Elkhorn Slough Sea Otters Offset Agricultural Runoff

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.

Otters not only keep kelp forests healthy, they also contribute to healthy eelgrass habitat. Photo: Bart Selby
Otters help keep kelp forests healthy and also contribute to healthy eelgrass habitat. Photo: Bart Selby

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).

Everyone’s favorite keystone predator, the California sea otter. Photo: Bart Selby
Everyone’s favorite keystone predator, the California sea otter. Photo: Bart Selby

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 pup will learn to eat crabs from its mom, indirectly protecting eelgrass habitat. Photo: Bart Selby
This pup will learn to eat crabs from its mom, indirectly protecting eelgrass habitat. Photo: Bart Selby

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.

The trophic cascade at Elkhorn Slough. The top predator is the sea otter (E. lutris), the mesopredators are crabs (Cancer spp. and Pugettia producta), the epiphyte mesograzers are primarily an isopod (I. resecata) and a sea slug (P. taylori), and algal epiphyte competitors of eelgrass primarily consist of chain-forming diatoms, and the red alga Smithora naiadum. Solid arrows indicate direct effects, dashed arrows indicate indirect effects, and the plus and minus symbols indicate positive and/or negative effects on trophic guilds and eelgrass condition. C, competitive interaction; T, trophic interaction. Image:  Original artwork by A. C. Hughes and used with permission of the Proceedings of the National Academy of Science 2013 (see full citation below)
The trophic cascade at Elkhorn Slough. The top predator is the sea otter (E. lutris), the mesopredators are crabs (Cancer spp. and Pugettia producta), the epiphyte mesograzers are primarily an isopod (I. resecata) and a sea slug (P. taylori), and algal epiphyte competitors of eelgrass primarily consist of chain-forming diatoms, and the red alga Smithora naiadum. Solid arrows indicate direct effects, dashed arrows indicate indirect effects, and the plus and minus symbols indicate positive and/or negative effects on trophic guilds and eelgrass condition. C, competitive interaction; T, trophic interaction. Image: Original artwork by A. C. Hughes and used with permission of the Proceedings of the National Academy of Science 2013 (see full citation below)

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.

Otters hard at work, keeping Elkhorn Slough’s ecosystem balanced.
Otters hard at work, keeping Elkhorn Slough’s ecosystem balanced. Photo: Bart Selby

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.

 Keep any eye out for otters when you visit the slough. Photo: Bart Selby
Keep any eye out for otters when you visit the slough. Photo: Bart Selby

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).

The MPAs in Santa Cruz County. Image: California Marine Sanctuary Foundation
The MPAs in Santa Cruz County. Image: California Marine Sanctuary Foundation

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.

Image: Courtesy the Ocean Conservancy via the California Marine Sanctuary Foundation.
Image: Courtesy the Ocean Conservancy via the California Marine Sanctuary Foundation.

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.

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Go to Mobile Ranger Guides in the Apple App Store
Go to Mobile Ranger Guides in the Google Play Store
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About The Author

Molly Lautamo is a content strategist and writer in Santa Cruz, California. She loves exploring and researching her surroundings and then writing about her discoveries to inspire others to get out and explore too. You can check out more of Molly's writing at mollylautamo.com.

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