Tag Archives: Sargassum

Kelptastrophe! Mystery of the Missing Kelp

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Catalina Island’s kelp forests are a wonder to witness. The amber-green kelp provides a home for countless animals along the California coastline, from gray whales to juvenile invertebrates. Snorkeling underneath giant kelp is like swimming through a kaleidoscope of stained glass windows and finding animals around every corner. Unfortunately, the kelp is in trouble and it has been struggling to grow in Southern California since 2014.

Giant kelp (Macrocystis pyrifera) is the fastest growing organism on the planet. In ideal conditions, Macrocystis can grow up to 2 feet in a single day! Like other brown algae, Macrocystis is an autotroph, meaning that it makes its own food from the sun using photosynthesis. Kelp forests produce as much oxygen using photosynthesis as the Amazon rainforests with 100 times less biomass. Although giant kelp can grow up to 110 feet in length, it prefers to grow in waters shallower than 30 feet. Air bladders along the kelp help float the translucent blades to the surface to collect sunlight while a branching network called a holdfast anchors the algae to hard substrate below. Giant kelp forms canopies on the water’s surface, providing an island for marine birds and a shelter for countless fish, marine mammals, and invertebrates. Furthermore, rotting kelp washing up on beaches provides a moist cover for fish eggs or ready buffet for insectivorous shorebirds.

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Anatomy of typical algae

Image: The Root Stock

Macrocystis also has critical economic importance to California and the Pacific coast. Many fisheries (rockfish, Dungeness crab, Red abalone, Red urchin) rely on the kelp for habitat, and those species are a food source for larger macrofauna like pinnipeds and seabirds. Giant kelp is widely used in cosmetic and food industries as a thickening agent in products like toothpaste, ice cream, and makeup. Its natural beauty makes it a highlight of tourism for the region as divers and snorkelers travel from all over to the world to frolic in the amber blades. Giant kelp is also being targeted as a possible bio-converter into ethanol as a fuel source for cars and boats.

Macrocystis is deeply integrated into coastal ecology. It is a critical food and habitat source for a wide network of sea creatures. It is an ecosystem in itself, supporting a wide network of consumers and producers. Sea otters and the ochre sea star (Pisaster ochraceus) are two keystone species, meaning that their removal has wide-reaching effects that greatly impact coastal populations relative to other animals. In the case of the kelp forest, they both curb the destructive effects sea urchins have on kelp abundance. Sea urchins are kelp’s worst enemy as they nibble on the stipes until the entire algae breaks off from the holdfast. If too many urchins are present then they will essentially mow down the forest until only barrens remain. Scores of animals that relied on the kelp for food or shelter move away from their former home as a result and the ecosystem could collapse.

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Simplified kelp forest food web, typical for Southern CA

Image: Michelle Frantellizzi

The kelp forests of Southern California are currently in the stages of an ecological emergency. In 2003 another species of brown algae, Sargassum horneri, was accidentally brought over to the Port of Long Beach by a cargo ship from Japan. The spores of this plant then traveled to Catalina Island via a recreational vessel and quickly took root in the hard substratum. Sargassum flourishes in a wide range of temperatures (50-75ºF) and is commonly known as “Devil Weed.” It is an annual alga and can self-fertilize, meaning that multiple generations can exist in a single space. This is the ideal time for Sargassum reproduction: winter and early-spring where the waters are colder and currents are stronger.

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Macrocystis struggling to grow among Sargassum

Image: Alyssa Bjorkquist

Scientists and divers began to notice tangled webs of Sargassum invading spaces where Macrocystis used to flourish. Invasive species can be detrimental to an ecosystem because they outcompete other native species for critical food, habitat, and resources. Once Sargassum was present in the environment, it essentially took three strikes to hurt the giant kelp populations:

Strike 1: While Sargassum began to develop its presence in Southern California, a virus infected essentially erased 90% of Pisaster populations. With sea otters absent from the region, giant kelp lost one of its greatest protectors to Sea Star Wasting Disease. Without a top predator curbing population growth, urchins began clearing the way for Sargassum to grow through intense kelp grazing.

Strike 2: A hurricane in 2014 brought powerful waves, currents, and winds to a relatively stable environment. Although Macrocystis is a hardy algae, massive swaths of kelp washed away in the face of the intense storm. Sargassum seized the opportunity for ideal real estate and claimed the land, weaving dense webs that made it impossible for Macrocystis to resettle.

