Kim Westerskov

Gardens under the sea

Found only in that narrow strip of Earth where ocean meets land, seaweeds soften the angles of the rocks and build forests and gardens beneath the wave. Richly diverse in colour and form, they are the fundamental food source of marine animals, and contribute more than we generally realise to everyday human life. Tide pool at Army Bay, 25 km north of Auckland, provides both a playground and a window of fascination into the sea.

Written by       Photographed by Darryl Torckler

As a child, I paid little atten­tion to seaweeds. Summer days on the wide South­land beaches of my youth are remembered more for the tangy-fresh smell of the sea and the tingling thrill of jumping into the cool Foveaux Strait surf. When my friends and I hunted in rock pools for crabs or starfish, we scarcely noticed the di­verse plant life around us. It was all just seaweed, an unnoticed back­drop to the animal life swimming or crawling amongst it.

We only noticed seaweed when it had a use: rubbery kelp thicker than a man’s leg which we carved with pocket knives into bouncy golden-brown balls for beach cricket; oval seaweed floats which popped loudly when we jumped on them.

When I returned to these beaches recently from the North Island where I now live, I saw a different picture from that of my childhood.

I now knew more about seaweeds, and saw before me at low tide a colourful seaweed garden with a forest of larger varieties be­yond. Some were fine and feathery, and some like pink icing on the rocks. Others were rows of beads or brown tresses waving in the surf. Some were just scungy brown slime. The forgotten background of child­hood had become a fascinating fore­ground.

Even so, there is much that re­mains hidden about seaweeds. Most lie out of our sight under the waves. While it is easy to see on land how grass is eaten by sheep, tracing the contribution of seaweed to the food chain for fish and shellfish is less straightforward. (Nature is full of surprises: sheep on the tiny west Scottish isle of Soay live almost en­tirely on seaweed they graze from the rocks at low tide, and some farm­ers on our own Stewart Island feed bull kelp to their cattle in winter.)

The uses we humans make of sea­weed are not that obvious either, although more than three million tonnes are harvested annually worldwide. While in some countries certain seaweeds are popular as food, in most western countries they figure in our diets as anonymous ex­tracts, doing everything from mak­ing instant puddings set to stabilis­ing the foam on beer [see box page 80]. The paper, textile, cosmetics and petrochemical industries also use substantial amounts of these same extracts.

Another seaweed extract under­pins the modern science of biotechnology and is an indispensa­ble tool in medical research.

The story of seaweeds begins, naturally enough, in the sea—and we creatures of the land don’t al­ways appreciate how different that environment is from our own.

Sea water is not just water, salt and a few other minerals. The ocean’s top 30 meters contain in­credible numbers of microscopic single-celled plants called phytoplankton (plant plankton) which are the base of the ocean’s food chain. These microalgae, which drift around with the currents, are the pastures of the sea, grazed by small animal plankton (zooplankton) which are eaten by small fish and devoured in turn by larger fish and mammals.

In shallow water, where sea meets land, grow the much larger macro-algae, commonly known as sea-weeds. All these plants of the sea have, like those on the land, a vital role in sustaining life on earth. They possess the ability to use sunlight as fuel to change water and carbon di­oxide into carbohydrates, the build­ing blocks of life in the sea as well as on land.

The world’s oceans are four kilometers deep, on average. Yet it is the precariously thin layer of plant life at the surface which ultimately feeds almost all marine animal life.

Like land plants, seaweeds need light, nutrients, a place to grow and a reasonable climate. Any or all of these factors can govern their growth and distribution.

The depth limit for seaweed growth is set by light. Sunlight has difficulty penetrating the sea, and one third of it is usually soaked up by the top metre of water. Less than one per cent of light striking the sur­face penetrates to 50 metres.

Where sediment and phyto­plankton cloud the water, seaweeds may not be able to survive below 15 metres, but in clear subtropical wa­ter they can grow much deeper. The deepest growing seaweed in New Zealand waters was found at a depth of more than 70 metres on the sea floor near the Kermadec Islands.

