Key Takeaways
- Kelp forests are sometimes called the coral reefs of cold water because both are highly productive, three-dimensional habitats that support huge marine biodiversity.
- Kelp are large brown algae (not plants) that anchor to rocky seafloors and grow upward in cool, nutrient-rich coastal waters.
- Kelp forests face pressure from warming oceans, storms, poor water quality, overfishing, and sea urchin outbreaks that can transform forests into urchin barrens.
- Sea otters act as a keystone species in kelp ecosystems by eating urchins and helping keep grazing pressure in balance.
Meet the Ocean's Fastest-Growing Underwater "Forest"
If your first encounter with a kelp forest was watching filmmaker and free diver Craig Foster descend into one in My Octopus Teacher, you already know these ecosystems can feel almost magical. Kelp forests are often compared to coral reefs because both are highly productive, three-dimensional habitats that shelter a huge diversity of marine life.
Kelp are large brown algae that live in cool, nutrient-rich coastal waters such as the Pacific shoreline. Although they aren't trees, they form dense underwater forests that can look and function like a marine version of a woodland canopy. They're also among the most dynamic and productive ecosystems in the ocean.
What is Kelp?
Kelp belongs to the order Laminariales and includes about 30 species. These large brown algae anchor themselves to rocky seafloors using root-like structures called holdfasts, then grow upward toward the sunlight with flexible stems called stipes and leaf-like blades.
Many species also have gas-filled bladders called pneumatocysts, which help keep their blades suspended in the water so they can capture more sunlight. Because kelp depends on photosynthesis for food and energy, it grows best in shallow, sunlit coastal waters where nutrients are plentiful.
Where Kelp Forests Grow
Kelp forests mostly occur along temperate coastlines, including on the Pacific shoreline from Alaska to Baja California. They thrive in cold, nutrient-rich waters and are rarely found deep below the surface.
In many places, kelp forests grow where underwater currents bring nutrient-rich water up from the deep ocean. This helps fuel their rapid growth and supports the huge diversity of life that depends on them. Although kelp forests are most common in cool waters, some species grow in unusual places such as tropical deep-water areas.
How Kelp Grows
Kelp has a fascinating life cycle. It begins as one of millions of spores released by mature kelp, known as sporophytes. These spores develop into tiny male or female gametophytes, which produce either sperm or eggs. After fertilization occurs, free-floating embryos attach to the sea floor where they'll eventually develop into mature kelp.
Once established, kelp can grow remarkably fast. In ideal conditions, giant kelp can grow up to 18 inches per day. Bull kelp can also grow quickly, often adding about 2 inches per day. This rapid growth helps kelp form tall, layered canopies that shelter countless marine species.
Why Kelp Forests Matter
Kelp forests are critical because they create structure in the ocean. Their towering canopies, blades, and holdfasts form a three-dimensional habitat that protects and feeds a wide range of marine life, including fish, shellfish, sea urchins, and snails.
Larger marine mammals like sea lions, harbor seals, and sea otters use kelp forests for feeding and protection from predators. 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.
Like coral reefs, mangrove forests, and seagrass meadows, kelp forests support biodiversity and help stabilize coastal ecosystems. They also buffer waves, which can reduce the force of incoming surf along shorelines. That makes them important not only to wildlife, but also to the coastal communities that live near them.
Kelp forests also support fisheries and local economies by providing habitat for commercially important species. Because they absorb carbon dioxide as they grow, they may also help reduce local ocean acidification.
What Threatens Kelp Forests?
Knowing that kelp can grow several inches per day, you might think they’re virtually invincible. Unfortunately, kelp forests face intense pressure from warming ocean waters, storms, poor water quality, overfishing, and outbreaks of sea urchins. When predator populations decline, urchins can multiply and graze heavily on kelp, sometimes transforming a forest into an urchin barren.
A remarkable phenomenon of marine ecology, "urchin barrens" occur when urchin populations grow to such extraordinary densities that the entire seafloor is cleared of vegetation, leaving behind an ecological wasteland. And as they gather and multiply, a virtually impenetrable barrier is formed, which prevents anything else from coming back.
In some places, these barren states can persist for years or even decades.
Climate change is making these pressures worse. As oceans warm, some kelp species are shifting toward cooler waters, and in certain regions, forests have declined sharply in recent decades.
How Kelp Forests Recover
Scientists have been investigating and experimenting with different ways to help for some time. One thing they’ve discovered is that sea otters are to kelp forests what wolves are to Yellowstone. One of the most important players in kelp forest recovery, sea otters eat sea urchins and help keep grazing pressure in balance.
Other predators, including rock lobsters in some regions, can also help control urchin populations. When these ecological relationships stay intact, kelp forests are far more likely to remain healthy and resilient.
Why This Ecosystem Deserves Attention
Kelp forests have been called the coral reefs of cold water because they perform many of the same ecological jobs in temperate oceans that coral reefs perform in the tropics. They create habitat, support biodiversity, and help sustain coastal biodiversity.
If you've ever imagined a forest beneath the waves, kelp forests are the closest thing: living, swaying ecosystems full of movement, complexity, and life. They're not just mesmerizing to look at; they're essential to ocean health.
