Episode Details

Kelp is not a plant. It is a brown macroalga in the kingdom Protista with no roots and no vascular plumbing: the holdfast only anchors it, the flexible stipe gives it structure, and each frond absorbs nutrients straight from the water while gas-filled pneumatocysts lift it toward the light. Fed by cold upwelling water, giant kelp grows 30 to 60 centimeters a day. Kelp forests and coral reefs cover one tenth of one percent of Earth's surface yet account for almost a full percent of global primary productivity. As a passive ecosystem engineer, kelp builds a layered city: a canopy of nudibranchs, an understory sheltering juvenile rockfish and perch, and holdfasts packed with crabs and brittle stars, while winter storms tear light gaps that let the lower layers grow.

Balance depends on predators. When fur hunters nearly wiped out Alaska's sea otters, sea urchins exploded, chewed through holdfasts, and left urchin barrens. Urchins stay passive while drift kelp keeps them fed and march only when it runs out, and in Southern California spiny lobsters and California sheephead stand in for otters. The Kelp Highway hypothesis argues that early people reached the Americas by boat along an unbroken kelp coast from Japan to Chile that offered consistent food and calm water. In the 2010s, marine heat waves known as the blob stratified the Pacific, starved the kelp, and wiped out 95 percent of Northern California's kelp ecosystems.

• A predictive model led a submarine team to deep tropical kelp forests near the Galapagos, once thought impossible.
• Kelp-derived alginic acid forms the gel that keeps toothpaste from separating.
• Macroalgae lock away roughly 200 million tons of carbon dioxide a year as blue carbon on the deep seafloor.
• Sea star wasting disease wiped out sunflower sea stars, the urchin hunters of Northern California.
• Divers now smash urchins, and Bodega Marine Laboratory, Humboldt State, and Ocean Rainforest near Santa Barbara grow kelp to replant.