Seaweeds: an idiosyncratic account

What are seaweeds?

A bit of background information for those of you who need to understand the what/why/where & how of things:

Seaweeds are algae that live in the sea or in brackish water and are called ‘benthic marine algae’, which means ‘attached algae that live in the sea’. They come in three basic colours: red, green, and brown.  Red and brown algae are almost exclusively marine, whilst green algae are also common in freshwater (rivers and lakes), but are also terrestrial, living in damp places such as rocks, walls and tree bark. Many of these algae are very ancient and are far more complex organisms than generally realised. Many seaweeds have specialised tissues and growth forms, with many of them producing sex pheromones with many different types of sex organs.

There is growing evidence of root-like structures in some wracks that reach deep into rocks. Generally, seaweeds and many algae have holdfasts or basal structures that ‘hold fast’ to the rock. Seaweeds produce adhesives to enable quite small holdfasts to keep  quite large plants attached to rocks.  A small holdfast (about 1 cm across) can attach an Ascophyllum nodosum (Egg Wrack) clump about 2 m in length.

There are about 10,000 species of seaweeds, of which 6,000 are red algae (Rhodophyta). There are about 1800 species of brown algae, mostly marine, generally larger with more species found in colder waters. Virtually all the seaweed biomass worldwide comes from a relatively small number of species in the orders Laminariales and Fucales. Laminaria and Saccharina species are found on rocky shores at low tide and in the subtidal zone to depths of 8-30 metres in the N. Atlantic and N. Pacific; some species occur at depths of up to 120 metres, but require extraordinary water clarity to survive.


Drift seaweed accumulates in lines left behind by the receding tide. As the height of the high tide recedes during the spring-neap cycle (which happens twice a month) successive lines are left behind. The highest line is left behind by the highest spring tide. You can work out what the current phase of the spring-neap cycle is by noticing the state and dryness of these lines of drift seaweed.  As the stranded seaweed dries out it is blown up the shoreline and acts as fertiliser for coastal plants.

Seaweed lines are active habitats, with large numbers of sand-hoppers and seaweed flies (Coelopidae) and many birds, such as Turnstones, feed on these during low tides. Numbers of seaweed flies are said to exceed a billion per kilometer. Two species of seaweed flies are found in north-eastern Europe (Coelopa frigida and C. pilipes)

On wave-exposed western Atlantic coasts much of the drift lines are made of fucoid algae and/or kelps. In more sheltered localities green algae may be more evident, particularly where there’s a lot of excess nutrients in the water, either from agricultural runoff or from sewage outfalls.  In some places, heavy casts of seaweed accumulate, as happens locally at Worthing. One explanation for the name of Worthing is based on this natural annual phenomenon. Seaweed beds off nearby Bognor Regis are ripped up by summer storms and prevailing currents deposit it on the beach. A rich source of nitrates, decaying seaweed, it is suggested, was sought by farmers from the surrounding area, thus giving the town its name, from Wort (weed) -inge (people).

Kelps are  known as oarweed in Ireland and Scotland, and are eaten as Kombu in Japan and Haidai in China, and in a number of south-east Asian countries. The most eaten species, Saccharina japonica and Undaria pinntifida, are now cultivated in China, Japan and Korea.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.