seaweeds

seaweeds are the most familiar of all marine plants. They are large algae, non-flowering plants. With the exception of sargasso weed, seaweeds are restricted to shallow inshore waters where they can anchor themselves to the seabed. They are limited to depths that are shallow enough for sufficient sunlight to penetrate to support their photosynthesis. They are classified according to their pigmentation into reds, browns, greens, and blue-greens. On rocky shores around Europe ebbing low spring tides reveal a zonation of the brown algae, mostly wracks. The species most tolerant of desiccation flourish at the top of the shore but are replaced down the shore by a succession of more vigorously growing species that are less and less tolerant of being exposed at low tide. Lowest down the shore are the kelps or oarweeds (e.g. Laminaria) that are anchored to rocks by their rootlike holdfasts from which grow their stipes (stalks) and leaflike blades. They form dense forests in the sublittoral (the zone just below the lowest tide mark), which are home to a rich fauna of invertebrate animals and fishes. Off California and Chile giant kelps can grow to lengths of 100 metres (325 ft), each year renewing their blades, whereas the stipes are perennial, They are grazed by sea urchins, types of echinoderms, which in turn are eaten by sea otters (see marine mammals). When the sea otters have gone into decline because of over-exploitation or pollution, the urchins are unchecked and graze down the kelp forest so the numbers of other species inhabiting the region decline markedly. Molecular techniques have shown that clumps of seaweeds can be surprisingly old. Off the coast of Brittany quite modest clumps of bladder wrack (Ascophyllum nodosum) have been shown to be as much as 70 years old.

On coral reefs eutrophication, one of today's environmental issues, can lead to seaweeds overgrowing and destroying the coral by shading it. So what can be good for a rich diversity of species in one habitat can be bad for it in others.

Some seaweeds are used for food and they are extensively cultured in China. In Britain the red alga (Chrondrus crispus), known as carragheen or Irish moss, is traditionally used to make blancmanges and jellies. In south Wales another species, Porpyra umbilicalis, is eaten fried with bacon or rolled in oatmeal as laverbread. Alginin, extracted from kelp, is used as an emulsifier in the manufacture of ice cream.

See also marine biology; marine pharmaceuticals.

M. V. Angel