The term seaweed refers to the large marine algae that grow almost exclusively in the shallow waters at the edge of the world's oceans. They provide home and food for many different sea animals, lend beauty to the underwater landscape, and are directly valuable to man as a food and industrial raw material.

Seaweeds are plants because they use the sun's energy to produce carbohydrates from carbon dioxide and water (this is called photosynthesis). They are simpler than the land plants mainly because they absorb the nutrients that they require from the surrounding water and have no need for roots or complex conducting tissues. Some large seaweeds such as the kelps have root-like parts called holdfasts, but these only serve to attach them to the rock. Most seaweeds have to be attached to something in order to survive, and only a few will grow while drifting loose in the sea.

Three groups of seaweeds are recognised, according to their pigments that absorb light of particular wavelengths and give them their characteristic colours of green, brown or red. Because they need light to survive, seaweeds are found only in the relatively shallow parts of the oceans, which means around the shores. Here they occur in a variety of shapes and sizes, from the large kelps (certain brown seaweeds) that form forests on temperate (cooler) coasts, to the hard "encrusting corallines" that look like pink icing, but are so important in building and cementing coral reefs in the tropics. Some seaweeds, especially many of the larger reds, are showy and attractive, while others may be small and inconspicuous, and grow in a low "turf" on the rocks.

Of the five to six thousand seaweed species that occur worldwide, about 720 have been recorded on the coast of South Africa. Some of these grow only in the warm waters of Natal, while others are restricted to the cool waters of the Western Cape. Many occur only on South African shores, and the high proportion of these endemic (occurs nowhere else) species makes our seaweed flora unique.


To call these marine plants "weeds" is of course incorrect, because they are essential in nature and directly valuable to humans. Together with phytoplankton (microscopic floating plants) seaweeds form the basis of the food chain in the sea. The myriad small animals that feed on seaweeds are in turn eaten by larger animals, and so on through to fish and perhaps to man. Seaweeds are also vital as a habitat for all sorts of other marine organisms, and a shore without these algae would truly be a desert.

Nowadays we use extracts from certain seaweeds as stabilizers, gelling agents or emulsifiers, in thousands of everyday products from pet food to dental moulds. Cloth dyes, toothpaste, salad dressings, flavoured milks, cosmetics, welding rods, and pizza toppings are just a few of these. Agar, the jelly found in some red seaweeds, is irreplaceable as a medium on which to culture fungi and bacteria for medical testing and research in microbiology.

Whereas the seaweed industry in the West is based mainly on these chemicals, in the East there are vast farms where seaweeds are grown for food. There "Nori" (the red seaweed Porphyra) is worth one and a half billion US dollars annualy - more than three times the entire South African fishing industry. This crop and others like the brown seaweeds called "Wakame" and "Kombu" are regarded as delicacies and sold for high prices.
It is a pity that Westerners have so little interest in eating seaweeds, for so many are highly nutritious and particularly rich in some vitamins and trace elements. In Chile and Fiji (in the south pacific), coastal dwellers include many local seaweeds in their diet and there is a thriving trade in these "sea vegetables".


Locally some red seaweeds are collected in the Eastern Cape (several Gelidium species) and at Saldana Bay (Gracilaria) and exported for the production of agar. Large beds of kelp ("sea bamboo") grow on Western Cape shores, and from these many plants are lost during storms, later to wash up on beaches. Much of this material is collected and either exported for the production of a seaweed concentrate that has a good international market in agriculture. The local seaweed industry is worth about R15 million annually, and has the potential to expand, both by greater use of existing natural resources and by farming of some of these marine plants.

The conservation and utilisation of South Africa's marine resources are controlled by the Department of Environmental Affairs, according to the Sea Fishery Act of 1988 (presently undergoing revision). Seaweed resources are no exception, and are managed and protected according to the results of research carried out by the Sea Fisheries Research Institute and contracted research organisations. So far, commercial activities have had negligible ecological effects, but monitoring is continually carried out, and new activities are only allowed if research shows that the environment will not be harmed.

At present, marine life is being studied more than it ever was in the past, and with our increased understanding of life in the sea should come a greater appreciation of the importance of seaweeds in nature and to man.


The original text is by Rob Anderson of the Sea Fisheries Research Institute. Most of the slides and photographs for this version are by Derek Keats with additional photographs and editorial input by Gavin Maneveldt.



a virtual slide show introducing algae presented by the Botany Dept.

World of Algae

an introduction and educational tour of this unusual mixture of organisms

Non-geniculate Coralline Algae

Our research website on coralline algae


information resources from the University of Galway, Ireland

Vegetable Aquatics

seaweed recipes

Developed & maintained by: Jocelyn Collins
Last Updated: Thursday, February 01, 2001