California's Kelp Forests: Urchin Barrens, Collapse, and Recovery
- Bret Whitman
- 14 hours ago
- 9 min read
Drop into a healthy California kelp forest and you understand, in about ten seconds, why divers get obsessed with them. Shafts of sunlight break through a canopy far overhead. Long amber stipes rise from the reef like the trunks of some drowned redwood grove, swaying in the surge. Baitfish flicker in silver clouds. A calico bass slides out of the shadows, and somewhere in the tangle a lobster tucks itself deeper into a crack. It is one of the richest habitats on the planet, and along much of our coast it has been in serious trouble. If you dive California, understanding what is happening to the kelp is not just ecology homework. It changes where you go, what you find, and how you dive.
A healthy giant kelp forest in the Channel Islands — the cathedral of California cold-water diving.
What a Healthy Kelp Forest Actually Is
There are two headliners on the California coast, and they are worth telling apart. In the south and central coast, the star is giant kelp, Macrocystis pyrifera — the fast-growing giant that can add up to a couple of feet a day in good conditions and reach the surface from reefs 60 or more feet down. It anchors to rock with a root-like holdfast, sends up multiple stipes, and floats its blades with gas-filled bladders so the whole plant reaches for the light. That surface canopy is the ceiling of the forest, and the structure beneath it is what holds everything together.
In Northern California, the dominant species is bull kelp, Nereocystis luetkeana — a very different animal. Bull kelp is an annual. It grows explosively each spring and summer from a single long stipe topped by a round float the size of a grapefruit, with a spray of blades trailing off it, then dies back in winter. Because it lives and dies in a single season, a bull kelp forest is far more fragile than a giant kelp forest. A giant kelp bed that takes a beating can regrow from a surviving holdfast. A bull kelp forest that fails to recruit in the spring is simply gone that year, and that fragility turned out to matter enormously.
Bull kelp, the annual canopy species of Northern California. Because it lives and dies in a single season, it had no reserve to fall back on when the urchins arrived.
Either way, the forest is not really about the kelp itself so much as everything the kelp makes possible. The canopy softens swell and light. The stipes and holdfasts give juvenile fish somewhere to hide from predators. Rockfish, kelp bass, sheephead, señoritas, perch, and countless invertebrates all key on that structure. For a diver, kelp is a habitat map you can read from the surface: where there is standing forest, there is usually life, cover, and a reason for game to hold. That is exactly why a stripped reef feels so wrong when you find one.
Urchin Barrens: When the Reef Goes Bald
Swim off the edge of a good bed in the wrong stretch of coast and the forest can just stop. The kelp thins, then vanishes, and what is left is rock carpeted wall-to-wall in urchins — mostly purple urchins, packed shoulder to shoulder like a spiny purple pavement. Almost nothing else. Divers call this an urchin barren, and once you have seen a real one you never mistake it for anything else. It is quiet in a way a healthy reef never is.
Urchins are always part of a kelp forest. In balance, they graze on drift kelp — bits and blades that break loose and tumble down to them — and their numbers are held in check by predators and by the sheer effort of finding food. The barren happens when that balance breaks. When urchin numbers explode and their usual food runs short, they stop waiting for drift and go on the offensive, mowing down living kelp and even chewing through holdfasts. Strip the holdfasts and the kelp can no longer regrow. The urchins, meanwhile, can drop into a kind of starvation standby, surviving for years on almost nothing, empty inside but still guarding the rock. That is the cruel trick of a barren: it is self-locking. The urchins that created it can hold it open long after the kelp is gone.
Purple sea urchins packed shoulder to shoulder. When predators vanish and their numbers explode, this spiny pavement is what replaces the forest.
The Cascade: How Northern California's Forests Collapsed
The purple urchin explosion did not come out of nowhere. It was the tail end of a chain reaction, and the story is worth knowing because it explains why the northern coast got hit so much harder than the south.
It starts with a predator you may never have noticed: the sunflower sea star, Pycnopodia helianthoides. This is a giant among sea stars — up to two dozen arms, a body that can span a yard, and a fast, roving appetite for urchins. Around 2013 and 2014, a devastating outbreak of Sea Star Wasting Disease swept the Pacific coast. Sea stars of many species essentially disintegrated, and the sunflower star was hit about as hard as any animal can be — functionally wiped out across huge stretches of California. With that predator gone, one of the main checks on urchin numbers vanished almost overnight.
The sunflower sea star, a voracious urchin predator — functionally wiped out by sea star wasting disease around 2013–14, removing one of the urchins’ main checks.
At nearly the same time, a marine heatwave settled over the eastern Pacific. Warm water is stressful for kelp; it carries fewer of the nutrients the plants need, and bull kelp in particular is sensitive to it. So the forests were weakening from heat stress at the exact moment their grazers were multiplying unchecked. The result on the north coast was catastrophic. Bull kelp, being an annual with no holdfast bank to fall back on, collapsed on a scale that is hard to overstate — well over ninety percent lost across large areas, some of the most productive reefs in the state reduced to purple barrens in just a few years. It was one of the most dramatic marine ecosystem shifts ever documented on our coast.
