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...

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