The name seaweed is typically applied to attached algae, to distinguish them from the phytoplankton which drift with the currents. Though masses of the large brown Sargassum seaweed are found adrift in the Atlantic’s Sargasso Sea, and in New Zealand free-living Hormosira and Macrocystis may be encountered in calm harbours, most seaweeds need to attach themselves to a solid surface—rocks, wharf piles, even boats will do.

Seaweeds do not usually grow on sand because the unstable, shifting surface does not provide an adequate hold. For the same reason, the soft papa rock of north Taranaki, which crumbles easily in the pound­ing surf, fails to support an abun­dant flora.

Suitable seaweed substrates need to be hard, but they do not have to be stationary. Many limpets and other molluscs carry a personal gar­den on their shells, and the decora­tor crab Notomithrax ursus cuts and entangles algae in its body hairs to disguise itself.

Availability of nutrients is not usually the problem for seaweeds that it can be for land plants. Seaweeds simply absorb minerals and water directly through their sur­face tissues from the nutritious sea around them. Unlike plants on land, seaweeds have no need for roots or internal canals to conduct water and nutrients. What look like roots in some types in fact serve only as an anchor, called the holdfast.

The temperature of the sea has a significant influence on where many seaweeds grow. Temperature-sensi­tive species may have only a limited geographical range—perhaps a few hundred kilometres of coastline. More tolerant types range over a much wider area. The common kelp Ecklonia radiata, for example, is found from the Three Kings Islands, north of Cape Reinga, to The Snares, south of Stewart Island. As a gener­alisation, the seaweeds of the cooler southern coasts such as Stewart Is­land tend to be larger and therefore more conspicuous than those of warmer northern regions such as the Bay of Islands, and the overall sea­weed biomass is larger in the south.

Sea currents, which move large bodies of heat around the coastline, are the primary determinants of wa­ter temperature. One of the most im­portant is the warm current from Australia that sweeps around Southland and Stewart Island, gradually mixing with a cold subantarctic current to move cool water right up the east coast of the South Island to Hawkes Bay.

Currents can also cause sharp lo­calised effects. Cold water upwelling north of Cape Reinga has allowed the growth on the Three Kings Islands of a number of seaweeds otherwise not found north of Cook Strait. Similarly, warm cur­rents that flow down the eastern coast of Northland transport to the offshore Poor Knights Islands a number of semitropical seaweeds and animals never recorded on the mainland just 20 km away.

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The world’s oceans and seas contain many different plant groups. Although they all tend to be grouped under the umbrella term algae, they actually vary widely in their evolutionary relation­ships, and are more diverse in their origins and form than the great vari­ety of flowering plants found on land. In fact, a kelp has more in com­mon with diatoms floating in the surf than with the green alga lying along side it. Botanically, seaweeds belong to three major groups of al­gae: the brown algae (Phaeophyta), red algae (Rhodophyta) and green algae (Chlorophyta). In spite of their differences, they have evolved simi­lar ways of coping with surging waves, salty water and the constant abrasion of rocks and sand.

Regrettably, colour alone is not a foolproof guide to the identity of a seaweed. So-called brown seaweeds may be any colour from olive green to dark chocolate, and the reds can be yellow, green or purple, although red is most usual. Greens are always a shade of green.

Seaweeds get their colour from the pigments they use to “harvest” light for photosynthesis. Although, like land plants, all seaweeds con­tain chlorophyll, they possess an ad­ditional range of pigments to har­ness wavelengths of light not effi­ciently absorbed by chlorophyll. This is necessary because not only is there less light under water, but its spectrum differs considerably from daylight. Sea water absorbs red light most readily, then green and finally blue. (It is the absence of red light that makes everything more than a few metres deep look bluish.)

Brown algae include the largest plants found in sea water down to the soft brown fuzz that makes sea­side rocks slippery. One of the most dramatic is bull kelp, Durvillaea antarctica—the surfer of seaweeds. It grows on the most wave-exposed coasts in both the North and South Islands, though it is more common in the south. Bull kelp is a massive plant with thick yellow-brown stem and rubbery fronds up to 10 metres long that float on the surface, swirl­ing back and forth with the waves.

