Waves mightier than sun, otter or urchin: storm disturbance shapes California kelp forests

This post contributed by Liza Lester, ESA communications officer.

As winter storms pick up along the California coast, a harvest of giant kelp comes ashore with the tides, torn from seafloor anchorages by the rough action of waves. Waves are the most powerful force shaping the kelp forest, superseding the influence of temperature, nutrients, and hungry animals, say University of California, Santa Barbara (UCSB) researchers in the November issue of Ecology.

From Alaska to Baja California, kelp undulates in the currents of rocky coastal shallows, feeding and sheltering a host of sea creatures and birds. Americans harvest kelp for food and fish feed, and the kelp forest harbors commercially valuable fish and shellfish. In central and southern California, the giant kelp predominates. Macrocysits pyrifera anchors at depths of 6 to 150 feet, and is the largest alga in the world, reaching underwater heights of nearly 150 feet in a single season. Conversion of sunlight into kelp fuels an ecosystem.

“Primary production is the amount of plant material produced per unit area of the Earth’s surface per unit time. It’s really the basis of all life on Earth for the most part,” said Dan Reed, research biologist at the Marine Science Institute at UCSB, and principle investigator of the Santa Barbara Coastal Long Term Ecological Research project. In the kelp forest, the primary producer is the kelp itself. Reed and his colleagues wanted to know how periodic disturbances from large waves stacked up against other influences on kelp forest growth.

Lack of nutrients, particularly nitrogen, slows the kelp’s exuberant expansion, as do the teeth of small, but numerous, sea animals. Kelp is the favorite food of the sea urchin, as commercial harvesters of the fist-sized, spiky animal well know. Urchins do not climb the kelp stalks. They forage across the seafloor, devouring fallen kelp blades (analogous to leaves) and chunks. But their powerful, self-sharpening teeth can also chew through the holdfasts of the kelp, releasing the giants to the mercies of the ocean currents, as graphically exhibited by time-lapse footage in the BBC’s documentary Planet Earth. In concentrated herds, unchecked urchins have been known to raze entire forests.

The check on the urchin is the sea otter, a top predator of the kelp forest. The demands of the otters’ high metabolisms drive them to eat up to a fourth of their body weight in invertebrates daily, and they like sea urchins. The otters are a classic example of a keystone species, an animal whose eating habits tip a crucial balance in a cascade of consumer-and-consumed reactions. The arrival of otters in new territory has changed relatively barren, stony seafloor into a canopy of kelp, seals, birds, and a multitude of small shelled creatures.

The relative importance of “top-down” consumption and “bottom-up” nutrition is a perennially popular conversation in ecology.

“Most scientists have realized at this point the both are important. That said, I think there are still scientists out there who are beholden to the fact that they study consumers, and that the world is controlled by consumption,” said Reed. The contrary can be true of researchers focused on the physical influences on primary production. “They are vying for recognition that their theories are more important. What we need to do is understand which factors are important in specific conditions.”

How do stormy seas shape the kelp forests, and by consequence, the animals and microorganisms they feed? The UCSB group couldn’t create a perfect experiment, controlling all variables—sea otters, urchins, kelp, nutrients and waves. The scale of the experiment would be impossible (as well as unethical, Reed added). But they could take advantage of natural variation along the California coast.

Upwelling cold water on California’s central coast brings nutrients up from the ocean floor that help kelp grow. The central coast of California is also home to nearly 3000 sea otters, the carefully protected descendents of a small group that escaped extinction from the nineteenth century fur trade. So from both “bottom-up” and “top-down” perspectives, the warmer, urchin-ridden coasts of southern California would be expected to host less kelp.

But Reed and Siegel’s group saw the opposite, and they think the reason is waves. While southern California does get storms, it does not have the consistently stormy weather of central California.

“In central California big waves happen every year. Large storms come in during the winter and pretty much take out the forest,” Reed said. “We found that the proportion of the canopy that’s lost is on average twice as high in central California as southern California.”

The team measured the mass of the kelp by calibrating satellite images with dive data from 17 long term research sites, a project made possible when Landsat 5’s 25-year image library became available to the public, free of charge. David Siegel, director of the Earth Research Institute at UCSB, specializes in understanding the oceans through remote sensory data and imagery. His graduate student, Kyle Cavenaugh, sifted the Landsat 5 images to track fluctuations in kelp mass and match them with diver’s measurements of kelp, wave height measurements from buoys, numbers of sea urchins, concentrations of nitrogen in the water, and ranges of sea otters. With the satellite data, the group could extrapolate data from long term research sites to hundreds of miles of California coast, over an eight year span.

Reed said the giant kelp’s prodigious growth rate is handy for investigating disturbance as a force shaping ecosystems. In forests on land, major disturbances such as fire are infrequent. Storms reshape kelp forests yearly. “You can get a kelp forest wiped out, and within a couple of years, it’s back,” he said, so the timescale is practical. But the group is more interested in interpreting kelp forest dynamics than scoring a theoretical point.

“In terms of looking at future climate change, there are predictions for the ocean to get warmer,” Reed said, “but also for the ocean to get stormier. If it does, then that’s going to affect the kelp.” And everything that depends on it, from urchins to otters to humans.

• Photo: Plate 1 from Reed et al.’s Ecology paper. “Mounds of beached giant kelp (Macrocystis pyrifera) torn loose by large waves during a winter storm off Santa Barbara, California, USA.” Credit, Shane Anderson.
• Want to learn more about giant kelp? Listen to ESA’s Field Talk podcast with marine population geneticist Filipe Alberto, “Stepping stones of diversity: the Santa Barbara landscape and giant kelp genetics.”