In A Nutshell
Seaweed is a very, very broad term that is used to describe the many marine plants and algae that live in the world’s waters. Kelp is actually a subgroup of seaweed and is also the largest form of seaweed. Seaweeds range in size from the microscopic to the massive, while kelp are so large and complex that they form massive underwater forests.
The Whole Bushel
Seaweeds do have some very specific characteristics. Whether they’re microscopic organisms of massive of structures many, many feet long, seaweeds don’t have the same internal structures that land-dwelling plants have. Seaweeds may or may not have roots, but when they do these roots (called “holdfasts”) are solely to anchor the plant to an underwater surface rather than to absorb any nutrients. Seaweeds absorb their nutrients through their leaf-like tissues. Those without holdfasts simply float; a single type of seaweed or kelp can spread around the world, carried on tides and on the bottoms of ships; there are literally hundreds and hundreds of different types of seaweed.
Kelp is one of those types. Among the largest types of seaweed, what makes them unique is their tendency to grow in large, thick kelp forests that can be home to hundreds of different species of marine life. Some small seaweeds aren’t anchored to anything, and live suspended and floating in the water; but kelp anchors itself in one spot, forming these massive underwater forests that create ecosystems all their own. Kelp forests are home—and food—to countless species of fish and marine animals like seals and sea otters, providing them a place to hide, sleep and feed all in one. A kelp forest tends to support a wide variety of smaller forms of seaweed and algae within it.
Seaweed is broken down into three different types—red, green, and brown; their different colors reflect the different type of light they will readily absorb for photosynthesis, which in turns dictates how deep or how close to the surface they generally grow. Officially, kelp is considered a brown seaweed, despite its variance between browns, reddish-greens, and purplish-brown.
Seaweed can grow in any type of marine environment, from the largest saltwater oceans to the smallest freshwater lakes and the narrowest of rivers. Kelp is found in saltwater, usually along rocky coastlines that provide a place for the kelp strands’ holdfasts to anchor itself. Because they are so large and require more nutrients than smaller seaweeds, kelp require nutrient-rich waters with at least a moderate amount of movement to ensure the continuous supply of nutrients.
There are different varieties of kelp. Bull kelp grows along coastal fronts around the world, and in some places a single strand of bull kelp can live for 10 years and reach a length of almost 45 meters (150 ft). Bladder kelp can be even larger, with strands reaching 50 meters (165 ft) in length and weighing hundreds of pounds. Other types of kelp tend to be much smaller—common kelp and Asian kelp average between 1–2 meters (roughly 3–6 ft).
Many types of seaweeds, including kelp, have long been a dietary staple. They have been a major food source in Japan, and more recently their true nutritional value has become more well-known. Types of seaweed have been found to contain cancer-fighting and anti-microbial agents, and can also have anti-inflammatory properties. Some types have practical uses, too; strands of kelp are so strong and so durable that they have been traditionally used as materials to make everything from bags for food storage to canoes.
Show Me The Proof
NOAA: What is seaweed?
Capital Regional District: What is kelp?
Encyclopedia of New Zealand: Story—Seaweed