This species is a classic example of how a seaweed has adapted to living in an extremely stressful envi­ronment. Its huge fronds receive maximum light by floating above the surf, using their own internal buoy­ancy in place of the air bladders found on some other species. Break open a frond and a hollow honey­comb structure is revealed that is tough, flexible yet buoyant [see fold­out after page 80].

To survive in surf conditions, a seaweed needs to be able to bend with the pressure generated by large waves and then bounce back. The rigidity of the timber tree would be a liability in this environment. The bull kelp stalk is particularly strong and elastic, to cope with these harsh physical forces. Individual plants can survive for up to seven years, but they lose their grasp on the wave battered rocks when their huge cup­like bases are weakened by the bor­ing worms and molluscs that shelter there. Bull kelp fronds torn off in a storm are known to drift for thou­sands of kilometres, ensuring wide­spread distribution of this species around the southern oceans.

Like many seaweeds, bull kelp is slimy to the touch. This surface mu­cilage helps protect the plants from abrasion against rocks and other or­ganisms, and improves the stream‑lined flow of water over the blades, reducing whiplash tearing. Also, be­cause the slime is being constantly produced and then sloughed off, it prevents other small plants getting a toehold on the surface and growing as epiphytes. Mucilage also helps in sexual reproduction by assisting in the liberation of eggs and sperm. A large female bull kelp plant may re­lease tens of millions eggs during its winter spawning.

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Seaweed communities have many parallels with the lay­ers of a land forest. Just as in the bush there are large canopy trees, tiny forest floor plants and everything in between, so in the sea plants are spread over a similar range of habitats right down to coral-like crusts, fuzzy filaments and sin­gle cell algae.

The large brown kelps are the canopy trees of the marine habitat, giving shelter, providing surfaces to settle on and holdfasts to burrow un­der. The kelp which most commonly fills this niche is Ecklonia radiata, a species which grows up to a metre tall as a round firm stalk with a clus­ter of flat fronds on top and lives from the low tide mark down to 15 metres. Ecklonia forests are an im­portant habitat for juvenile fish and rock lobster, and provide a food source for kina and other echinoderms, gastropods and butterfish (which bite beautifully clean holes in the fronds).

Macrocystis pyrifera, another brown kelp, is the giant of our sea­weed flora and the fastest growing plant in the world. It can grow up to half a metre a day, faster than bam­boo, and can reach lengths of 35 me­tres—about as high as a mature kauri tree—in only three months. Macrocystis has very strong vines like power cables. Every few centi­metres along the vine there are pear-shaped bladders attached to ribbon­like fronds. The bladders enable the plant to float up to the surface, where the fronds catch the maxi­mum amount of light energy.

Macrocystis prefers less exposed waters than does bull kelp or Ecklonia, and is common from Stewart Island north to the Wairarapa. Large beds of the potas­sium-rich kelp are found off the north Otago coast, in Foveaux Strait and around the subantarctic islands. Macrocystis was investigated during World War II as a source of potash fertiliser, and its use in this regard is well known to gardeners.

Red seaweeds have the largest number of species and the widest range of forms. Most are less than 30 cm tall, and a few are microscopic. Some form broad sheets a single cell thick, others exist as crusts that coat rock with a bright pink “paint.” The group also has representatives at the extremes of seaweed habitat. Species such as the edible Porphyra, known as karengo by the Maori and nori in Japan, will tolerate being sunbaked on a hot rock for hours at low tide, while the deepest seaweed yet found is a red growing in near blackness at a depth of 270 metres in the Caribbean.

Almost all of New Zealand’s com­mercially gathered seaweeds are reds. Besides the agar weed Pterocladia [see box, page 91] and karengo, Gracilaria—resembling dark brown vermicelli—is used for food grade agar elsewhere in the world, and may prove useful here as a diet for cultured paua.

The red seaweeds also provide the most beautiful and delicate forms—as well as a few uninspiring shapes. The fleshy blobs of Apophloea sinclairii on the intertidal ledges of northern shores have been compared with patches of dried blood.

