Seawater is not just water, salt and a few other minerals. The ocean’s top 30 metres contain incredible numbers of microscopic single-celled plants called phyto plankton (plant) plankton which are the base of the ocean’s food chain. These microalgae, which draft 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 seaweeds. 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 kilometres 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.
Sunlight is Important
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 percent 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 the depth of more than 70 metres on the sea floor near the Kermadec Islands.
How Does Seaweed Grow
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 seaweed 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.
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).
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 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, thought is more common 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.
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. Bell kelp fronds torn off in a storm are known to drift for thousands of kilometres, ensuring widespread distribution of the 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 streamlined 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 o 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.
Macrocystis pyrifera, found in most cooler seas, is considered the world’s fastest growing plant: 0.5 metre per day and 25 metres in three months. Bladders provide buoyancy to hold the plant upright in the water, drawing it up towards the life-giving sun. Absence of light limits seaweeds to shallow water, mainly down to 15 metres, with very few below 50 metres. top of page
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 (mentioned above), 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 length 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 ribbon-like 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 to the Wairarapa. Large beds of the potassium-rich kelp are found off the north Otago coast, in Foveaux Strait and around the subantartic 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 groups 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 the depth of 270 metres in the Caribbean.
Almost all of New Zealand’s commercially gathered seaweeds are reds. Besides the agar weed Pterocladia 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 edelicate forms – as well as a few uninspiring shapes. The fleshy blobs of Apophloea sinclairii on the inter-tidal 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 unusually 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.
Sea lettuce, Ulva lactuca, stands out on most shores because of its lime-green coloure. 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-dollar-a-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 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.