Plants of the Sea

Forest and Bird, Issue 309, 1 August 2003, Page 40

Plants of the Sea

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ANN GRAEME

a storm — you will see lacy red fronds, glistening brown straps and the occasional handkerchief of green, torn from their rocky reefs. Seaweeds are sea plants, large algae which first appeared in the oceans more than a billion years ago. Algae range from minute single cells to large seaweeds like the bull kelp. However they do not have roots, stems and leaves. Instead a ‘holdfast’ like a root, anchors the at the seaweeds on the beach after

seaweed to a rock. Unlike a root, it does not need to take in water, for seaweeds can absorb water and minerals from the salt water they are bathed in. They have no need for the water-conducting tissues of roots and stems. Most seaweeds are brown or red. Green algae grow mainly in fresh water rather than the sea. Green is the colour of chlorophyll, that miraculous pigment which can convert light energy into chemical energy so that plants can create sugars and generate

oxygen from water and carbon dioxide. Green is the colour of the land plants and of the algae of freshwater ponds and lakes. Why then are most seaweeds brown or red? The answer lies in the nature of visible light. Sunlight reaching the earth is made up of all the colours of the rainbow. Chlorophyll itself looks green because it reflects green light. It doesn’t reflect longwavelength red or short-wavelength blue, but absorbs them, providing the energy that drives the chemical process of photosynthesis. As sunlight penetrates water, first the red, then orange and yellow rays are filtered out. Deep down the light is blue-green; a wavelength which green chlorophyll cannot absorb. Now the brown and red seaweeds come into their own. They too have chlorophyll but it is masked by other pigments, brown and orange, reddish and blue. None of these pigments takes part directly in photosynthesis but each can absorb the energy of the only available green and blue light. In clear water and with the help of these ancillary pigments, seaweeds can grow deep in the ocean, the record being a red coralline algae growing 250 metres below the surface.

So seaweeds of different colours grow at different depths or zones on the rocky shores to make the most of the available light. Light penetration falls off quickly in the turbulence where the tides rise and fall. Water motion shapes their form too, for life in this zone is tough. Pounded by the waves when the tide is low, and wrenched by the surge when the tide is high, the brown seaweeds have to be robust. They make a forest, clamped to the rocks by

gnarled holdfasts. Their leathery stalks and flattened fronds bend and flex as the waves drag them to and fro. The biggest of all, the bull kelp Durvillea, look like fallen trees when their trunks and glistening fronds are exposed by the low tide. Deeper down, beneath the turbulent

waves and the tumbling forests of kelp, the water is much calmer. This is where the red seaweeds grow. Delicate, like lace, their thin and divided fronds help them make the most of the meagre light. In still, clear water, short-wavelength light can penetrate for hundreds of metres. Around the coasts, where nutrients are rich and sediments wash off the land, the water is cloudier and seaweeds do not grow beneath the illuminated zone. Thus green, brown and red seaweeds are beautifully adapted to the factors of light and surge. The seaweeds are the algae we know best so it is easy to forget the army of microscopic algae in plankton. They are single-celled algae, prodigious in species and countless in numbers that float in the surface layers of the sea. Like grass in a paddock, plant plankton is the pasture of the sea; the beginning of the food web that feeds all the living things in the ocean. It seems absurd that such claims can be made for a microscopic food source, yet oceanographers have found tonnes of plant plankton in a hectare of sea, made up of millions of single-celled algae. Not all of the sea’s surface layers are as prolific as this. Plant plankton grows well at colder temperatures because cool oceanic water is much more nutritious than warmer, tropical waters. Cold water wells up from the deep ocean floor laden with minerals from the ages-old erosion that has collected there. This means that the southern oceans support vast tonnes of plant plankton, which in turn support fish and mammals, like whales, seals and penguins. By comparison, the warmer tropical seas are poor in plant plankton and their food webs rely on special algae living in the tissues of coral and on the productivity of mangrove forests for support. — ANN GRAEME is the national co-ordinator of Forest and Bird’s junior arm, the Kiwi Conservation Club. She lives in Tauranga.

An Edible Seaweed

Seaweeds contain some proteins and good supplies of vitamins and trace minerals which are often lacking in New Zealand soils. Seaweeds make great garden fertilizer. If you want to taste an edible seaweed, try karengo, Porphyra columbina. It looks like dark greenish-purple cellophane and grows high on rock shores. Maori have long used it as a food supplement. Quantities were dried and sent to the Maori Battalion during the Second World War. Karengo is also pleasanttasting as a chewing gum. It is quite safe to try as (should you make a mistake), none of the seaweeds are poisonous. Karengo is related to the Japanese nori, which is used in sushi.

Sea Lettuce a Signal

From time to time, blooms of sea lettuce plague our harbours. Enrichment of the waters by human activities such as farming and urban development is implicated in their growth. So too is the El Nino weather phenomenon. This weather pattern blows the surface waters off the eastern coastline and deeper, cooler, more nutritious water upwells and floods into the harbours.