The Kelp Brings All the Isopods to the Yard

Seagrass meadows are productive, but food from next door can be irresistible.

We often think big when considering how energy and nutrients move between ecosystems: a dead whale falling to the ocean floor, or salmon carcasses taken into the forest by a bevy of messy predators. Or kelp washing into a seagrass meadow? Okay, so that last example’s not famous, but that doesn’t mean it isn’t important.

“A lot of work has focused on resources moving from productive to nutrient-poor habitats,” says Carolyn Prentice, a scientist with the Hakai Institute and coauthor on a new study out in Frontiers in Plant Science. “But less is known about how nutrients move between relatively productive habitats, and that could have unknown importance to the ocean ecosystem.”

So scientists from the Hakai Institute and the University of Victoria waded in to investigate these lesser-studied subsidies on the British Columbia Central Coast by looking into just how connected seagrass meadows are to the kelp forests next door.

Seagrass meadows are considered to be productive habitats full of life in their own right. The seagrass itself isn’t particularly palatable to most herbivores, but the tiny diatoms and algae that grow on seagrass blades are appetizing to smaller grazers like snails, crabs, and other crustaceans. These grazers play an important role in seagrass meadows. Like goats munching weeds from a field, they gobble up other kinds of algae that grow on the blades of seagrass and hinder the marine plants’ potential growth. 

One such grazer is the eelgrass isopod. It’s a common crustacean roughly the size of a Mike and Ike candy that resembles a hulked-out lesser-legged centipede. This emerald-colored critter dines out in a seagrass meadow at a perfectly adequate underwater buffet, where it munches away half-heartedly at the somewhat bland, repetitive fare. All of a sudden, a mouthwatering smell wafts in as a new food truck pulls up outside—with delectable kelp on the menu. Does it bolt for the door, abandoning its usual ecosystem fare in favor of the new offerings?

Neighboring kelp forests also teem with life, including ample edible goodies that can drift into the seagrass meadow—and onto the grazers’ radar—on occasion. So given the choice, what does a grazer in the eelgrass choose—the dependable food that’s always there, or an episodic rush of grub from next door?

To find out, the researchers set up feeding trials where they gave eelgrass isopods and a species of kelp crab—two of the main grazers in eelgrass beds—a choice: bits of bull kelp, or their regularly available meals of other algae and seagrass. While kelp crabs weren’t all that picky, the choice was clear for eelgrass isopods. They ate kelp at 20 times the rate of the other local food sources.

“Kelp is known to be more palatable and nutrient rich,” says Prentice, “but it was quite striking how much more the isopods consumed kelp.”

It may seem like a small interaction—a minor blip in the grand scheme of wider ocean food webs. But it’s indicative of the complexity of these coastal ecosystems. 

“This kind of work is just emerging,” says Angeleen Olson, a scientist with the Hakai Institute and lead author on the new study. “Seagrass meadows are studied really well on their own, but it’s critical to know the connections and what we might be missing in these dynamic, important ecosystems.”

Not only are there countless connections within habitats but also between habitats, even when both are relatively productive. There are no hard barriers. Ocean habitats don’t exist in a vacuum. And eelgrass isopods won’t turn down a kelp buffet.