What Is Seaweed?

Sunlight through kelp forest
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'Seaweed' is a general term used to describe plants and algae that grow in waterways such as the ocean, and rivers, lakes, and streams.

Learn basic facts about seaweed, including how it is classified, what it looks like, where it is found, and why it is useful.

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A Common Name

Seaweed at Shore
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Seaweed isn't used to describe a certain species - it's a common name for a variety of types of plants and plant-like creatures, from tiny phytoplankton to enormous giant kelp. Some seaweeds are true, flowering plants (an example of these are seagrasses). Some aren't plants at all but are algae, which are simple, chloroplast-containing organisms that don't have roots or leaves. Like plants, algae do photosynthesis, which produces oxygen.

The algae shown here have pneumatocysts, which are gas-filled floats that allow the blades of the seaweed to float towards the surface. Why is this important? This way the algae can reach the sunlight, which is crucial for photosynthesis. 

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Assorted Seaweed
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Algae are classified into three groups: red, brown, and green algae.  While some algae have root-like structures called holdfasts, algae do not have true roots or leaves. Like plants, they do photosynthesis, but unlike plants, they are single-celled. These single cells may exist individually or in colonies. Initially, algae were classified in the plant kingdom. Classification of algae is still under debate. Algae are often classified as protists, eukaryotic organisms that have cells with a nucleus, but other algae are classified in different kingdoms. An example is blue-green algae, which are classified as bacteria in the Kingdom Monera.

Phytoplankton are tiny algae that float in the water column. These organisms lie at the foundation of the ocean food web. Not only do they produce oxygen through photosynthesis, but they provide food for countless species of other marine life. Diatoms, which are yellow-green algae, are an example of phytoplankton. These provide a food source for zooplanktonbivalves (e.g., clams) and other species. 

Plants are multi-cellular organisms in the kingdom Plantae. Plants have cells that are differentiated into roots, trunks/stems and leaves. They are vascular organisms that are capable of moving fluids throughout the plant. Examples of marine plants include seagrasses (sometimes referred to as seaweeds) and mangroves.

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Dugong and Cleaner Fish on Seagrass
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Seagrasses like those shown here are flowering plants, called angiosperms. They live in marine or brackish environments worldwide. Seagrasses are also commonly called seaweeds. The word seagrass is a general term for about 50 species of true seagrass plants.

Seagrasses need lots of light, so they are found at relatively shallow depths. Here they provide food for animals such as the dugong, shown here, along with shelter for animals such as fish and invertebrates.

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Sun Shining Through Kelp Forest
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Seaweeds are found where there is enough light for them to grow - this is in the euphotic zone, which is within the first 656 feet (200 meters) of water. 

Phytoplankton float in many areas, including the open ocean. Some seaweeds, like kelp, anchor to rocks or other structures using a holdfast, which is a root-like structure that "

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Bowl of Seaweed
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Despite the bad connotation that comes from the term 'weed,' seaweeds provide a lot of benefits for wildlife and people. Seaweeds provide food and shelter for marine organisms and food for people (have you had nori on your sushi or in a soup or salad?). Some seaweeds even provide a large portion of the oxygen we breathe, through photosynthesis.

Seaweeds are also used for medicine, and even to make biofuels.

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Sea Otters in Seaweed
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Seaweeds can even help polar bears. During the process of photosynthesis, algae and plants take up carbon dioxide. This absorption means that less carbon dioxide gets released into the atmosphere, which lessens the potential impacts of global warming (although sadly, the ocean may have reached its capacity to absorb carbon dioxide).

Seaweeds play a crucial role in maintaining the health of an ecosystem. An example of this was shown in the Pacific Ocean, where sea otters control the populations of sea urchins. The otters live in kelp forests. If sea otter populations decline, urchins flourish and the urchins eat the kelp. The loss of kelp not only impacts the availability of food and shelter for a variety of organisms but impacts our climate. Kelp absorbs carbon dioxide from the atmosphere during photosynthesis. A 2012 study found that the presence of sea otters allowed kelp to remove much more carbon from the atmosphere than scientists originally thought. 

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Red Tides

red tide

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Seaweeds can also have adverse impacts on humans and wildlife. Sometimes, environmental conditions create harmful algal blooms (also known as red tides), which can cause illness in people and wildlife. 

'Red tides' aren't always red, which is why they're more scientifically known as harmful algal blooms. These are caused by a profusion of dinoflagellates, which are a type of phytoplankton. One effect of red tides can be paralytic shellfish poisoning in humans. Animals that eat red tide-impacted organisms can also become ill as effects cascade up the food chain. 


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Kennedy, Jennifer. "What Is Seaweed?" ThoughtCo, Sep. 9, 2021, thoughtco.com/what-is-seaweed-2291912. Kennedy, Jennifer. (2021, September 9). What Is Seaweed? Retrieved from https://www.thoughtco.com/what-is-seaweed-2291912 Kennedy, Jennifer. "What Is Seaweed?" ThoughtCo. https://www.thoughtco.com/what-is-seaweed-2291912 (accessed June 6, 2023).