Satellite tracks 25 years of giant kelp

UC SANTA BARBARA (US) — A quarter century of images are giving an unprecedented view of the ebb and flow of giant kelp forests, which can be entirely wiped out in days, but fully recover in months.

Along with data collected underwater, scientists used satellite images of kelp canopies taken by the Landsat 5 Thematic Mapper every six weeks for 25 years to track the effects of climate and the environment on the world’s largest alga throughout the entire Santa Barbara Channel. Landsat 5 was originally planned to be in use for only three years.


“I’ve been heavily involved in the satellite game, and a satellite mission that goes on for more than 10 years is rare,” says David Siegel, professor of geography at University of California, Santa Barbara. “One that continues for more than 25 years is a miracle.”

The research is reported in the journal Marine Ecology Progress Series.

Forests of giant kelp, located in temperate coastal regions throughout the world, are among the most productive ecosystems on Earth, providing food and habitat for numerous ecologically and economically important near-shore marine species.

Giant kelp also provides an important source of food for many terrestrial and deep-sea species, as kelp that is ripped from the seafloor commonly washes up on beaches or is transported offshore into deeper water.

Particularly sensitive to changes in climate that alter wave and nutrient conditions, the dynamics of giant kelp growing in exposed areas of the Santa Barbara Channel are largely controlled by the occurrence of large wave events. Meanwhile, kelp growing in protected areas is most limited by periods of low nutrient levels.

Images from the Landsat 5 satellite provided the research team with a new “window” into how giant kelp changes through time. The satellite was designed to cover the globe every 16 days and has collected millions of images. Until recently these images were relatively expensive and their high cost limited their use in scientific research. In 2009, the entire Landsat imagery library was made available to the public for the first time at no charge.

“In the past, it was not feasible to make these longtime series, because each scene cost over $500,” says Kyle C. Cavanaugh, graduate student in marine science and the study’s first author. “In the past, you were lucky to get a handful of images. Once these data were released for free, all of a sudden we could get hundreds and hundreds of pictures through time.”

Giant kelp grows to lengths of over 100 feet and can grow up to 18 inches per day. Plants consist of bundles of ropelike fronds that extend from the bottom to the sea surface and live for four to six months, while individual plants live on average for two to three years.

Giant kelp forms a dense floating canopy at the sea surface that is distinctive when viewed from above. Water absorbs almost all incoming near-infrared energy, so kelp canopy is easily differentiated using its near-infrared reflectance signal.

The satellite images allowed researchers to see how the biomass of giant kelp fluctuates within and among years at a regional level for the first time. “It varies an enormous amount,” Cavanaugh says.

“We know from scuba diver observations that individual kelp plants are fast-growing and short-lived, but these new data show the patterns of variability that are also present within and among years at much larger spatial scales. Entire forests can be wiped out in days, but then recover in a matter of months.”

Kelp occurs in discrete patches that are connected genetically and ecologically, says Daniel C. Reed, co-author and research biologist who has spent many hours as a research diver.

“Having the satellite capability allows us to look at the dynamics of how these different patches are growing and expanding, and to get a better sense as to how they are connected. We can’t get at that through diver plots alone. The diver plots, however, help us calibrate the satellite data, so it’s really important to have both sources of information.”

The research team received funding from NASA and the National Science Foundation.

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