.
How Deep Kelp Can Grow
Kelp is a photosynthetic organism, so it needs sunlight. That limits how deep it can grow. Along the California coast, giant kelp typically attaches to rocks from the low tide line down to about 25 meters (82 feet), with a maximum depth of around 30 meters (98 feet). Bull kelp tends to grow a bit shallower, usually down to 17 meters (56 feet), though it can reach 40 meters (131 feet) in exceptionally clear water.
Water clarity is the controlling factor here. In murky coastal waters with heavy sediment, kelp might only grow in the first few meters. In crystal-clear water, enough light penetrates to support kelp at much greater depths. This relationship between clarity and depth becomes especially important in tropical regions, where kelp has been found growing far deeper than anyone expected.
Kelp in Unexpected Places
For decades, scientists assumed kelp was strictly a temperate and polar organism. Then researchers predicted, and later confirmed, that kelp forests exist in tropical waters between the Tropics of Cancer and Capricorn, growing at depths of 30 to 200 meters. At those depths, water temperatures drop low enough to support kelp even when surface waters are far too warm.
One of the earliest clues came from Brazil, where two kelp species were discovered near Rio de Janeiro growing at 70 meters depth in 15°C water. The surface above them hovered around 22 to 23°C, too warm for kelp. Similar deep-water kelp populations have been documented near Morocco, Qatar, and the Philippines. Some scientists believe these deep populations are relics from ice ages, when cooler ocean temperatures allowed kelp to grow in shallow water at those latitudes. As the climate warmed, the kelp retreated to deeper, cooler refuges.
How Warming Oceans Are Shifting Kelp Habitat
Rising ocean temperatures are reshaping where kelp can survive. Long-term surveys along a 600-kilometer stretch of Baja California, Mexico, revealed three strikingly different outcomes after the extreme marine heatwaves of 2014 to 2016. In the southern part of the region, giant kelp showed remarkable resilience, recovering 95% of its historical canopy area despite experiencing the highest absolute temperatures. Healthy populations of predators that eat sea urchins helped keep the ecosystem in balance.
Further north, the picture was grimmer. In the mid-region, giant kelp canopy dropped by 96%, replaced by smaller brown seaweeds that filled the gap. In the northernmost area, the ecosystem flipped entirely into what scientists call an “urchin barren,” a seafloor stripped bare by exploding populations of purple sea urchins. Urchin numbers surged by nearly 2,500% between 2016 and 2023, and giant kelp canopy shrank by 95%. That area had fewer predators to keep urchins in check, a problem potentially linked to fishing pressure.
These patterns illustrate a broader global trend. As ocean temperatures climb, kelp forests at the warm edges of their range face collapse, while populations in cooler waters may persist or even expand. In parts of Australia, Norway, and Japan, kelp forests have already retreated toward the poles. The speed of this shift depends not just on temperature, but on the health of the broader ecosystem, particularly whether enough predators remain to prevent urchin populations from devouring kelp before it can recover.