If you’ve ever spent time by the sea, you’ve probably seen seaweed—either floating in the water, washed up on the beach, or draped over intertidal rocks. But what exactly are these strange organisms? And how do they differ from algae?

An ocean scene with waves crashing onto rocks that are covered with red, green and brown seaweeds.
Waves crashing onto seaweed-covered rocks at low tide in Kejimkujik National Park, Nova Scotia. Image: Amanda Savoie © Canadian Museum of Nature 

The short answer is that seaweeds are a kind of algae. If that is the case, what are algae?  

The term “algae” refers to a group of organisms defined by ecological traits. Algae are usually photosynthetic, meaning that they convert light from the sun into chemical energy—carbohydrates like sugar and starch. Algae are aquatic and can be found almost anywhere that there is water and sunlight. They occur in a wide range of sizes: some are tiny single cells, while others such as giant kelp can be more than 45 metres long. This group includes seaweeds, diatoms, dinoflagellates, phytoplankton, cyanobacteria, and many more photosynthetic organisms; it does not, however, include land plants, even those that live in lakes and oceans like seagrass or water lilies.

An ocean scene with blue sky and seaweed-covered rocks. A person is reaching into the water, looking for seaweed to collect.
Collecting seaweed almost always involves getting your hands and feet wet. Here, Amanda is looking for specimens to bring back to the Museum in the intertidal zone at Kejimkujik National Park, Nova Scotia. Image: Tobi Bailey © Canadian Museum of Nature

So, seaweeds are algae, but what makes them different from other types of algae? The answer lies in two important characteristics: they are macroscopic and marine. Seaweeds are big enough that they can be seen without a microscope. Additionally, they live in marine environments, such as oceans and estuaries, in the intertidal zone or shallow subtidal zone. The depth at which they can be found in the ocean is determined by how much sunlight penetrates the water—in clear tropical waters this can be surprisingly deep! 

A bright green, leafy seaweed in the middle of a tide pool. It is suspended in water and is surrounded by algae of many different colours: pink, red, brown, olive. There are also snails and barnacles around the algae.
Different types of seaweeds found in a tide pool near Point Lepreau, New Brunswick. Tide pools are formed when water is left behind in a depression as the tide goes out. It can host a wide variety of algal and animal life. The bright green seaweed in the centre is an alga known as Ulva lactuca, or sea lettuce. Image: Amanda Savoie © Canadian Museum of Nature 

Seaweeds are divided into three main groups based on colour: brown, green and red algae. A well-known example of the first group is the giant kelps that form massive and ecologically important forests off the western coast of Canada and the United States. They are also edible and can be used to make kelp lasagne! Another example of a delicious seaweed is nori, also known as laver. This is the familiar dark green wrapper used to make sushi, but you may be surprised to learn that nori is actually a red alga—the green colour is a result of it being dried and toasted. 

A woman is holding up thin, filmlike red seaweed near the ocean. She stands in shallow water against a misty backdrop.
This collection of Porphyra sp. (a red alga) from near Saint-Siméon, Québec, is a great example of how different living seaweeds can look compared to when it’s dried. This example is very closely related to the species used to make nori in Japan, but you can see how different the colour and texture are when it’s alive. Image: Nicolas Séguin © Canadian Museum of Nature 

Finally, why are algae—and seaweed—so important? One reason is that they produce up to 70% of the oxygen in the air we breathe. Another is that they are an essential component of aquatic food webs, providing a source of nutrients for invertebrates and fish—freshwater and marine. They are also an important food source for humans (e.g. nori, wakame, dulse, Irish moss). In fact, there are too many reasons to list in this blog post, but you can click here to learn more!  There’s an entire world of algae out there waiting to be discovered.