A group of sea otters laze at the edge of Elkhorn Slough. They float on their backs in the steel-gray water, paws folded against their chests, gazing at the small boat steered by ecologist Brent Hughes of the University of California, Santa Cruz. Hughes has documented a profound shift in the slough’s ecology, triggered by the otters.
Sea otters were nearly driven to extinction by fur hunters in the nineteenth century and were gone from Elkhorn Slough for decades. In 1984, when the first sea otters recolonized, Elkhorn Slough’s once bountiful eelgrass beds had dwindled to a few small, scattered patches. Now, more than 30 years after the sea otters’ return, expanding eelgrass beds grow lush beneath the water’s surface, the dense leaves sheltering juvenile fish and feeding an array of invertebrate grazers.
The slough, on the central California coast, is one of the most severely polluted estuaries on the planet. Artificial fertilizer applied to 2.69 million acres of farmland in the neighboring Salinas Valley runs into its waters. The excess nutrients lead to eutrophication: Masses of algae bloom, die, and sink to the bottom, where bacteria devour them, draining the water of oxygen in the process. The heavy nutrient load also fuels the growth of the epiphytic algae that thrive on the surface of eelgrass leaves, blocking the sunlight the grass needs and smothering whole beds.