Strike 3: Incredibly warm oceans and strong currents during the 2016 El Niño Seasonal Occurrence (ENSO) swept away Macrocystis spores or freshly settled growths. Giant kelp can only grow in a narrow range of temperatures (59-65ºF), and the 2016 ENSO provided the perfect recipe for inhibiting giant kelp growth with a peak water temperature of 80ºF. Sargassum, on the other hand, flourished under the warm waters and used the strong currents to spread to other kelp forests in the region.

With warming oceans and increasing boat traffic throughout the region, it is unknown whether the giant kelp will be returning any time soon. Juvenile Macrocystis have a difficult time settling and growing among the dense webs of Sargassum and those that do settle are quickly torn apart by the invasive algae. Divers conducting surveys of Sargassum have observed that fish and other grazers are not eating the algae. Preliminary research suggests that the algae’s thick stipe, tiny blades, or potential presence of toxins deter animals from grazing.

The Macrocystis kelptastrophe is not hopeless. Many scientists, divers, and citizens who care deeply about Macrocystis are working hard on restoration projects in the region. Many students are using Macrocystis restoration as an opportunity to connect with their coastlines. Scientists and dive companies are collaborating to conduct surveys of where the algae are distributed and how they are changing over time. Nancy Caruso, a marine biologist in Southern California, has been working with local high school students to raise juvenile Macrocystis in their classrooms and then plant the kelp near their local beaches. Regardless of perspective, it is undeniable how much we love our giant kelp and we hope it comes back soon.

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The underwater forests are changing. (Sargassum on left, Macrocystis on right)

Images: Alyssa Bjorkquist

To track the spread of Sargassum or to report a sighting, visit: http://www.marineinvasives.org

To learn more about Nancy Caruso and how you can help the kelp, click here: http://loe.org/shows/segments.html?programID=14-P13-00045&segmentID=5

To learn more about Southern CA fisheries, click here: http://www.pressdemocrat.com/news/5487602-181/collapse-of-kelp-forest-imperils?artslide=0

Written By: Alyssa Bjorkquist


  1. Harbor Seal in kelp forest: http://www.rsmas.miami.edu/assets/w144-overall-KyleMcBurnie.jpg
  2. Kelp anatomy: http://www.therootstock.org/
  3. Kelp forest food web: https://prattf10.wordpress.com/author/michellefrantellizzi/


  1. http://www.pressdemocrat.com/news/5487602-181/collapse-of-kelp-forest-imperils?artslide=0
  2. http://www.eeb.ucsc.edu/pacificrockyintertidal/data-products/invasive-species/Sargassum%20horneri%20Information%20Sheet_8.27.15.pdf
  3. http://californiadiver.com/the-invasion-of-the-devil-weed-sargassum-horneri-invading-california-waters/
  4. https://www.montereybayaquarium.org/animal-guide/plants-and-algae/giant-kelp
  5. https://caseagrant.ucsd.edu/project/the-spread-and-ecological-consequences-of-the-invasive-seaweed-sargassum-horneri

Sargassum is it a Weed?

What makes a weed a weed? It grows fast, it takes up resources of other native species thereby reducing diversity, and it causes ecological problems because it is so abundant – sometimes called a “monoculture”. Did you know there are “weeds” in the ocean, too? 

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Sargassum horneri, or as it is not-so-fondly known by some California divers, “devil-weed”, is an invasive algae from Asia that has been steadily taking over Catalina since 2006, when it was discovered by divers who began a citizen science project to document its abundance. In some places, including our coves here on the leeward side of the island, it is outcompeting our native algae, including the giant brown kelp that once formed dense forests home to a diverse abundance of invertebrates and fish.

Invasive species traverse the world’s oceans by many means. The most common is through shipping channels. A ship’s hull can hold thousands of species, accumulating them in one port and depositing them thousands of miles away. Ships also hold and empty water called ballast, used to counteract the weight of offloaded cargo to keep the vessel balanced. Sargassum likes shallow, temperate rocky reef habitats, and as of today can be found as far north as and as far South as Baja California, Mexico. and as ocean temperatures rise, it is predicted to continue to expand its range. Sargassum grows ten feet tall and forms “fields” on the ocean floor, and thick carpets on the water’s surface when its holdfasts, root-like structures that keep it anchored to the rocky bottom, break free. While sargassum is displacing kelp populations, it is also increasing fish abundance.


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