Kelp and Seaweed


From the tiniest of bacteria to the massive blue whale, the world’s oceans and freshwater support a tremendous variety of life. Often, a beachcomber will find rubbery plants washed up on the shoreline. These exotic-looking plants are seaweed. A dive below the surface of coastal waters in some areas of the world, such as California, reveals a world of towering plants that sway gently in the ocean current. These giants are one form of seaweed called kelp.

Kelp make up only about 10% of all the known seaweed species. The many varieties of seaweed present in the world’s fresh- and saltwater provide a habitat and even a food source for creatures. Humans benefit from seaweed as well. For thousands of years in Far East countries like Japan, seaweed has been an important part of the diet, in the form of soup stock, seasoning, and as an integral part of sushi. In addition, seaweed is useful in the laboratory. The artificial growth surfaces used to raise bacteria rely on a seaweed component as a thickening agent, similar to that found in gelatin.

Characteristics of seaweed

The leafy-looking seaweed that grows in ocean waters is a type of algae. Other forms of seaweed look grass-like or feathery. Algae are plants; that is, they contain the chemical chlorophyll that converts energy from the Sun into food substances that the plant uses to grow. Algae range in size from microscopic single cells (the fundamental unit of all living things) to huge numbers of cells assembled together to form a much bigger organism. Seaweed is the large collection of algae cells, or macroalgae.

The many types of seaweed come in all shapes, sizes, and colors. Depending on the species, shapes range from the mighty tree-like kelp to the smaller and more delicate leafy or ribbonlike seaweed varieties. Most types of seaweed are found in shallow water, from just a foot or so to depths of 100 to 200 feet below the water’s surface, as it needs sunlight for growth.

Also, most seaweed is found where there are rocks, as the seaweed clings onto the rock at one end using a structure called a holdfast. Some seaweed can attach to the sandy ocean bottom using a specialized structure that appears similar to the roots of plants that grow on land.

Like plants, seaweed can convert the energy from sunlight into the compounds needed for its growth. In order words, seaweed is a photosynthetic organism. Some seaweed contains the light-absorbing compound chorophyll, which gives seaweed its green color. Other species of seaweed contain different light-absorbing chemicals that are colored red, brown, blue, or gold.

While similar to land-bound plants in its light-absorbing ability, seaweed is distinct from its land cousins in other ways. It can contain a holdfast, a part that anchors it to the seafloor. Many types of seaweed also have hollow, gas-filled structures called floats that help buoy the leaves up nearer to the sunlight.

The three categories of seaweed

red algae
red algae
Seaweed is often grouped into three categories based on its color. These groups are the brown, green, and red algae. Brown algae range in size from forms that are a few inches (centimeters) in size to the giant kelps that can reach over 150 feet (46 meters) long.

This type of seaweed lives only in salt water, and cannot grow in waters where the temperature varies much. Brown algae is found in waters that stay cold all year, such as the coastal waters of Alaska, or in tropical waters that stay warm all the time.

The chemicals that make up brown seaweed are useful in the manufacturing of cosmetics and some medicines. For example, kelp can be used in medicine to treat high blood pressure, thyroid problems, and even arthritis. Seaweed is also a rich source of such minerals as iodine, zinc, copper, sodium, calcium, and magnesium, which are important in a healthy diet.

Additionally, brown seaweed has been an important part of the Japanese diet for centuries; it is used in soups, as an additive to change the taste of other foods, and as a wrap for the raw fish and rice combinations known as sushi.

The red algae group dominates in ecosystems such as coastal regions of California, where they can comprise 70% of all the seaweed species present. The group contains about 3,800 different species of seaweed. Because they can absorb even tiny amounts of sunlight, red algae can live deeper in the water than other kinds of seaweed. Many red algae can live at depths of 150 to 200 feet (46 to 61 meters) below the ocean surface, and some species have been found growing even 600 feet (183 meters) below the surface of the ocean.

At these depths, the ocean waters are calmer, and the red algae that live there tend to have a more delicate structure. These algae are more easily broken than seaweed that grows in the churning waters nearer to the surface. A component of red algae is also used to make the solid food (agar) that is used in laboratories to grow many types of bacteria.

Green seaweed can be found in both freshwater and the ocean. These types of seaweed feed on water that contains chemicals such as nitrogen and phosphorus that flow into the water from farmer’s fields, or that are in sewage. When some types of green algae are present in high concentrations, this may indicate that the water is polluted with too many of these chemicals.



Kelp are a type of brown seaweed that often appear as big leaves swaying in the underwater current. Some types of kelp can grow to be almost 100 feet (30 meters) long, and can form an underwater forest. Kelp are important to life in the sea.

The thick masses of kelp that grow off the coasts of New York, California, Australia, the Arctic, and the Antarctic are home to a variety of creatures including lobsters, snails, octopuses, seahorses, starfish, fish, and seals. These sea creatures use the seaweed forests as a protective haven as well as a source of food. Thus, kelp is important in establishing and sustaining the complex ecosystems that can form.

A kelp plant can grow to be dozens of feet (meters) long and grow quickly. It is anchored to the bottom of fairly shallow waters by means of a holdfast and reaches up toward the surface, forming an underwater forest. The leaf-like structures (fronds) that are near the surface have pockets of air built into them, which act as balloons to hold the leaves nearer to the surface where they can capture the Sun’s energy.

The material that makes up kelp is also part of peoples’ everyday lives. Kelp helps thicken ice cream and jelly, and provides the smooth texture present in some frozen drinks.