Different Types of Seaweed

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Seaweed, more scientifically known as marine algae, can be found throughout the oceans of the world. There are more than 9,000 species of algae, of which seaweeds contribute a large part. Each species has its own unique characteristics but they are generally divided into three main categories: green, brown and red algae. These seaweed types can be found growing at different depths. The effects of light conditions, and in turn photosynthesis, play an important role in their development.


Green seaweeds are generally found in the intertidal zone, the area between the high and low water mark. Green seaweeds are very reliant on photosynthesis, needing plenty of light in order to grow. This makes shallow water the perfect environment for green seaweed. This is the least species-rich of the three seaweed types. Common types of green seaweed include the sea lettuce, gut weed and sea rimu. These are the types of seaweed that beachgoers will often find washed up on the sand.


Brown seaweed grows at greater depths than green seaweed but not as deep as red seaweed. These seaweeds can normally be found growing at depths of 50 to 75 feet. Brown seaweeds are generally larger than the greens, with the largest varieties commonly known as kelps. Approximately 2,000 species of brown algae have been recorded, a large majority of which are marine algae, or seaweeds. Neptune's Necklace is one of the most recognisable brown seaweeds, identifiable by the series of water-filled bladders or bubbles that help it to survive when out of the water. Brown seaweeds are found in both temperate and polar regions.


The red seaweed category is the largest and most species-rich of the three seaweed groups. Red seaweeds grow at deeper levels than other seaweed types; they are adapted to photosynthesise in deep waters where light levels are significantly lower. In clear waters, red seaweed can be found growing at depths of more than 650 feet. However, they can also survive and flourish in shallower waters and rock pools if sufficient shade is available.

Usage Classification

Seaweeds are sometimes classified by their use. Such classifications include food, fertiliser, medicinal, commercial and industrial categories. Seaweed is perhaps best known for its edible varieties, particularly in Japanese sushi dishes. Agar, a food additive often used in confectionery and desserts, is also extracted from seaweed. For agricultural purposes, seaweed is an effective organic fertiliser. It is also an ingredient in commercial beauty products such as moisturising creams and lotions.

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