Put a number on it and the urchins’ role stops being abstract: along the north coast, purple urchin densities exploded by an estimated ten thousand percent in just a few years. This wasn’t kelp quietly fading in warm water — it was urchins actively eating the forest faster than it could grow. That is why Northern California, more than anywhere else in the state, became the epicenter of the collapse, and why urchins are the single factor a diver up north cannot ignore.
The southern and central coast fared better, and the reasons for that are the same reasons a diver should care about who else lives on the reef.
The Urchin Eaters: Why Predators Keep Reefs Green
Where urchins have healthy predators, barrens are far less likely to form and far easier to reverse. California has three heavyweights worth knowing.
California sheephead are one of the reef's great urchin controls. Big males, with their blunt heads and powerful jaws, crunch urchins and other hard-shelled prey. A reef with plenty of good-sized sheephead is a reef where urchins get kept honest. This is also why size and take matter — remove all the big sheephead and you remove a natural brake on the very problem that ruins the diving.
California spiny lobster are the night shift. They are significant predators of urchins in the south, and research on marine reserves has shown that where lobster are abundant and large, they can help hold urchin populations down and keep reefs in the kelp-forest state rather than the barren state. Another reason a healthy, well-managed lobster population is worth more to a diver than one great night of overharvesting.
And then there are sea otters, mostly along the central coast. An otter eats a staggering amount of urchin to fuel its metabolism, and where otters are established they are among the most effective kelp guardians on the planet. It is no accident that some of the most resilient kelp forests in the state sit in otter country. The otters never recolonized the far north in numbers, which is one more reason the northern collapse ran so deep — the north lost its sea stars and never had the otters.
The lesson for a diver is simple and a little humbling: the fish and game we value and the forest we love to dive are held up by the same predators. Steward them and you are stewarding the whole reef.
Fighting Back: Restoration and Cautious Recovery
The good news is that people have refused to accept the barren as permanent, and divers have been at the center of the fight. Because a barren is self-locking, the most direct tool is blunt: get the urchins off the rock. Volunteer divers and organized crews have culled or removed purple urchins from targeted reefs, clearing patches to give kelp a chance to recruit again. It is slow, cold, unglamorous work — hours of prying spiny animals off rock — but on cleared plots the kelp has come back where it had been absent for years.
Other efforts run alongside the culling. Kelp reseeding and outplanting projects raise young kelp and give it a foothold on cleared reef. Marine protected areas let predators like sheephead and lobster rebuild to sizes and densities that help keep urchins in check on their own. Researchers are even working on the urchins from the other end — the starving urchins in a barren are commercially worthless because they are empty, but there is interest in ranching and fattening them so there is an economic reason to pull them off the reef. None of this is a silver bullet, and recovery is patchy. But there are real, verified signs of it: reefs that were purple gravel a few years ago now carrying young kelp again, and glimmers of the sunflower star beginning to reappear in places. It is fragile, and it is worth protecting.
Reading the Reef: Healthy Bed vs. Barren
For the practical diver, learning to read the difference is one of the highest-value skills you can build, because it tells you where the fish and game will be. A healthy bed has vertical structure — standing kelp, a canopy or at least midwater stipes, drift kelp tumbling along the bottom, and a general busy-ness to it. You will see bait, you will see reef fish working the edges, and you will find lobster and game holding in the cover. That structure is the whole point: game holds where there is food and shelter.
A barren reads the opposite way. The rock is exposed and often looks almost clean except for a dense, even carpet of urchins. Little standing kelp, little bait, an eerie stillness. You can burn a whole dive over a barren and see almost nothing worth the swim. The takeaway is not that a barren is worthless to look at — it is fascinating, and worth understanding — but that it is telling you to move. Find the edge where the barren gives way to living forest, or find the next reef that still holds a bed, and that is where your day is. Kelp on the surface, read from the boat or the beach, is one of the best free scouting tools you have.
Diving Like a Steward
All of this adds up to a way of diving, not just a set of facts. The forests are recovering in places and still fragile everywhere, and the predators that hold them together are the same animals many of us hunt. That is a responsibility, not a contradiction. Take within the rules and take conservatively — leaving the big breeders, especially the large sheephead and lobster that do the most to keep urchins in check, pays you back in better reefs down the line. Do not overharvest a good spot just because you can. If you have the chance to join an urchin removal day or support a kelp restoration group, that is some of the most direct good a diver can do for the water we all share.
And know your protected species. If a giant sea bass materializes out of the kelp — a slab of a fish that can be the size of a person — the rule is simple: look, do not touch, and count yourself lucky. They are protected, they are curious, and a calm, respectful encounter with one is a highlight of a diving life, not a target.
The kelp forest gave California divers some of the best cold-water diving anywhere, then showed us in a single hard decade how quickly it can come apart. The barrens are a warning. The green patches coming back are a promise. Learn to tell them apart, dive the living forest, protect the animals that keep it standing, and you are doing your part to make sure there is still a forest to drop into the next time you go.
Photo credits: Giant kelp forest — NOAA Photo Library (public domain). Bull kelp — Peter Pearsall / U.S. Fish and Wildlife Service (public domain). Purple sea urchins — James St. John (CC BY 2.0). Sunflower sea star — Steve Lonhart / NOAA (public domain).
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