Kelp forests are underwater areas where large amounts of kelp grow, covering many parts of the world's coastlines. Smaller groups of kelp that are attached to the ocean floor are called kelp beds. These areas are considered some of the most productive and changing ecosystems on Earth. Although kelp forests and coral reefs together cover only 0.1% of Earth's surface, they produce 0.9% of the food made by plants and algae worldwide. Kelp forests are found in temperate and polar coastal waters around the globe. In 2007, scientists also found kelp forests in tropical waters near Ecuador.

Kelp forests are physically made by brown seaweed, which creates a special home for many sea animals. They help scientists study how ecosystems work. Over the past 100 years, researchers have studied kelp forests closely, especially how animals and plants depend on each other for survival. These forests also affect ocean currents near coasts and provide many benefits to the environment.

However, human activities have often harmed kelp forests. Overfishing in coastal areas can cause problems, such as allowing too many animals that eat seaweed to grow unchecked. This can lead to large areas where kelp and other seaweed are gone, leaving few species to survive. Because of overfishing and climate change, kelp forests have nearly disappeared in places like Tasmania’s east coast and Northern California’s coastline. One way to help protect these areas is by creating marine protected zones, which can reduce the harm from fishing and help ecosystems deal with other environmental challenges.

Kelp

The term kelp refers to a type of marine algae in the order Laminariales (phylum: Ochrophyta). Even though this group is not very different in terms of species, kelp plants vary greatly in how they look and function. The most well-known species are giant kelps (Macrocystis spp.), but other genera, such as Laminaria, Ecklonia, Lessonia, Nereocystis, Alaria, and Eisenia, are also described.

Kelp forests support a wide range of sea life, including fish, invertebrates, and marine mammals and birds. In the North Pacific, species like rockfish, amphipods, shrimp, marine snails, bristle worms, and brittle stars live in kelp forests. Marine mammals such as seals, sea lions, whales, and sea otters, as well as birds like gulls, terns, snowy egrets, great blue herons, cormorants, and some shorebirds, are also found there.

Kelp is often called an ecosystem engineer because it creates physical structures and habitats for other organisms. In algae (kingdom Protista), the body of an individual is called a thallus, not a plant (kingdom Plantae). A kelp thallus has three main parts:

• The holdfast is a root-like structure that attaches the kelp to the seafloor, but it does not absorb nutrients.
• The stipe is like a plant stalk, growing upward from the holdfast and supporting other parts of the kelp.
• The fronds are leaf-like parts that grow from the stipe. These are where the kelp takes in nutrients and performs photosynthesis.

Many kelp species have pneumatocysts, which are gas-filled bladders near the base of fronds. These bladders help kelp stay upright in the water.

For kelp to grow, it needs hard surfaces (like rock or sand), plenty of nutrients (such as nitrogen and phosphorus), and enough light (at least 50 E m annually). Kelp forests often grow in areas where deep, nutrient-rich water rises to the surface, a process called upwelling. Water movement helps kelp take in nutrients, and clear water allows enough light to reach deeper areas. Under ideal conditions, giant kelp (Macrocystis spp.) can grow 30–60 cm per day. Some species, like Nereocystis, live only one year, while others, like Eisenia, live more than 20 years. In perennial kelp forests, growth is fastest during upwelling months (spring and summer), and plants die back when nutrients are low, daylight hours are shorter, and storms are more frequent.

Kelp is mainly found in temperate and Arctic waters worldwide. The genus Laminaria is common in the Atlantic Ocean, China, and Japan. Ecklonia grows in Australia, New Zealand, and South Africa. Macrocystis is found in the northeastern and southeastern Pacific Ocean, the Southern Ocean, and parts of Australia, New Zealand, and South Africa. The most diverse kelp region is the northeastern Pacific, from north of San Francisco, California, to the Aleutian Islands, Alaska.

Kelp forests are not found in tropical surface waters, but some Laminaria species live in deep tropical waters. The lack of kelp in tropical areas is likely due to low nutrient levels in warm, nutrient-poor waters. A recent study mapped kelp’s required conditions with ocean data and predicted that kelp might grow in subsurface tropical waters up to 200 meters deep. In the Galapagos Islands, researchers tested their model and found thriving kelp forests in all eight sampled sites, matching the model’s predictions. This suggests their global model may be accurate, showing that kelp forests could exist in tropical subsurface waters worldwide. This discovery has led to new research on kelp forests, highlighting their potential to provide shelter for marine life during climate change and explaining patterns in kelp evolution.

