California Kelp Forests

Along the cold, rocky coast of California, underwater forests of giant kelp sway in the current. Giant kelp (Macrocystis pyrifera) is one of the fastest-growing living things on Earth. In good conditions it can grow up to two feet in a single day. Even though it looks like a tree, kelp is not a plant — it is a large brown alga.

Kelp anchors to rocks on the seafloor with a root-like structure called a holdfast. A long, flexible stalk called the stipe rises toward the surface. Leaf-like blades grow along the stipe, and this is where photosynthesis happens. Small gas-filled bladders called pneumatocysts keep the blades floating near the sunlit surface, forming a thick canopy.

Kelp forests need cold, nutrient-rich water. Along California, a process called upwelling brings deep, cold water full of nutrients up to the surface, fueling rapid kelp growth. These forests are among the most productive ecosystems in the ocean, sheltering fish, invertebrates, and marine mammals.

One famous resident is the sea otter. Sea otters eat sea urchins, spiny animals that graze on kelp. When otters are abundant, urchin numbers stay low and the kelp thrives. Because the otter has such a large effect on the whole community, it is called a keystone species. When otters are removed, urchin populations explode. The hungry urchins mow down the kelp, leaving bare rock called an urchin barren.

Today these forests face new threats. Marine heatwaves warm the water and slow kelp growth. A sickness called sea star wasting disease killed billions of sea stars, including the sunflower sea star — a major urchin predator. With fewer predators, purple urchins have multiplied and large stretches of kelp forest have collapsed into barrens. Scientists, divers, and tribes are now working to restore the forests by removing urchins and replanting kelp.