Underwater landscaping: Foster Scholar Lindsay Marks removes invasive seaweeds around Channel Islands National Marine Sanctuary

By Yaamini Venkataraman

February 2018

There’s an invasion beneath the waves at Channel Islands National Marine Sanctuary. It’s not an alien invasion — although the invaders themselves are green. Sargassum horneri, an invasive seaweed from Japan, has been replacing native algae in the sanctuary since 2009. In the past six years, its abundance has shot through the roof. With this seaweed sprawling over the seafloor, who’s the sanctuary going to call?

They call Dr. Nancy Foster Scholar and “underwater landscaper” Lindsay Marks. For her Ph.D. at the University of California, Santa Barbara, Marks extensively studies the fundamental ecology of S. horneri to try and predict how the ecosystem will be affected by its growing presence in Southern California.

According to Marks, S. horneri has several characteristics that make it a “really great invader.” It reproduces and matures quickly, and can also self-fertilize to reproduce faster. If the parent plant gets dislodged, it can easily drift long distances and drop offspring as it floats along. The invasive algae can also thrive at depths where other species do not.

While the species living within ecosystems can change naturally, invasive species change the abundance and diversity of the native species. This change can leave an ecosystem less stable, which in turn can impact its economic benefits (like serving as a nursery for juvenile fish) and recreational offerings (like acting as an intriguing dive site). S. horneri is no exception. Averaging 10 feet tall, it’s shorter than the 60-foot native giant kelp (Macrocystis pyrifera). The invasive algae is unable to provide the same extensive kelp forest habitat giant kelp does. Giant kelp houses several species in its kelp forest canopies, including the valuable kelp bass. If S. horneri continues to replace the native seaweeds, these kelp forests will look very different.

Marks sums it up well: “It does not taste good to herbivores like sea urchins, it grows really fast, and it reproduces like gangbusters.” How, then, can the sanctuary stop S. horneri from spreading?

Marks tests how effective removing the invader is out at Santa Catalina Island. Santa Catalina Island is just outside of Channel Islands National Marine Sanctuary, and is one of the first places S. horneri became really abundant. In 2016, Marks removed seaweed before the reproductive season. Not only did she see a 25 percent reduction in the adult population, but she also saw a 50 percent reduction new S. horneri plants.

While removal as a management strategy seems promising, the 2016 El Niño year tells a different story. The warmer waters it brought to Southern California favor the invasive seaweed, and the corresponding increase in S. horneri abundance may provide a glimpse into future ocean conditions. With the steady increase of sea surface temperatures, it is possible that S. horneri will start to engulf the sanctuary, even with removal efforts.

There is still a reason for ocean optimism. Long-standing marine protected areas and no-take reserves, like those found within Channel Islands National Marine Sanctuary, are more resilient to the spread of S. horneri.

“In historical [marine protected areas] within the sanctuary, we see less of the invasive algae than we do in newer reserves that have not had enough time to recover from overexploitation,” Marks says. “There is some hope that well-preserved, healthy kelp forest communities do have the ability to resist these species.”

The logic goes like this: with protection and conservation, ecosystems can find balance within their food webs. For example, areas that have been protected longer have higher populations of kelp forest predators, like lobsters. The lobsters, in turn, prey on sea urchins. Sea urchins are notorious for their ability to consume vast quantities of kelp, but they are kept in check when lobsters are around. But if lobsters are fished excessively, the urchin population balloons out of control. The many urchins can eat all of the native algae, making space for the invasive S. horneri to proliferate. In protected areas like national marine sanctuaries, resource managers can work closely with fishermen to ensure that enough lobsters and other predators remain to keep the ecosystem healthy.

Instead of waiting for Southern California waters to recover over time, Marks is taking action. She worked with researchers at the University of California, Santa Cruz to create an information portal that documents the spread of S. horneri. She also created promotional materials — like fliers and videos — with California Sea Grant for recreational divers and boaters to help them identify the invasive algae, report it, and avoid spreading it. Marks has presented this information to the Channel Islands Sanctuary Advisory Council and stakeholders like dive clubs, in addition to posting fliers around harbors.

Currently, citizens can only harvest up to 10 pounds of invasive algae a day. But by educating the public, Marks is building an army of citizen scientists who care about the ocean and marine conservation. In the future, Marks hopes she can take citizen groups out into the water to participate in her underwater landscaping projects.

“People are very excited to learn about it. Every time I talk to divers about S. horneri, they are all gungho to go out there and start removing it,” Marks says. “It’s definitely a dream of mine to keep working to control marine invasive species and harnessing the energy of citizens who want to help.”

Yaamini Venkataraman is a volunteer social media intern for NOAA’s Office of National Marine Sanctuaries and a graduate student at the University of Washington.