June 08, 2020 4 min read

Trees Under the Sea: How Kelp Forests Grow Underwater

When you think about forests, massive, swaying stands of kelp probably aren’t the first thing that come to mind. And while kelp isn’t technically a tree, scientists call large growths “kelp forests” because they form dense groupings, tower above the ocean floor, and perform many of the same functions that traditional forests do. And they’re no less important — or threatened — than their terrestrial counterparts. So in honor of World Oceans Day, we’re sharing all things kelp. Get your scuba gear ready, because we’re diving deep!

So what exactly IS kelp?

The short answer? A variety of seaweed. The (slightly) longer answer? Large brown algae that live in cool, relatively shallow waters close to the shore. They provide nutrition and homes for biodiversity just like terrestrial forests.

And how does it grow?

Reliant on light for photosynthesis, kelp grow along rocky coastlines at depths ranging from 6 to 90+ ft. Tiered like a terrestrial forest with a canopy and several layers below, kelp forests do best in cold, nutrient-rich upwellings (areas where the ocean layers overturn, bringing cool, nutritious bottom waters to the surface). And similar to trees, they have root-like “anchors” called holdfasts that keep them from floating away. But unlike trees, their “roots” don’t burrow deep under the surface, but rather grip tightly onto the ocean floor. 

Kept afloat by pneumatocysts (gas bladders), kelp grow rapidly toward the ocean's surface — and when we say rapidly, we mean it! Although their typical growth rate is around 10 inches a day, giant kelp can grow up to 2 ft in ideal conditions. Bull kelp, another common variety, grows 4 inches a day — which is still pretty impressive if you ask us! 

A perennial, Giant Kelp can live for up to 7 years if it isn’t uprooted by a storm or devoured by hungry sea-dwelling herbivores. Bull kelp, an annual, goes through its entire life cycle in just 1 year, but both have a two-stage growth trajectory. 

In the beginning, they exist as spores, released by the millions by their parent kelp, sporophytes. These spores then grow into tiny male or female gametophytes, which can produce either sperm or eggs. After fertilization occurs, these free-floating embryos quickly grip the ocean floor and develop into mature plants.

Source: Smithsonian Photographed by Kyle McBurnie

Home sweet home

Forming a dense barrier between coastlines and damaging waves, kelp forests harbor a higher diversity of plants and animals than almost any other ocean community. Providing food and shelter to thousands of marine mammals ranging from shellfish to grey whales, they’re incredibly important to ocean ecosystems. Larger marine mammals like sea lions, harbor seals, and sea otters use them for feeding and protection from predators. Connected to water and sky, sea birds like gulls, terns, snowy egrets, great blue herons, and cormorants also benefit. And on rare occasions, even grey whales have been spotted seeking refuge from predatory killer whales. 

Similar to coral reefs, mangrove forests, and warm-water seagrass beds, kelp forests provide an important three-dimensional, underwater habitat to thousands of species of invertebrates, fish, and other algae. Some species use them as spawning grounds, and others stay to raise their young in the relative safety there. We say relative, because sharks and other marine mammals hunt in the long corridors that form between rows of individual plants.

Trouble in paradise

Knowing that kelp can grow up to 2 ft per day, you might think they’re virtually invincible. But unfortunately, that isn’t the case for this incredible underground ecosystem. In fact, from Tasmania and Australia to Norway, western Europe, and the west coast of North America, kelp forests are under dire threat. And while warming ocean waters are taking a toll (and will continue to do so if climate change continues unchecked), it’s the dang sea urchins that are really doing a number on them. See, just like you love pizza, sea urchins LOVE kelp and will eat through an entire forest in no time. 

And unfortunately, when the last kelp strand has been cut down, large herds of sea urchins respond to the stress of starvation by becoming aggressive and growing larger, more powerful teeth. These new teeth allow them to crunch through just about anything in their path, which only compounds their destructive influence. And once things have progressed to this point, there’s virtually no chance of kelp coming back.

A remarkable phenomenon of marine ecology, "urchin barrens" occur when their populations grow to such extraordinary densities that vegetation on the entire seafloor is annihilated, leaving behind an ecological wasteland. And as they gather and multiply, a virtually impenetrable barrier is formed, which prevents anything else from coming back. Unfortunately, once established urchin barrens will persist indefinitely. 

Is there hope?

It depends. While there is little hope for kelp forests that have already been annihilated and turned into urchin barrens — or for those that grow in rapidly warming waters — scientists have been investigating and experimenting with different ways to help for some time. And what they’ve discovered is that sea otters are to kelp forests what wolves are to Yellowstone. 

Because as it turns out, sea otters love urchins just as much as urchins love kelp! And if left alone, they will happily keep the urchins in check. But when they decline, urchin populations explode. For this reason, sea otters are considered a keystone species (similar to the stone at the top of an arch that prevents the other stones from falling down, keystone species prevent the collapse of entire ecosystems) in kelp forests. Another species that loves to dine on sea urchins? Rock lobsters.

So there you have it — everything you didn’t know you needed to learn about kelp forests! The next time you visit the coast, keep your eyes peeled — if you’re lucky, you might just spot their characteristic tendrils swaying slowly with the tides.

Check out even more weird and otherworldly forests on planet Earth!

We plant trees on 4 continents around the world. Want to choose where yours are planted?

by Meaghan Weeden

Forest Whisperer