Floating around our waters are small microscopic celled plants called phyto plankton (plant plankton) which are the base of the ocean’s food chain. Just on the edge of the oceans there grows a much larger macro algae commonly known as seaweeds.
Like plants on our land have a purpose to full fill, like sustaining life on earth.
These seaweeds 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.
Not just the ocean around New Zealand but all the worlds oceans are well over several kilometres deep, yet it is the relatively small layer of plant life at the surface that eventually 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 very Important because the depth limit for seaweed growth is set by the light.
The sea being so dense it is very difficult for the sunlight to penetrate. One third of it is usually soaked up by the top metre of water, and there is less than one percent of this light hitting the surface that penetrates to below 50 metres (160 ft).
When you have constant sediment or dirty water these seaweeds may not be able to grow 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 near the Kermadec Islands.
There are three major groups of seaweeds, these being red, green and brown.
Seaweed Groweth:
Seaweeds do not usually grow on sand or soft papa rock because the unstable surface does not provide an adequate hold.
Most seaweeds need to attach themselves to a solid surface such as rocks, wharf piles, even boats will do (some surfaces are not always stationary).
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.
Seaweeds simply absorb minerals, nutrients and water directly through their surface tissues from the nutritious sea around them.
Sensitiveity:
The temperature of the sea has a significant influence on where many seaweeds grow.
The most common type of kelp, Ecklonia radiata, for example, is found North of cape Reinga at the Three Kings Islands, down to as far as the Snares, south of Stewart Island.
Seaweeds in the southern parts of the country tend to be larger than in the north.
Warm sea currents from Australia mixing with cold currents from the south mix and send cool water right up the east coast of the South Island to the Hawkes Bay in the north.
Seaweeds get their colour from the pigments they use to gather light for photosynthesis. Like land plants, all seaweeds contain a green colouring matter (chlorophyll) contained in the flattend body essential to their growth by photosynthesis. They possess an additional range of pigments to harness 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 the most, then green and finally blue. (It is the absence of this red light that makes everything look a bluish colour after a couple of metres.
Some seaweeds need to be able to survive the constant pounding surf conditions, so they ned to be able to bend with the pressure generated by large waves and then bounce back.
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 after a while they lose their grasp on the wave battered rocks when their huge holdfast bases are weakened by 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 helps protect the plants from abrasion against rocks and other organisms, and improves the streamlined flow of water over the fronds, helping reduce whiplash and tearing.
The largest seaweed that grows around our coasts and maybe around the world is the Macrocystis pyrifera. It can grow faster than bamboo, infact it grows up to half a metre a day. It can reach length of 35 mts ( 115ft) in only three months, thats about as high as a mature kauri tree.
Brown Seaweeds: Brown seaweed starts from the soft brown algae fuzz that makes seaside rocks slippery,
and include the largest plants found in sea water.
Large brown kelps provide shelterd canopys of the marine habitat, giving shelter, providing surfaces to settle on and holdfasts to burrow under.
Red seaweeds:
Red seaweeds have the largest number of species and the widest range of forms. Most are less than 30 cm (1 ft) 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 colorization.
These red seaweed groups also have great extremity of seaweed habitat. Species such as the edible Porphyra, known as karengo by the Maori, will tolerate being sunbaked on a hot rock for hours at low tide.
while the deepest red seaweed found is growing in the dark cold depths of 270 mts (885ft) in the Caribbean.
Almost all of New Zealand’s commercially gathered seaweeds are reds.
The red seaweeds also provide the most beautiful and distinct forms as well as a few uninspiring shapes. Some group structures can sometimes be expressed with calcium bones and flexible joints. Or they can be simply pinkish colours and knobbly lumps on rocks, shells or any other firm surface.
Green Seaweeds
There are fewer green seaweeds than reds or browns. Many are tolerant of lower salinity water, and are able to survive much higher on the shore.
The green Sea lettuce, stands out on most shores because of its lime green colour. Thick masses of this type can be found along the shores around Tauranga Harbour. It becomes a nuisance when thick masses wash up on beaches.
click to see a larger image
There is another species that looks like a carpet of green velvet while another looks like horns from deer.
This seaweed does not like exposed waters like bull kelp and is most common the Wairapa coast down to Stewart Island. Large beds of this kelp are found off the north Otago coast, in Foveaux Strait and around the subantartic islands like the Auckland islands.
One of the most dramatic is bull kelp, It grows on the most rougth exposed coasts in both the North and South Islands.
Bull kelp is a massive plant with thick yellow brownish stem though sometimes blackish, and rubbery fronds up to 10 metres or 30 ft 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.
The most common brown kelp canopys is Ecklonia radiata, a species which grows up to a metre tall. It has a round firm stalk with a cluster of flat fronds on top and lives from the low tide mark down to 15 mts (50ft).
Ecklonia forests are an important habitat for juvenile fish and crayfish, and provide a food source for kina and other gastropods.
This small edible alga grows up to 20 cm across, and has an irregularly shaped, broad frond that is membranous but tough. The plant attaches to rock via a minute hold fast. It is greenish when young becoming a purplish red.
