Seaweeds at the South Coast

Seaweed garden

Seaweed garden

(For photos of  most of the seaweeds noted in this article, click here) 

Snorkel ‘floating’ over a South Coast reef is akin to flying slowly over a New Zealand terrestrial high forest. The ecological elements are strikingly similar, with major kelp forests harbouring epiphytes and a host of small invertebrates and bacteria. Secondary forests and shrubs abound. Burrowing animals attack holdfasts and fish fly/swim amongst the forest fronds. On a micro scale there are coralline algae turfs also carry epiphytes[1] and critters as mini-ecosystems vital to marine food chains. The seaweed provide the forest structure and the shelter and habitats for so many marine creatures. 

By contrast with our terrestrial ecosystems however one striking feature of our marine habitats is how little is really known. Nearshore habitats have been explored well enough but there is much work to do to further identify, classify and understand how everything fits together. What is available suggests some fascinating ecological relationships and mutual dependencies every bit as complex as those on the land. 

New Zealand has a rich macroalgal flora with about 1000 species, of which some 850 have been studied and documented. We have considerable endemism in our macroalgae, while others are very widespread around southern oceans. All seaweeds depend on sunlight for food, through cellular chlorophyll. They live on the nearshore from the tidal fringes down to the limits to which sunlight can penetrate. On the South Coast this is about 20 to 30 m at the best of times and very much less when the storms have stirred up the sediments. At the Kermadec Islands sunlight penetrates to about 200 m.

Seaweeds are important organisms in marine food chains, providing nutrients and energy for marine animals.  Seaweeds may be grazed alive, or when decomposing parts break down into fine particles and are taken up by filter-feeding animals. Beds of seaweed provide shelter and habitat for scores of coastal animals for all or part of their lives. They are important nurseries for many commercial species such as the rock lobster, pāua and green-lipped mussel. Beach cast seaweed plays a role in the shore ecology as food and shelter for a host of invertebrates and foraging and nesting material for sea birds.

Seaweed structure varies at the lower end as single cell species and the ‘paint’ of crustose coralline algae, with turfs forming from the more complex jointed coralline species. At the other end of the spectrum is the country’s largest kelp, Macrocystis pyrifera, known as bladder kelp in New Zealand, but giant kelp elsewhere. It can grow rapidly to 50 metres in length and 100 kilograms in mass. Bladder kelp forms large forests in the deep sheltered waters of southern New Zealand, and is easily identified by the gas-filled floats at the base of each frond. On the South Coast we have Macrocystis forming ‘forests’ in sheltered waters such as Island Bay, as well as bull kelp, Durvillaea antarctica, at the outer reaches of more exposed reefs. Some species live as annuals; others are perennial and long lived – until they are torn apart in storms. These large kelps can twist and flow in a whiplash like action in high seas, often clearing neighbouring substrates of all but the toughest of smaller seaweeds. 


The major groups of seaweeds are noted by colour, though it may be hard to decide whether greens are browns or reds are really brown or green.Brown seaweeds can be golden yellow-brown to dark olive. Red seaweeds have the greatest range of tone – pink to purple, red, and brown to nearly black. Brown and red seaweeds have additional pigments that enable them to photosynthesise where little light penetrates.These three groups have quite distinct evolutionary origins, however, with browns related to diatoms, the greens to terrestrial plant origins, and the reds have very ancient origins with no swimming stages in their life cycle.

Until quite recently seaweeds were described as simple plants that grew in the sea, but scientists now consider that the structure and chemistry of seaweeds is so distinctive that they warrant their own classification. They are simple in structure, with the whole known as a thallus usually consisting of a holdfast that anchors them to a surface, and a blade which may be divided into fronds. Some of the larger seaweeds have a flexible stalk or stipe connecting the blades to the holdfast. Unlike plants, seaweeds do not have roots or internal tissues to conduct water. They absorb minerals and gases directly from sea water through the surface of their blades.


The Wellington South Coast, in common with Cook Strait and the Kaikoura Coast is one of New Zealand’s seaweed hotspots, with some 370 species of macroalgae present, or about 30% of the NZ total. The South Coast lies at a meeting of currents, cold southern currents moving north and the warmer currents around the North Island circulating south. At this meeting point we find species at their northern and southern limits overlapping, forming a richness and diversity which makes Taputeranga Marine Reserve such a special place for biodiversity protection. The macroalgae provide the structure for habitats of great diversity.  

Seaweeds have to demonstrate high flexibility in form and cellular structure to cope with the dynamic randomness of the forces in waves, tides, currents, storms and swells. Most seaweeds exude mucilages onto blade surfaces to prevent drying out in the sun and as a means of dealing with the abrasion of rocks and sand. These exudates are also an important feature in food chains, feeding a variety of critters including bacteria. 

Green seaweeds:

Green seaweeds are found in the shallowest of nearshore habitats where there is plenty of sunlight. A commonly found green species is sea lettuce, Ulva lactuca, which forms quite heavy masses of apparently unattached greenery at the tidal edge. As the name indicates, it can be edible, but it is often growing in abundance where there is pollution from land sources. Shore-tossed masses can rot with unpleasant odours to spoil your day at the beach.Velvet weed, Codium convolutum forms dark green to black brain-like cushions in the intertidal zone. Further out along reefs in the lower intertidal can be found Caulerpa species, including sea rimu, (C. brownii), for all the world like fronds of NZ’s native forest giant. Grapeweed, (C. geminate), is often seen in close association with its relative. Green gut weed (Enteromorpha intestinalis), forms as a tubular seaweed, also favours high-nutrient sites including rock pools that have become oxygen depleted in the summer sun. 