Ecosystem architecture

The structure of a kelp forest ecosystem is shaped by the physical arrangement of its plants, which determines the types of animals and plants that live there. This ecosystem has three main groups of kelp and two groups of other algae:

• Canopy kelps are the largest species and often form floating layers that reach the ocean surface (e.g., Macrocystis and Alaria).
• Stipitate kelps grow a few meters above the ocean floor and may form thick clusters (e.g., Eisenia and Ecklonia).
• Prostrate kelps grow close to the ocean floor (e.g., Laminaria).
• The benthic assemblage includes other algae, such as filamentous and foliose types, and organisms that live on the ocean bottom.
• Encrusting coralline algae cover rocks and other hard surfaces on the seafloor.

Many kelp species can live together in one forest. The understory canopy refers to the stipitate and prostrate kelps that grow below the main canopy. For example, a Macrocystis canopy may stretch high above the seafloor, while the understory kelps Eisenia and Pterygophora grow only a few meters upward. Below these, a group of foliose red algae may live. The tall, layered structure of the kelp creates different areas, like a sunny top layer, a partly shaded middle, and a darker bottom, similar to a forest on land. Each group of plants and algae supports different animals, which rely on the habitat in varying ways. For instance, in California’s Macrocystis pyrifera forests:
– The nudibranch Melibe leonina and skeleton shrimp Caprella californica live near the surface canopy.
– The kelp perch Brachyistius frenatus, rockfish Sebastes spp., and other fish are found in the stipitate understory.
– Brittle stars and turban snails Tegula spp. live near the base of the kelp.
– Sea urchins and abalone live under the prostrate kelp.
– Seastars, hydroids, and benthic fish live among the benthic algae.
– Solitary corals, gastropods, and echinoderms live on encrusting coralline algae.
Pelagic fish and marine mammals are loosely connected to kelp forests, often visiting the edges to feed on the animals that live there.

Trophic ecology

Classic studies of kelp forest ecology have mostly focused on trophic interactions, which are the relationships between organisms and their food webs. These studies often examine top-down processes, where predators influence the numbers of species lower in the food chain. Bottom-up processes depend on non-living environmental factors, such as the availability of light and nutrients, that allow primary producers like kelp to grow. These nutrients then support higher levels of the food web. For example, kelp often grows in areas where ocean currents bring nutrient-rich water to the surface, called upwelling zones. This helps kelp thrive, which in turn supports herbivores (plant-eaters), who then support animals higher in the food chain.

In contrast, top-down processes occur when predators reduce the numbers of species lower in the food chain by eating them. If predators are absent, these lower-level species may grow too large because their food supply is not limited. In Alaska, sea otters (Enhydra lutris) control sea urchin populations by eating them. When sea otters are removed, urchin numbers increase rapidly. This leads to more urchins eating kelp, which harms the kelp forest. The loss of kelp weakens the physical structure of the ecosystem, causing other species that rely on kelp to disappear. In Alaska, sea otters are a keystone species, meaning they play a critical role in maintaining the balance of the ecosystem through these interactions.

In Southern California, kelp forests survive without sea otters because other predators, such as lobsters and large fish like the California sheephead, control urchin populations. Here, the ecosystem has more predator species to regulate urchins, so the loss of one predator does not cause major problems. However, if many predators are removed, urchins may grow unchecked, leading to kelp forest decline. Similar patterns have been observed in Nova Scotia, South Africa, Australia, and Chile. Scientists continue to study how much top-down and bottom-up processes affect kelp forests and how strong these food relationships are.

Kelp forests can change into barren areas dominated by sea urchins, called "urchin barrens," through trophic cascades. These two states—kelp forests and urchin barrens—are considered alternative stable states of the ecosystem. Kelp forests can sometimes recover after major disruptions, such as urchin disease or sudden changes in water temperature. Recovery from less severe damage is harder to predict and depends on both environmental conditions and interactions between living organisms.

While sea urchins are the main herbivores in kelp forests, other animals like seastars, isopods, kelp crabs, and herbivorous fish also play important roles. Many of these animals eat kelp that has been knocked loose from the ocean floor and floats near the surface, rather than searching for intact kelp. When enough floating kelp is available, herbivores do not harm attached kelp. However, if floating kelp is scarce, herbivores may directly damage the physical structure of the ecosystem. Studies in Southern California show that the amount of floating kelp affects how sea urchins forage. Floating kelp and particles from kelp also help support other nearby habitats, such as sandy beaches and rocky areas near the shore.

