seaweed

thumb|upright|Ascophyllum nodosum exposed to the sun in [[Nova Scotia, Canada|alt=Photo of seaweed with small swollen areas at the end of each frond]] thumb|Dead man's fingers (Codium fragile) off the [[Massachusetts coast in the United States|alt=Photo of detached seaweed frond lying on sand]] right|thumb|The top of a kelp forest in [[Otago, New Zealand|alt=Photo of seaweed with the tip floating at the surface]] Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae. Seaweed species such as kelps provide essential nursery habitat for fisheries and other marine species and thus protect food sources; other species, such as planktonic algae, play a vital role in capturing carbon and producing at least 50% of Earth's oxygen.

Natural seaweed ecosystems are sometimes under threat from human activity. For example, mechanical dredging of kelp destroys the resource and dependent fisheries. Other forces also threaten some seaweed ecosystems; for example, a wasting disease in predators of purple urchins has led to an urchin population surge which has destroyed large kelp forest regions off the coast of California.