Feb 042018

Mature s. horneri

Sargassum horneri (S. horneri) is a large alga native to the shallow reefs of eastern Asia. As a key species in the Northwest Pacific ecosystem, S. horneri is a primary producer, a biofilter of nutrient runoff, and a traditional food source for the people who live in Japan, Korea, and China. The seaweed beds also provide a habitat for fish, sea urchins, abalones and turban shells. But in California, it grows more densely than many native species, creating a canopy that blocks sunlight from other plants, including the giant kelp (Macrocystis pyrifera) which is native to the California coast. This may impact animals relying on the kelp to hide or for food.

For example, in 2015, local researcher and oceanographer Jon Council said in The Catalina Islander that sea lions “use giant kelp like duck blinds and dart out into anchovies and smelt. Without the kelp, they don’t have that cover.” Because of warmer-than-normal water temperatures brought on by a late-season El Nino and tremendous surge generated by Hurricane Marie in August of 2015, the kelp was devastated. While adult sea lions can adjust, pups are a lot slower and have to work a lot harder to get to the bait balls.  Also, some species, such as smelt, lay their eggs in the fronds or leaves of the kelp. So for sea lion pups, the smelt are now fewer and harder to catch.

Commonly referred to as sargassum, S. horneri was first detected in Long Beach Harbor in 2003 – at that time the only place outside its native range. By April 2006 it was found off Santa Catalina Island north of Big Fisherman’s Cove near Two Harbors and USC’s Wrigley Institute. In 2009, it was also found in the marine park in Anacapa Island. It has since spread rapidly across the Channel Islands and halfway down Baja California, and is still actively spreading. Since 2013, it has become more dominant on the reefs which have historically been dominated by giant kelp.

The problem is twofold: the invasive seaweed grows very fast, especially in the winter and spring; and once established, eradication is impossible. Hence its nickname, “devil weed.” It is highly fecund as well as monecious, which means it has male and female gametes. Theoretically, a single individual can start a whole new population. It is well suited to short and long range dispersal; and if ripped off the substrate while reproductive, the buoyant plants can drift long distances on currents and drop gametes as they go. Once established, populations continue to spread locally.

Lindsay Marks

Lindsay Marks, a PhD candidate at the University of California, Santa Barbara (UCSB), spearheaded recent investigations on the invasion. In May of 2017, she gave a presentation about the ecology of S. horneri and her attempt to remove it with the help of a giant underwater vacuum tube known as the Super Sucker.

To find out how its stages of growth and relative biomass compared to native species in the same area, Marks and her team have been sampling the native algal community at Isthmus Reef, located on the front side of Catalina Island, every three months since 2014.

What they found was an inverse seasonal pattern between the native kelp and the invasive algae—the native species have higher biomass in the summer and fall, and as they later decline, S. horneri -increases. “You could look at this sargassum as just filling an empty niche,” Marks said, “or taking advantage of this opportunity where the native species may be underutilizing their limited resources like space and light.”

To find out whether marine protected areas (MPAs) can resist invasion, she collaborated with researchers Jenn Caselle and Katie Davis at UCSB to study it at Anacapa Island because it has three marine protected areas: the no-take old reserve established in 1978, the new state marine conservation area (SMCA) established in 2003 which allows fishing of spiny lobster, and the no-take new reserve established in 2004.

Sargassum Survey

In the old reserve, there is a much larger number of predators like lobsters than in the unprotected areas on the backside of the island. Because of the higher number of lobsters, there are less sea urchins which are lobsters’ preferred prey. Sea urchins eat algae, so there’s a lot less kelp in the unprotected areas. Conversely, fewer predators live outside the protected areas, which results in many more herbivorous urchins and less native kelp.

The researchers suspected that this biological difference in communities might also be responsible for the differences in the S. horneri biomass. Since the invasive alga is very opportunistic and prospers in places where native algae do not, they surmised that perhaps there would be less S. horneri in areas with healthy robust kelp communities and more of the sargassum in degraded areas.

