Can I say "seaweed" instead of "kelp"? Or are they totally different?

Native speaker’s answer
Rebecca
Good question! Seaweed is considered to be an umbrella term for many different species of underwater plants and algae. Meanwhile, kelp is the name for a subgroup of large, brown-colored seaweed, so kelp can be considered to be a type of seaweed. Thus, they are slightly different and should not be used interchangeably without knowing exactly what type of seaweed is being spoken about! Ex: Many people like to use kelp as fertilizer. Ex: Seaweed is becoming more popular as a health food. Among these, kelp is especially popular.