Today, much of the land surrounding Elkhorn Slough near Monterey, California, is managed to reduce nitrogen input. Nevertheless, there is still 150 times more nitrogen in the slough than a century ago. Because of those levels, Elkhorn Slough mudflats are often covered in a thick, green sludge of nitrogen-loving algae. Algae could also be growing on eelgrass, blocking its access to light. Instead, the slough harbors beds of healthy, dark green eelgrass. So what’s the secret? It turns out to be everyone’s favorite keystone predator, the California sea otter.

Brent Hughes from the University of California, Santa Cruz was first to make the connection between a charismatic carnivore and an unassuming perennial marine plant. He looked at 50 years of historical data that compared areas with and without otters and discovered that areas with larger populations of sea otters had healthier beds of eelgrass (Zostera marina).

The reason: Otters love to eat crabs, which love to eat grazing invertebrates such as sea slugs, which love to eat the algae that grow on eelgrass.

This top-down management of species is known as a trophic cascade. A top predator alters the abundance of its prey, which in turn reduces predation on the next trophic level down the food chain.

You can see the difference between an area with otters and one without when you look at Elkhorn Slough compared to Tomales Bay. Even though Tomales Bay has lower levels of nutrients (such as nitrogen from fertilizer), it has significantly fewer otters. The result is brown eelgrass covered in algae. Without as many otters preying on the crabs, the crustaceans are four times as abundant and 30% bigger than the crabs in Elkhorn Slough. This leaves very few sea slugs to keep the eelgrass grazed clean of algae.

Where to See the Sea Otters

For the best view of sea otters, visit the Moss Landing Wildlife Area (on the east side of Highway 1) or Moss Landing State Beach (on the west side of Highway 1, just off Jetty Road). At the beach on the same side as the harbor, you can often see a raft of male otters floating together out on the open water.

Elkhorn Slough – A Protected Place for Otters and More

Elkhorn Slough is one of the few coastal wetlands remaining in the state, second in size to San Francisco Bay. At nearly 7 miles long, this estuarine waterway shelters an abundance of marine life, providing essential habitat for over 700 species, including aquatic mammals, birds, fish, invertebrates, algae, and plants. It is also part of California’s statewide program to establish Marine Protected Areas (MPAs). MPAs are a lot like our landlocked state parks except that they extend many miles off our coastline. These “underwater parks” are established in areas that are biologically diverse (lots of different marine species), culturally rich (think sunken ships like the S.S. Palo Alto and buried anthropological treasure), and economically important (fishing and tourism are a big deal to these coastal communities).

Elkhorn Slough is home to three marine protected areas—Elkhorn Slough State Marine Conservation Area (SMCA), Elkhorn Slough State Marine Reserve (SMR), and the Moro Cojo Slough SMR. There are several different types of MPAs, and each one has its own set of rules. See the California Department of Fish and Wildlife website for details.

Take the Self-Guided Mobile Tour

This piece is part of the Santa Cruz Marine Protected Areas Beaches Tour made possible by the Santa Cruz Collaborative with support from the California Marine Sanctuary Foundation and the Resources Legacy Fund. Download the free app with many tours of the Santa Cruz area and beyond.

1. Sources

   • Sea Otters: Your Defence Against the Algal Apocalypse. Ed Young. National Geographic. August 26, 2013.
   • Sea otters promote recovery of seagrass beds. Tim Stephens. August 26, 2013. University of California Santa Cruz Newscenter Website.
   • Recovery of a top predator mediates negative eutrophic effects on seagrass Brent B. Hughes, Ron Eby, Eric Van Dyke, M. Tim Tinker, Corina I. Marks, Kenneth S. Johnson, and Kerstin WassonProc Natl Acad Sci, September 17, 2013. Issue 38:15313–15318.