Brown seaweeds: 

While these grow generally below the greens and above the reds in the intertidal-subtidal continuum, there are exceptions. Neptune’s necklace, Hormosira banksii, can be found often at the edge of the sea in rock pools and the intertidal zone, with ‘beads’ which store water to withstand drying. The brown seaweeds occupy two main groups, the Laminariales (true kelps) and the Fucales (which confusingly includes bull kelp). The commonest brown seaweeds at the Snorkel Trail include strapweed, Lessonia variegata, clearly a laminarian seaweed. This forms extensive ‘secondary forests’, each individual anchored by a firm holdfast and calcareous stipe. Holdfasts of many species provide food sources for burrowing invertebrates and it is a combination of holdfasts weakened by grazing and the ferocity of storms which see kelps and seaweeds cast up on beaches. The rotted kelps often leave behind a ‘skeleton’ of whitened stipe and holdfast material on beaches above high tide.

Strapweed is usually found with the accompaniment of another true kelp, stalked kelp, Ecklonia radiata, and a fucalean, flapjack, (Carpophyllum maschalocarpum and flexuosum). Flapjack also is very evident in the pools and channels of the Snorkel Trail reef, along with slender zigzag weed, Cystophora retroflexa. This last species is especially to be seen in the ‘lead in’ channel at the Trail, distinguishable by small round float bladders. Ecklonia is more common further north and is indeed found in most of the world’s cooler oceans. It is about 1 metre in length and has a bunch of fronds arising from a central supporting stipe. It is a preferred food of the sea urchin, kina, which sometimes mass together and munch through an entire forest. Following such large-scale habitat destruction, other seaweeds may dominate for a while, but in northern New Zealand, at least, common kelp grows rapidly and soon replaces itself, especially where snapper are able to re-establish and reduce the kina population.

The large bladder and bull kelps, Macrocystis and Durvillaea, are further out in the subtidal, the former with characteristic pear shaped float bladders to maintain structure and position in the sunlight. Durvillaea, also known as rimurapa, relies on air filled blades to similarly provide such structure. These blades are easy to split and are still harvested and used in southern NZ as pots or bags for mutton birds to be packed and stored. Bladder kelp has been added to the NZ Quota Management System as it has a commercial value – apparently no longer seen as just a nuisance species to fishers. Blades can grow up to 60 cm a day. 

Many smaller brown seaweeds may be found in the low intertidal. Halo weed, Halopteris spp are tufted and resemble bushes. They are a golden brown colour and maintain a healthy infestation of epiphytes and invertebrates. Zonaria aureomarginata have fan shaped, golden tipped blades. Xiphophora gladiata is a light brown, dichotomously branched species, not common around the South Coast, but has been spotted in the Snorkel Trail area. Finally, Splachnidium rugosum, is a golden brown seaweed with cigar shaped form. Known as gummy weed, it takes mucilage production to the extreme – its swollen tubes ooze profuse quantities of sticky slime when touched.

Red seaweeds:

There are about 550 species of red seaweed, making them the largest group, though many are small and at depth appreciated more by the divers than snorkelers.

One of the best-known reds is the edible karengo (Pyropia spp), which grows on rocks near high-tide level and resembles cellophane sheets of light purple (and many other colours). This is a highly prized genus and is listed as a taonga of Ngai Tahu. Pyropia and related genera are high in proteins and vitamins. Apparently supplies of karengo were sent to the Maori Battalion in WW2, which they chewed while on the march. Pyropia cinnamomi is like a large floppy disc of cellophane. There is a wild harvest in NZ but it is not regulated as a commercial species. It is a close relative of the Japanese nori, used for sushi. One phase of the karengo life cycle involves a form which bores into the surfaces of shellfish and rocks. 

Another familiar red is the fern-like agar weed (Pterocladia lucida) which has been harvested for agar production in New Zealand since 1943. The coralline seaweeds are a group of reds that deposit calcium carbonate in their cell walls, forming pink skeletons or paint-like crusts on coastal rocks. Scientists have discovered that some crust and turf forming red coralline seaweeds release chemicals that encourage pāua larvae to settle and mature.

Weeds amongst seaweeds:

Asian kelp (Undaria pinnatifida) arrived in New Zealand waters in the 1980s and quickly made itself at home in sheltered harbours. Although it is farmed and eaten in Japan, where it is known as wakame, this fast-growing, 2 metre-tall seaweed is unwelcome in New Zealand as it can change the structure and composition of native marine communities. It is present in the marine reserve.


This note just scratches the surface of the seaweeds of the marine reserve. There are many other fascinating species to be explored and studied. Get yourself into a wetsuit and into the marine reserve and have a look!


I am indebted to Dr Wendy Nelson, of NIWA, a leading NZ expert on seaweeds. The material for this note is drawn from Dr Nelson’s Chapter in ‘The Taputeranga Marine Reserve’, published by Victoria University, and from an interview at  National Radio’s ‘Our Changing World’ (Alison Ballance) on 30 August 2012. I also value material obtained from Schools At Look4. My apologies to these sources if I have misinterpreted any of their excellent material. MH

[1] An epiphyte is a plant that grows upon another plant (such as a tree or large kelp) non-parasitically, or sometimes upon some other object (such as a roof lichen). Seaweed epiphytes grow on other seaweeds. Neither host nor epiphyte gains nutrients from each other.