Within the Rhodophyta are a group called the corallines—named for their similarities to corals, and usually pink in colour. These algae lay down calcium carbonate in their cell walls, providing them with a sort of skeleton. The structures can sometimes be articulated with calcium “bones” and flexible “joints.” Or they can be simply pink paint and knobbly lumps on rocks, shells or any other firm substrate. How many species of these crusts occur in New Zealand is unknown.

There are fewer green seaweeds than reds or browns. Many are toler­ant of lower salinity water, and are able to live higher on the shore.

Sea lettuce, Ulva lactuca, stands out on most shores because of its lime-green colour. High nutrient lev­els (resulting from fertiliser run-off or septic tank seepage) favour its growth. In Tauranga Harbour, it be­comes a nuisance when thick masses wash up on beaches.

Algae belonging to the genus Enteromorpha are coloured simi­larly to Ulva, and are almost invari­ably associated with stream mouths and fresh water seepages high up on beaches.

Codium is a genus with species having two quite different forms. One is a crust, a luxurious-looking green velvet cushion on rocks, while the other branched form looks like green deer horns.

The sex life of seaweeds is among the most Byzantine in the plant world. Reproduction in the red al­gae, particularly, is notoriously com­plicated. Indeed, the Japanese sea­weed industry honoured British botanist Kathleen Drew for disentan­gling the complex life history of Porphyra, and in so doing laying the scientific base for the billion-dollar­a-year cultivation of nori—a major constituent in the popular seaweed and rice dish sushi. She discovered the “missing link” in the plant’s life cycle: a threadlike branching stage called a conchocelis which grows into the surface of oyster shells over summer. This stage was so different from the plastic sheet-like form of the adult that it created something of a sensation among algologists.

With seaweeds, there is often not one life cycle—such as the egg/cat­erpillar/pupa/adult butterfly story—but rather several possible cycles, involving sexual, asexual and veg­etative means of reproduction. Within the life cycle of many spe­cies an asexual spore-producing plant—the sporophyte—alternates with a sexual gamete-producing plant—the gametophyte.

The large, familiar plants of Macrocystis or Ecklonia, for example, are sporophytes. These release vast numbers of spores that grow into microscopic filamentous male and female gametophytes which are seldom seen. It is these minuscule gametophyte plants that reproduce sexually to form the next generation of sporophytes.

In various red algae, crusts, shell-boring filaments (such as the conchocelis) or microscopic fuzz on rocks alternate with larger foliose plants. Some seaweeds, such as Ulva, have gametophyte and sporophyte plants that look identi­cal, and which can only be distin­guished by examining their repro­ductive structures or by counting cell chromosomes.

The simplest method of repro­duction is vegetative—the way new strawberry plants root from runners, or potatoes from tubers. Some seaweeds make new plants using stolon-like runners similar to straw­berries. One of these is Caulerpa geminata, which looks like bunches of small green grapes and can be eaten fresh in a salad. Others pro­duce plantlets on their fronds, which eventually fall off and start their own life.

Asexual reproduction occurs when specially produced cells termed spores grow into new plants which are genetic carbon-copies of their parents.

Sexual reproduction involves the making of male and female repro­ductive cells which are released into the ocean. The fusion of egg and sperm cells results in an exchange of genetic material.

Some seaweeds cunningly pro­duce cells which can be either sexual or asexual. If a cell does not meet a partner of the opposite sex, it can settle down quite happily as a spore and grow into a new plant.

The chain of events in the life history of a seaweed is not always a direct sequence, but is often closely cued into environmental stimuli like temperature, day length and light wavelengths—the same kinds of triggers that cause bulbs to bloom in spring or deciduous trees to drop their leaves.

To give some idea of the com­plexities of the life histories of red seaweeds, one New Zealand species of Porphyra has been found to use six different methods of propaga­tion, and each of the 16 species of  Porphyra so far discovered here has a distinct pattern of reproduction.

Although this genus is one of the most closely studied in the world, and has been under the microscope for decades, more is constantly be­ing gleaned about its life.

