Are Kelp and Seaweed the Same Thing?

One of the most common questions we get is something along the lines of, “what’s the difference between kelp and seaweed? Aren’t they the same thing?”  This is a totally understandable question—these words are often used interchangeably, and we’re even guilty of doing it ourselves! Technically, however, there is a difference.

Taxonomically speaking, kelp is a type of seaweed — so all kelp is seaweed, but not all seaweed is kelp. But more on that later. First, how do you tell what’s kelp and what isn’t when you’re out there on the coast looking at seaweed? It turns out there is no foolproof way to do this, but there are a lot of clues that something is probably kelp:

  • Generally speaking, the larger seaweeds that you find washed up on the beach will be kelp. Especially along the central California coast, if you find an unbroken piece longer than your arm, it’s probably a kelp (more specifically, it’s likely Bull Kelp, Feather Boa Kelp, or Giant Kelp).
  • Another thing to look for is the presence of air bladders, or pneumatocysts. These air bladders help keep the kelp blades afloat, ensuring that the plant always grows more or less vertically in the ocean, and allowing it to access the sunlight at the ocean’s surface. Some non-kelp seaweeds have them (the most notable in California being Bladderwrack), but for the most part pneumatocysts mean kelp!

 Pneumatocysts

Pneumatocysts

  • The last clue is how and where it’s growing. Many seaweeds tend to grow on the rocks in the intertidal zone, which is the relatively narrow area around the shoreline and into the shallows. However, if you venture a little farther out in the open water, you may see lush forests of marine plants housing entire ecosystems, with their canopies taking up a large amount of surface area. These forests are likely to be kelp, with each plant stretching all the way from the seafloor to the ocean surface!

Now that you know how to distinguish kelp from other seaweeds, let’s talk taxonomy. The first thing to note is that not all marine plants are actually seaweed.  For example, the marine grasses (surf grass, eel grass, etc.) that you may see in tide pools or along the rocks aren’t seaweed.

 Eelgrass (seagrass, not seaweed)

Eelgrass (seagrass, not seaweed)

“Seaweed” is an umbrella term for any macroscopic marine algae — that is, algae that grow in seawater and can be seen with the naked eye. Seaweeds are classified into three main subgroups: red algae, green algae, and brown algae.

  • Red algae like the bacon-tasting Dulse
 Dulse (red algae)

Dulse (red algae)

  • Green algae like sea lettuce
 Sea lettuce (green algae)

Sea lettuce (green algae)

  • And brown algae like kelp

But be careful—not every brown alga is kelp! “Kelp” only refers to brown algae that are classified under the order Laminariales.  So take something like macrocystis: It’s a macroscopic marine algae, making it seaweed; it is in the brown algae category; and it is of the order Laminariales, making it a kelp.  And, in fact, it is—its common name is Giant Kelp!

 Giant kelp (brown algae)

Giant kelp (brown algae)

On the other hand, ascophyllum nodosum, while also a macroscopic brown marine algae, is a seaweed but not a kelp, because it’s in the order Fucales. Taxonomy matters here: whether or not something is a kelp really depends on its scientific order! And, of course, anything that is a green or red algae is automatically not a kelp, even though they are seaweed.

Ocean SourceSeaweed Facts