As expected, Marks and her team did not find a lot of S. horneri in the old reserve. They think this was because the sargassum was experiencing more intense competition and shading from native algae and therefore had less resources available.

Lindsay Marks in the Field

However, on the backside of Anacapa where fishing is allowed, they saw very little of S. horneri as well. These sites were characterized by “urchin barrens” where enormous numbers of sea urchins graze all available algae, including both kelp and Sargassum. While this is an alternative scenario for resistance to invasion by sargassum, it is certainly a less desirable state of the ecosystem.

In the newer MPAs established in 2003 and 2004, there were much higher levels of S. horneri. The researchers surmised that urchins will eat anything if hungry enough, but given the choice they prefer the taste of kelp over S. horneri. A follow-up project Marks called “Algae Buffets” bore out these expectations. Given a choice between samples of giant kelp, southern sea palm (Eisenia arborea), and S. horneri, the urchins favored the two common native kelps. The reason?  S. horneri is a fucoid alga, and these typically have chemical defenses against herbivories, so it probably doesn’t taste very good.

Thus MPAs may be able to resist invasion only if they’re more established, with mature kelp forest communities and more predators controlling herbivore populations. “It will be interesting to see as these newer MPAs continue to mature, whether or not sargassum will impede that progress,” Marks said.

In an attempt to weed the seaweed, Marks used an underwater vacuum device called a Super Sucker as part of a National Oceanographic and Atmospheric Agency (NOAA) Fisheries pilot project. The gentle vacuum transports algae from divers to the surface where it can be sorted for bycatch and bagged for disposal. A similar device has already been used on coral reefs around the island of Oahu in Hawaii to reduce invasive algae there.

Vacuuming S. horneri using the Super Sucker

In February of 2015, Marks—along with Adam Obaza of NOAA Fisheries, Sam Ginther, a graduate student at Cal State Northridge, and a group of volunteer research divers—cleared fourteen 60 m2 areas of S. horneri on the leeward side of Catalina Island. Since it was too difficult to remove tiny recruits from seaweed beds, they chose February when the alga was biggest but also the least dense, and also before it was reproductive.

So understanding the life cycle of S. horneri is paramount to launching a successful removal. While native algae are perennial, meaning they live through multiple reproductive cycles, S. horneri is an annual species. It’s much more abundant in the summer time, sometimes reaching 1000’s of individuals per meter squared, due to the peak in recruits. As the season progresses from fall to spring, it becomes much less abundant.  But in terms of biomass, recruits contribute very little in the summer.  That changes in the winter and spring, when there’s a huge boom of biomass.

In all they removed more than four tons of material in three days. They returned in September 2015, when they expected to find the most of the new recruits based on their seasonality data. Initially, they had reduced recruitment by about 50% in the removal plots compared to control plots. Sounds impressive, but unfortunately that 50% reduction turned into a 25% reduction as those populations matured and grew up. Marks was disappointed. “We sort of sped up the self-thinning process but all we really did was slow population growth,” she said.

Wrigley Sunset

Marks surmised that the warmer waters deterred the growth of native kelp, which had essentially disappeared from the region where they were working on the leeward side of Catalina near Wrigley, which meant the native species were unable to colonize the spaces the reef we created by clearing sargassum. She thought they would have seen less of it the following year if native kelp had been able to reclaim those spaces.

Dr. Bill Bushing, a marine biologist who was one of the first to discover sargassum on Catalina Island, observed that there was very little of it during August and September of 2009, at which time the water was cooler than the previous five years when sargassum became a problem. “Possible reasons may include the limited light reaching the subtidal rocky reefs due to the thick native kelp canopy, or the cooler water temperatures directly affecting the growth of sargassum,” he wrote in California Diver back in 2014.