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Human use of seaweeds has never been great in New Zealand. Unlike the Japanese, for whom seaweed is an important component of the diet, the immigrants to Aotearoa, both Maori and Pakeha, largely overlooked seaweed as food in favour of the fruits of the land.

Southern Maori did, however, harvest karengo [see box, page 87] and made use of bull kelp as a means of stor­ing mutton birds caught off Stewart Island. Wide blades of Durvillaea antarctica were split and dried to form flexible airtight bags or poha titi to store the birds (titi) in their own fat—a method still used today.

Seaweeds are collec­tively called rimu or rimurimu in Maori. First Maori immigrants would have been familiar with a green alga now called Caulerpa which is common in tropical Polynesia. The rimu tree’s hanging tresses looked very like the green seaweed Caulerpa brownii (sea rimu) which is found from Hawkes Bay south. (The tree was named after the seaweed, not vice versa.)

Botanists were probably the only Europeans interested in local seaweeds in the first century of con­tact. Banks and Solander made the initial collections on Cook’s first voyage. The most recent illustrated book on the complete seaweed flora of New Zealand was published in 1855. Only now, 139 years later, is another illustrated flora, by botanist and botanical artist Nancy Adams, about to be published.

Nancy Adams had a remarkable introduction to seaweeds as a teen­ager working in the DSIR during World War II with botanist Lucy Moore, whose investigations into New Zealand seaweeds started an agar industry here.

Previously, Japan had supplied all agar, but with the advent of war new sources were urgently sought. Agar was needed as a microbiologi­cal culture medium, and also for meat canning to feed soldiers at the front.

Nancy maintained the collections of seaweeds made by Lucy Moore as she travelled around the coast in the early 1940s, looking for potential agar sources. After the war, Nancy joined the National Museum and be­gan building up what is now the biggest collection of seaweeds in the country. Some 300 new species of seaweeds have been found in the past two decades, giving a New Zea­land total of over 1000, and more are being found each year.

The difficulties of making com­plete collections from the tremen­dous variety of underwater habitats around an indented coastline longer than that of the continental United States are enormous, and many amateur collectors have played an important part in this knowledge gathering. Victor Lindauer, a sole charge teacher who lived in Northland and Taranaki in the 1930s and ’40s added enormously to this knowledge.

So too did Eileen Willa of Halfmoon Bay, Stewart Island. Now in her late 80s, and retired to Invercargill, she lived most of her life near perhaps the richest colleting beach in the country, Ringa Ringa Bay.

Lindauer named the kelp Durvillaea willana after her. She was the first to notice that some bull kelp had branches like trees and could not be split for mutton bird bags. The species is now recognised to be as common as Durvillaea antarctica around much of the South Island.

Since the war, the use of seaweeds in New Zealand for com­mercial applications has continued to be based largely on agar extrac­tion from Pterocladia lucida, though recently Ecklonia kelp has been used in the making of liquid sea­weed fertilisers and animal drenches.

Seaweed extracts are used in a tremendously wide range of prod­ucts from dogfood to DNA finger­printing. Seaweeds are in demand because of the gelatinous polysaccharides in the cell walls of many varieties. The ability of these polysaccharides—agar, carrageenan and alginate—to gel liquids, hold material in suspension and to act as thickeners makes them invaluable in myriad applications.

Agar was first used as a solid cul­ture medium for experiments on tu­berculosis bacteria in 1882. Amaz­ingly, 100 years later it remains the culture medium of choice for gen­eral microbiological work, and much growth and cloning of recombinant microbes and plants produced by gene technology is per­formed on it. Most DNA electrophoresis—a high resolution method for separating DNA frag­ments that has been central to the success of almost all modern genetic technologies—depends on agarose gels derived from seaweeds.

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Just along from Freyberg Pool, in Wellington’s fash­ionable Oriental Bay, I clam­ber down the rocks to look at an unusual brown seaweed.

I pull up one of the pro­fusely growing plants. It is a metre long, with a rounded sheet of fanlike frills growing out from a flat strap of a stalk. A handsome sea­weed, without doubt, but not a na­tive. It is a new immigrant from Asia, a descendant from a plant that jumped ship here about 10 years ago. Its name is Undaria pinnatifida, and it happens to be one of the most desirable edible seaweeds in the East. Known as wakame in Japan, it is sold here in health shops and ori­ental food markets.

