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Much like forests on land, kelp forests are a bonanza for biodiversity. The aptly named giant kelp can grow 2 feet (0.6 meter) per day, sometimes reaching a length of 175 feet (53 meters). Inside its leafy stipes live kelpfish, kelp crabs, sea lions, and seals. Sea otters twirl themselves in the tendrils so they don’t drift away while sleeping.
Kelp, which describes several species of large brown algae, is a crop and a carbon sink, too. Companies use the seaweed to make everything from shampoo to salad dressing, and its ecosystem services are valued at $500 billion annually, in large part because it captures nearly 5 million metric tons of climate-warming carbon dioxide each year.
But kelp forests are fickle. They can boom and bust under the influence of marine heat waves, the El Niño–Southern Oscillation (ENSO), or storms. New research shows that forest fires in coastal watersheds also play a role.
By comparing two data sets in an unexpected way, a team of researchers is exploring how forest fire size relates to kelp beds downstream. The team presented some of their findings at AGU’s Annual Meeting 2023 in San Francisco.
Calculating a Canopy of Kelp
After a forest fire, rain sweeps dirt and ash into the nearest waterway. When that excess terrestrial grit hits the ocean, it can both hurt and help the kelp.
For one, the rich nutrients that wash downriver can fuel the growth of marine life, but cloudy water also reduces the availability of sunlight, which can stunt growing kelp stalks. In addition, excess sediment can hinder giant kelp holdfasts from anchoring to the rocky seafloor.
“If enough of the reef is covered [in sediment], kelp can’t survive,” said Kyle Cavanaugh, a marine scientist at the University of California, Los Angeles.
The new research uses two public tools—Landsat and SWAT—to study the complex connection.
“We often end up getting siloed by discipline. We’re working to connect all the different pieces that would otherwise be disparate.”
Landsat is a joint program of NASA and the U.S. Geological Survey. Its satellite imagery has captured California’s coastline at 30-meter resolution since 1984. More recently, the kelp project has also incorporated imagery from NASA’s Sentinel-2. Kelp canopies are fairly easy to see on satellite images, said Cavanaugh, but creating kelp maps requires pixel-by-pixel analysis. His lab group and collaborators from Woods Hole Oceanographic Institution make their data available on kelpwatch.org.
The researchers also used the Soil and Water Assessment Tool (SWAT), a hydrology model developed by the U.S. Department of Agriculture. The tool tracks terrestrial traits like soil erosion and groundwater infiltration. But by combining satellites with SWAT, the researchers can better understand how forest fires and soil erosion affect giant kelp.
“We often end up getting siloed by discipline,” said Christine Lee, a scientist at NASA’s Jet Propulsion Laboratory, who coauthored the research with Cavanaugh, NASA colleagues Mandy Lopez and Lori Aznive Berberian, and Erin Hestir of the University of California, Merced. “We’re working to connect all the different pieces that would otherwise be disparate.”
Splash and Burn
Preliminary research shows there’s an impact. The Woolsey Fire, for example, burned nearly 940 million square meters in November 2018, sending sediment into Malibu Creek. Before the fire, the spring 2018 kelp canopy covered 46,606 square meters near Malibu. The following spring, kelp covered only 9,543 square meters and has yet to recover to prefire levels.
The SWAT model estimated that Malibu Creek carried up to 1,721 metric tons of sediment per month after the fire. That’s up to 1.3 times higher than prefire sediment loads and aligns with in situ observations from Los Angeles County.
The multiyear study shows that wildfires have a secondary impact on kelp, but there’s more to explore. The Woolsey Fire arrived after a marine heat wave and multiyear El Niño, and kelp is so naturally dynamic that “it can be difficult to detect the effects of long-term and local factors,” said Cavanaugh. He and his fellow researchers are currently reviewing decades of data to find control sites along California’s coast and isolate the effects of sediment.
Kelp and Coastlines
SWAT is a land surface hydrology model, but the study extends its results to the ocean. That’s just one thing that makes this project unique, said Adnan Rajib, a computational hydrologist at the University of Texas at Arlington who was not involved in the research. It’s one of the first studies to connect the dots between postfire hydrology and kelp, he said, and it does so on a scale larger than other SWAT studies.
“When you look at large watershed systems, that’s when you have the total picture of things.”
“Usually, studies on postfire hydrology are very local,” he explained. “When you look at large watershed systems, that’s when you have the total picture of things.”
The group is already studying several California watersheds and aims to scale their research globally. Giant kelp stipples the coastlines of Chile, Mexico, New Zealand, and Tasmania, and the frequency and severity of fires, marine heat waves, and ENSO are all expected to increase. Will nutrients from the forest offset warming in the ocean?
“I thought this was a really interesting study because it looks at the nontraditional sea surface–temperature–nutrient relationships for kelp,” said Meredith McPherson, an oceanographer at the University of Massachusetts Boston who was not involved in the research.
“I like the concept of the study,” she said. “I’m interested to see what plays out for the other sites.”
—J. Besl (@J_Besl), Science Writer