Patch dynamics

Another important area of kelp forest research focuses on how kelp patches change over time and space. These changes not only shape the physical environment but also influence animals that rely on kelp for shelter or food. Large environmental events have helped scientists understand how kelp forests respond to challenges and recover. Examples of such events include:

• Pollution events, such as those from sewage, industrial waste, and chemicals like PCBs, heavy metals (such as copper and zinc), pesticides from farms, and chemicals used in harbors (like TBT and creosote), can harm kelp forests. The effects depend on the type of pollutant and how long it remains in the environment.
• Strong storms can damage the top layer of kelp forests but often leave the lower layers intact. Storms may also remove sea urchins if they lack hiding places. Open areas created by storms allow more sunlight to reach the forest floor, helping some species grow. These open spaces can also provide room for other organisms to settle, sometimes competing with young kelp.
• El Niño events cause warmer ocean temperatures, fewer nutrients, and changes in weather patterns. These conditions can weaken kelp, making it more vulnerable to damage from storms or grazing by sea animals. In some cases, this leads to forests dominated by sea urchins. Ocean conditions, such as temperature and water movement, also affect how well kelp and its competitors grow, influencing the overall health of the forest.
• Overfishing top predators that control herbivore populations is another challenge for kelp forests. Removing these predators can disrupt the balance of the ecosystem, affecting how kelp and other species interact.

Scientists study kelp forests before, during, and after these events to understand their effects. They also conduct experiments on smaller areas to test how different factors influence kelp growth. For example, studies in southern Australia showed that the amount of a specific kelp species, Ecklonia radiata, in a forest can predict which other plants and animals are present nearby. This information helps scientists identify how changes in one species affect the rest of the ecosystem.

Human use

Kelp forests have been important to humans for thousands of years. Some scientists think that early people in the Americas followed kelp forests when they moved from Asia during the last ice age. One idea suggests that kelp forests, which once stretched from northeast Asia to the American Pacific coast, helped ancient travelers. These forests provided food and protected people from rough ocean waves. Researchers believe kelp forests might have also helped early travelers find their way, acting like a "kelp highway." Scientists also think these forests gave early people a stable lifestyle, reducing the need to adapt to new environments as they traveled long distances.

Today, many economies depend on fish and other sea life that live near kelp forests, such as lobster and rockfish. People also harvest kelp directly to feed sea animals like abalone and to make a substance called alginic acid, which is used in toothpaste and antacids. Kelp forests are also valued for activities like SCUBA diving and kayaking. The industries that support these activities and the enjoyment people get from them are examples of how kelp forests benefit humans. The Monterey Bay Aquarium was the first to display a living kelp forest in an aquarium.

Kelp forests grow in rocky areas along the shore where the land is constantly worn away and carried into the deep sea. When kelp dies, it sinks to the ocean floor and stores carbon in a place where it is unlikely to be disturbed. Scientists from the University of Western Australia found that kelp forests in Australia store 1.3–2.8 teragrams of carbon each year. This is 27–34% of the total carbon stored yearly by tidal marshes, mangrove forests, and seagrass beds in Australia. Every year, 200 million tons of carbon dioxide are stored by large seaweed like kelp.

Threats and management

Kelp forests are complex because they have different shapes, locations, and ways of interacting with other living things. This makes it hard for environmental managers to predict how they will change in the future. Ecosystems can behave unpredictably because not all relationships between living things are fully understood, and sudden changes can happen when conditions change.

Important problems affecting kelp forests include pollution, overfishing, invasive species, and climate change. Overfishing is a major threat because it removes top-level animals, leading to areas where too many sea urchins grow, which harms kelp. Keeping many different species in an ecosystem helps it stay stable by reducing the chance of sudden problems. A 2022 report said that kelp and other seaweeds are dying in large numbers because of rising ocean temperatures, as they cannot move quickly to cooler areas.

To help protect kelp forests, managers often limit how much kelp is harvested or how many animals are taken from the ecosystem. While these steps can help, they may not protect the whole ecosystem. Marine protected areas (MPAs) are special places where fishing and harvesting are limited. These areas help protect not only the animals people want to catch but also the entire ecosystem. Studies show that MPAs can help fish populations grow and spread to nearby areas. They also help ecosystems recover from damage. A 2023 report said that restoring kelp forests has become more common and may help coastal communities economically.

In the 2010s, Northern California lost 95% of its kelp forests because of warm ocean conditions. Efforts to restore kelp in California focus on removing too many sea urchins, which is done by divers and sea otters, which naturally eat sea urchins. An invasive seaweed called Sargassum horneri, first found in 2003, has also caused problems. The Sunflower sea star is a key species that helps control sea urchin numbers, but it has declined due to disease and climate change.

Researchers at the Bodega Marine Laboratory and volunteers from the Orange County Coastkeeper group are working to replant giant kelp. Humboldt State University started growing bull kelp in a research farm in 2021. In 2020, California announced plans to stop kelp forests from disappearing. At the federal level, a law called the KELP Act was introduced in 2021 to fund kelp restoration through NOAA. A company called Ocean Rainforest received $4.5 million in U.S. government funding to grow giant kelp on a large farm off the coast of California.