Kelp and seaweed are often used interchangeably, but they are not the same. Seaweed is a broad category, while kelp refers to a specific group within it. This distinction clarifies the diverse world of marine algae.
Understanding Seaweed
Seaweed is a broad term for thousands of species of macroscopic, multicellular marine algae. Unlike land plants, seaweeds lack true roots, stems, or leaves and a vascular system. They absorb nutrients directly from the water. Seaweeds anchor to surfaces like rocks using a holdfast, which provides attachment but not nutrient absorption.
These photosynthetic organisms convert sunlight, carbon dioxide, and water into sugars and oxygen. Seaweeds are classified into three main groups based on their primary pigments and color: green algae (Chlorophyta), red algae (Rhodophyta), and brown algae (Phaeophyceae). These pigmentations allow them to thrive at various depths with differing light penetration.
Understanding Kelp
Kelp refers specifically to a group of large brown algae in the order Laminariales. They are known for rapid growth, with some species growing up to two feet per day. This allows kelp to form dense underwater “forests” in cool, nutrient-rich waters.
Kelp has a distinct structure with specialized parts. A root-like holdfast anchors it to the seafloor, providing stability without absorbing nutrients. A stem-like stipe extends from the holdfast, supporting leaf-like blades that are the primary sites for photosynthesis. Many kelp species also have gas-filled bladders, called pneumatocysts, at the base of the blades, which help them float upright to access sunlight.
How They Relate
All kelp is seaweed, but not all seaweed is kelp. Seaweed is an overarching term encompassing thousands of marine algae species, including green, red, and brown varieties. Kelp is a specific subset within the brown algae group, possessing unique features that differentiate it from other seaweeds.
Just as all oaks are trees but not all trees are oaks, kelp is a type of seaweed, but many others, like red dulse or green sea lettuce, are not kelp. Kelp belongs to the order Laminariales, a classification excluding other brown algae such as rockweed. Thus, kelp is a specific kind of seaweed known for its large size and complex structure.
Why the Distinction Matters
The distinction between kelp and other seaweeds is meaningful due to their differing ecological roles, commercial applications, and nutritional profiles. Kelp forests, formed by dense kelp growths, are among Earth’s most productive and biodiverse ecosystems. They provide shelter, food, and nursery habitats for a wide array of marine organisms, including fish, invertebrates, and marine mammals. These forests also play a role in carbon sequestration, absorbing carbon dioxide from the ocean.
Commercially, kelp is harvested for food products, dietary supplements, and alginate extraction. Alginates are used as thickening agents in industries from food to pharmaceuticals. Other seaweeds also have culinary and industrial uses, like red algae providing agar and carrageenan, but kelp is noted for its unique composition. Nutritionally, brown seaweeds, including kelp, generally have higher iodine levels than red or green seaweeds, making the distinction relevant for diet.