Biosphere Daily Earth Science Environment Repost

California kelp forests: Ecosystems in distress

The phrase “sequoias of the sea” is a reference to California’s kelp forests. Kelp plants grow up to 175 feet above the ocean floor under ideal conditions. They are also fast-growing and thus very efficient at carbon sequestration. Kelp is the anchor species for local ecosystems along the southern California Coast and northward to Santa Cruz. But the kelp forests along the West Coast are ecosystems in distress. Increasingly warm coastal waters and over-predation by purple urchins have decimated many kelp forests. 

Over the years, pollution and overfishing along the southern California Coast have reduced kelp in some localities by 75 percent. Overfishing is a problem since it removes fish with critical roles in protecting the ecosystem. Then, starting in 2014, the kelp forests faced another threat when marine heatwaves wiped out 90 percent of the north coast kelp along a 200-mile stretch. Kelp survives in a limited temperature range, so heatwaves either wipe it out or reduce its reproductive capabilities.

Balanced ecosystems

Kelp forests are ecosystems supporting an intricate and interdependent web of life. Hundreds of marine species live in the kelp forests and rely on the kelp for their survival. Gray whales shelter their young there. Sea otters hunt for food among the towering kelp plants, and commercial abalone fisheries also depend on the kelp. The survival of the kelp depends on a healthy balance of marine species within the forests. When that balance is destroyed then the kelp weakens or disappears.

One of the natural predators of the purple urchin is the sunflower sea star. Disease decimated the sunflower sea stars in recent years, and in the absence of natural predators, purple urchin populations exploded. The purple urchin is a natural part of the kelp forest ecosystem. It feeds on the kelp. But without natural predation, these creatures devour all of the kelp in an area. 

The kelp forest in Honeymoon Cove near Palos Verdes was entirely consumed by the urchins several decades ago. So, areas like Honeymoon cove are referred to as “urchin barrens.” The problem with this particular species of urchins is that when their food source, the kelp, disappears, they enter into a hibernation period for up to 50 years. Therefore, any new kelp that returns to the area immediately becomes a meal for ravenous urchins.

Kelp and carbon

Healthy kelp forests also act as a chemical buffer. They represent fast-growing biomass that effectively sequesters carbon by removing carbon dioxide from the ocean water. The kelp use CO2 for photosynthesis and produce oxygen as a by-product. So, the high biomass of kelp in a forest ensures that the surrounding waters remain well oxygenated.

Absorption of CO2 by the kelp provides another added benefit to the ecosystem. It reduces the acidity of the local water. Carbonate buffering is the name given to the CO2 exchange process between the atmosphere and the oceans. When the CO2 enters the ocean, it reacts with water to form carbonic acid, and this causes ocean acidification. However, buffering involves a reversible set of chemical reactions. So, the removal of CO2 from the ocean waters lowers ocean acidity (raises the pH). Rising ocean acidity affects organisms from plankton to crabs. If acidity weakens species like plankton at the base of the food chain, then all of the species in the ecosystem come under stress. Lower acidity helps local ecosystems thrive.

Kelp forests are the foundation for healthy and diverse coastal ecosystems in California. Their role in maintaining viable commercial fisheries and productive coastal habitats should not be underestimated.


Hot ocean waters affecting New Zealand kelp (Source: ArcheanWeb) –  Also:

Heat affects the ocean’s food chain (Source: ArcheanWeb) –  Also:

Ocean Acidification (Source: ArcheanWeb) –  Also:

A crab crisis from coastal water acidification (Source: ArcheanWeb) –  Also

Acid and phytoplankton in the ocean’s food chain (Source: ArcheanWeb) –  Also:


California’s critical kelp forests are disappearing in a warming world. Can they be saved? (By Todd Woody; National Geographic) –  Also:

Giant kelp (Macrocystis pyrifera) (Source: Monterey Bay Aquarium) –  Also:

Saving California’s Kelp Forest May Depend On Eating Purple Sea Urchins (By Erika Mahoney; NPR – The Salt) –  Also:

Feature Image: Kelp Forest tank, Monterey Bay Aquarium  (Modified) – By Daderot – Own work, CC0,

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.