Native kelp and the invasive alga share a similar temperature range of what they can survive in—10° to 25°C (50 to 77°F) for S. horneri and 5° to 20° C (42° to 72°F) for giant kelp. But Marks said that it was possible that in the upper threshold the sargassum might have a little bit of a leg up. In the last few years, we’ve had unusually warm water in southern California and native kelps have all but disappeared from the leeward side of Catalina while sargassum has continued to thrive.

Two hurricanes in 2014, Marie in August and Odile in September, may have also deterred the growth of native kelp. While Marks was hesitant to comment on the role of those particular hurricanes, she suggested that any loss of native kelp would facilitate the sargassum.

“It’s well known that invasions do better in more disturbed habitat,” she said. “So maybe this former water event has removed what we would think would be primary competitors of Sargassum – the larger species of algae like kelp. And that frees up additional resources like space and light for the invasive species.”

Lindsay Marks with Mature S. horneri

To investigate whether increased wave disturbance facilitates the disturbance of the invasion of S. horneri, Marks has been turning herself into “Hurricane Lindsay”—ripping out the bigger species of algae, and then allowing the invasion to occur in order to see what grows back in its place. She hopes to understand what is excluded, and whether or not sargassum does better in these areas where the natives have been compromised.

For the Super Sucker’s next phase of abatement and management, NOAA has handed off the Super Sucker to Los Angeles Waterkeeper. Their target is Palos Verdes, specifically Malaga Cove where the sargassum is patchy in distribution (compared to Catalina where it has become ubiquitous in some areas), and the kelp populations are more established.

“The idea is that after removals, there’ll be enough of the native species to recolonize the space that’s been created, create a foothold, and then prevent the invasive algae from becoming abundant there,” Marks said.  “So hopefully LA Waterkeeper will have more success working in places that are not so heavily invaded.”

Though it seems like Catalina has been abandoned, Marks thinks the situation there is neither hopeless nor permanent. “I think the kelp will come back,” she said. “It’s not uncommon for there to be a few years of low abundance, especially following major, El Niños, but it’s cyclical. I think kelp may have a harder time coming back in areas where sargassum is really dense, but I think it will eventually. In the last year or so I’ve started seeing new kelp plants growing back here and there.”

LA Waterkeepers: (from left to right) David Coles, Alex Rappaport, Michael Murrie, and Ian Jacobson

Los Angeles Waterkeeper launched their Invasive Species Campaign in early 2016, treating invasive species as a form of biological pollution. As the proliferation of Sargassum horneri continued, its widespread distribution threatened potential lasting ecological consequences to native habitats. LAW decided to dedicate the resources of their dive program to address the current invasion with the hope of developing practical removal methods, establish a more informed dive community, and aid in the development of recommendations that will address future non-native introductions.

LA Waterkeepers: (from left to right-)Adam Obaza, Hannah Ake, and Katie Nichols

Their pilot study, headed by Dive Program Manager Ian Jacobson, began in September 2016. Five volunteer divers removed more than 100 lb. in two days in an area next to a healthy kelp forest, which is the reference site. There is one S. horneri reference site which is untouched, and two experimental sites, one of which sargassum has been removed once, and another in which sargassum will have been removed a second time by February 2018. The reason for the double removal plot is to find out if regular maintenance is more effective.

L.A. Waterkeepers: Hannah Ake surveys S. horneri

Jacobson started seeing new sargassum recruits in late September 2017, but it won’t be until May of next year before they have data. Jacobson will be looking at whether the removals reduce density over time. If successful, they will develop a strategy to control and abate the sargassum, as well as protect kelp forests in certain areas. For example, say a small sargassum population is found in a harbor and gets caught in boat hulls or anchors, and is then reintroduced in a new area.  “If we can effectively control both populations in a very coordinated way, then that could be one buffer to potential expansion and potential distribution to novel areas,” he said.