Its rapid acclimatisation to Wel­lington conditions and to harbours down the South Island’s east coast is causing some disquiet, tempered by modest enthusiasm for possible commercial gain.

Marine botanist Cameron Hay, who works for the Department of Conservation just a few blocks away from where Undaria grows in Wel­lington Harbour, has been charting its progress since it was first discov­ered at Oriental Bay in 1987.

Before making his first explora­tory dive, Cameron says he faced something of an ethical dilemma. Suppose he found only half a dozen plants? Should he remove them, knowing they might spread, or should he, as a detached observer, simply watch what happened?

“My moral dilemma soon ended when I jumped in the water. There were acres of it,” he laughs.

Undaria now extends for a few kilometres from Oriental Bay to the ferry terminal on wharf piles, rocky embankments and natural reefs. It probably arrived in the early 1980s on the hulls of Japanese or Korean fishing boats, or as spores in ship ballast water.

Japanese eat 100,000 tons of the kelp each year, making it the third biggest seaweed crop and a half-bil­lion-dollar business. In Korea, even more is harvested. Most is cultured on ropes, like mussels on mussel farms. Competition from fish farm­ing operations, together with con­cerns over coastal pollution, are prompting some Asian companies to consider sourcing Undaria from New Zealand, and Nelson’s Cawthron Institute is preparing to evaluate the possibility of a local in­dustry.

The kelp seems to have taken to its new home in New Zealand with alacrity. In Japan, it has only one crop a year; in our more temperate waters it is fertile and growing most of the year.

Cameron says that in the inner Wellington Harbour, previously un­remarkable for its seaweed, Undaria has filled an unoccupied niche by creating a dense kelp forest. But it has not become a monoculture. Many native seaweeds are growing amongst it. Paua, too, seem very par­tial to Undaria, and numbers are in­creasing where the plant flourishes.

A more vigorous variety of Undaria grows in Timaru Harbour. This is the commercial variety grown in the colder waters of north­east Japan, and in Timaru it is form­ing thick forests of up to 100 plants per square metre.

Undaria hitches a ride on the hulls of ships. So far, Cameron Hay has tracked the plant to Lyttelton, Oamaru, Port Chalmers, Porirua and Picton. It has not spread further north, as it prefers cooler waters.

Cameron believes the ecological impact of Undaria could be greatest in the Marlborough Sounds. Rocks in the inner and central parts of the Sounds are almost bare of seaweed, and the weed is already growing near Picton and on a salmon farm at the entrance of Pelorus Sound. If the Japanese seaweed takes off, the nearshore marine life of the Sounds might change from being mainly dependent on phytoplankton produc­tivity and a supply of river-borne detritus to communities that depend on seaweeds and the products of seaweed decay.

Undaria is just one of New Zea­land’s potentially valuable sea­weeds, says Cameron.

“New possibilities come up all the time. Not long ago, a Japanese research institute detected chemi­cals with food preservative proper­ties in samples of New Zealand Carpophyllum, and were very ex­cited about the discovery, claiming that these substances may be supe­rior to traditional preservatives.

“On another occasion, a macadamia nut grower from Queensland rang up, wanting to know if macerated bull kelp could be sourced from New Zealand. There is apparently insufficient available in Australia, and growers are finding that, used as a fertiliser, it confers resistance to fungi and im­proves yields. Unfortunately I don’t think that we can help him—no such product is made here.”

Science and commerce certainly seem to be riding a wave of interest in new products from the sea. A massage “oil”/body lotion under de­velopment is based not on sticky ol­ive oil, but on an oily derivative of seaweed polysaccharides which is readily absorbed into the skin. And recently Lower Hutt plastic surgeon Tony Tonks has been using dress­ings made from seaweed alginates to successfully treat the weeping sites that skin grafts have been taken from. Wounds heal in half the time that they take with conventional dressings, he says.

All this from humble undersea gardens that most of us have scarcely noticed.

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