Finding Floating Forests:
It Takes an Online Village to Map Massive Kelp
If you have ever walked along the California coast, you’ve likely had to navigate around clumps of seaweed. Fathom this: before it was thrown up by the surf and left to dry on the beach, that seeming jetsam was part of a majestic underwater forest just offshore.
Giant kelp forests are among Earth’s most productive habitats, and their great diversity of plant and animal species supports many fisheries around the world. The kelp, or Macrocystis, that make up these underwater forests truly are giant. They are the world’s largest marine plants and regularly grow up to 35 meters (115 feet) tall; the largest giant kelp on record stood 65 meters (215 feet) tall. Divers have compared swimming through mature kelp forests to walking through redwood forests.
Unlike redwoods, giant kelp are ephemeral. They live for seven years at most, and often they disappear before that because of winter storms or over-grazing by other species. As fishermen know, giant kelp forests can appear and disappear from season to season, from year to year. But is there a long-term trend or cycle at work?
A few years ago, Jarrett Byrnes was in a bit of a quandary over these disappearing forests. As part of his postdoctoral research at the University of California–Santa Barbara (UCSB), he was studying giant kelp at four National Science Foundation-funded sites off the coast. Since 2000, biologists had been using this Long-Term Ecological Research (LTER) site to make monthly in situ measurements of giant kelp. But Byrnes and his colleagues found that they often could not make measurements in winter because rough seas made the diving unsafe.
“Storms remove quite a bit of the canopy in the winter. Sometimes they even remove whole forests if the storms are large enough,” Byrnes explained. “But getting to those sites with regularity in the winter gets very challenging.” Most of the diving had to wait until summer, and by then the kelp had largely recovered or changed, making it difficult to measure how much damage the storms had done.
To complicate matters, kelp forests have different seasonality depending on where they are. For instance, the forests along the Central California coast are at their maximum size in the fall; in Southern California, they often reach their peak in the winter and spring. How could these dynamic habitats be monitored more frequently without putting divers at risk?
Kyle Cavanaugh, then a UCSB graduate student, had an idea. “These forests change so rapidly and on a variety of different time scales—months to years to decades—so we needed a long record with consistent, repeated observations,” Cavanaugh said. He devised a method to use Landsat satellite data to monitor kelp forests.
A few things made Landsat an obvious resource. Since the 1970s, the satellites have had a regular collection schedule (twice monthly). Their data and images are managed by the U.S. Geological Survey and are reliably stored in an archive that dates back more than forty years. And Landsat’s images are calibrated, or standardized, across different generations of satellites, making it possible to compare data collected across several decades.
Landsat measures the energy reflected and emitted from Earth at many different wavelengths. By knowing how features on Earth reflect or absorb energy at certain wavelengths, scientists can map and measure changes to the surface. The most important feature for the kelp researchers is Landsat’s near-infrared band, which measures wavelengths of light that are just outside our visual range. Healthy vegetation strongly reflects near-infrared energy, so this band is often used in plant studies. Also, water absorbs a lot of near-infrared energy and reflects little, making the band particularly good for mapping boundaries between land and water.
“The near-infrared is key for identifying kelp from surrounding water,” Cavanaugh explained. “Like other types of photosynthesizing vegetation, giant kelp have high reflectance in the near infrared. This makes the kelp canopy really stand out from the surrounding water.”
For Byrnes, the approach was a breakthrough: “This meant we could see the forests I was analyzing right after storms hit them.”