Neither Jacobson nor Marks is even thinking about eradication. “Management, control, abatement— those are the terms we’re using,” Jacobson said. “And it’s premature to say how effective any strategy is, because our studies aren’t complete yet. Call me next year at this time.”

I plan to take him up on his offer, so stay tuned. In the meantime—if you’re a certified diver, you can help track the spread of marine invasive species by reporting sightings of S. horneri and a new one not discussed here, the edible seaweed Undaria Pinnatifida. If you come across either of these species, take a close-up photograph so you can upload your observation to MarineInvasives.org or the iNaturalist app. Include the name of the species, its location, how many you saw, the water depth, life stage, and habitat. Make sure you don’t remove the invasive algae since it spreads easily. And before you leave the dive site, check your gear and anchor, and remove any hitchhiking seaweed.

Marineinvasives.org has useful resources, including information on both invasive species, and a map of all the places where these species have been recorded. iNaturalist is a user-friendly app you can download on your phone and also use on your computer. After you upload your pictures and observations of Sargassum and Undaria to the project named “Marine Invasives Reporting App,” fellow iNaturalist contributors can verify your species ID and provide useful feedback. LA Waterkeeper has trained 30 LAW divers how to identify sargassum, report it using iNaturalist, and conduct monitoring surveys for the Pilot Project. And rather than give your GPS location, you can just drop a pin on a map.

Also, LA Waterkeeper has been giving presentations to local clubs since 2016 so contact them if you’re interested.

As Marks said, “It’s a slow process, we’re building momentum, we’re moving in the right direction.” So get involved and help the effort to weed this seaweed.

Originally published in Catalina Marine Society’s OceanBights, p. 7



Jim Haw, USC Dornsife Scientific Diving: An Analysis of Sargassum Horneri Ecosystem Impact, Scientific American, May 22, 2013

Jim Watson, Invasive seaweed adds to sea lions’ woes, The Catalina Islander, March 20, 2015

Miller, Kathy Ann & M. Engle, John & Uwai, Shinya & Kawai, Hiroshi. (2007). First report of the Asian seaweed Sargassum filicinum Harvey (Fucales) in California, USA. Biol. Invasions. 9. 609-613. 10.1007/s10530-006-9060-2

Lindsay Marks,  New Research Studies Spread and Ecology of Invasive Seaweed, National Park Services, May 11, 2017

Lindsay Marks, PhD candidate at the University of California, Santa Barbara (UCSB), Interview October 17, 2017

Bill Bushing, The Invasion of the “DEVIL WEED”: Sargassum horneri invading California waters,California Diver, July 22, 2014

Lindsay Marks, Sargassum horneri Information Sheet, University of California, Santa Cruz (UCSC), Ecology and Evolutionary Biology

Kelp Forests – a Description, NOAA National Marine Sanctuaries

Marks, Lindsay & Reed, Daniel & Obaza, Adam. (2017). Assessment of control methods for the invasive seaweed Sargassum horneri in California, USA. Management of Biological Invasions. 8. . 10.3391/mbi.2017.8.2.08.

Ian Jacobson, Dive Program Manager, L.A. Waterkeeper, Interview October 22, 2017



Mature s. horneri – Lindsay Marks

Lindsay Marks –  Jessie Alstatt

Sargassum Survey – Lindsay Marks

Lindsay Marks in the Field – Jessie Alstatt

Vacuuming S. horneri using the Super Sucker – Adam Obaza

Wrigley Sunset – Lindsay Marks

Lindsay Marks with Mature S. horneri – Jessie Alstatt

L.A. Waterkeepers: (from left to right) David Coles, Alex Rappaport, Michael Murrie, and Ian Jacobson, Chris Glaeser – Chris Glaeser

L.A. Waterkeepers: (from left to right) Adam Obaza, Hannah Ake, and Katie Nichols – Ian Jacobson

L.A. Waterkeeper: Hannah Ake surveys S. horneri – Adam Obaza

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