New Zealand’s largest seaweeds are collectively called kelps and belong to the brown group. Because beds of kelp have such a complex structure and are able to modify their environments, Charles Darwin likened them to forests. Kelp forests may be over 20 metres tall, and support an understorey of smaller brown and red seaweeds as well as rich populations of both grazing and immobile animals.
The country’s largest kelp is Macrocystis pyrifera, known as bladder kelp in New Zealand, but giant kelp elsewhere. It can grow 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. These help to hold the kelp upright, maximising the amount of sunlight the blades receive.
In northern waters common kelp (Ecklonia radiata) dominates marine forests. This seaweed is about 1 metre in length and has a bunch of fronds arising from a central supporting stem. It is the preferred food of the sea urchin, kina (Evechinus chloroticus), 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.
Seaweeds have been stowaways to New Zealand on the bottom of vessels or in ballast water for years. A few arrived with sealers and whalers in the late 18th and early 19th centuries, but did not pose a threat to native marine life. The situation changed when Undaria arrived. At two months of age it can release millions of spores into the sea. Germinating spores will colonise any firm surface – ropes, buoys, vessel hulls, floating plastic as well as rocky reefs – and grow rapidly, displacing native seaweeds.
Bull kelp or rimurapa (Durvillaea species) is the most striking seaweed of the exposed coasts. Its tough, flexible fronds are secured to intertidal rocks by a solid disc-shaped holdfast capable of withstanding tremendous forces when storm waves crash onto reefs and cliffs. The seaweed grows to 10 metres in length and can live for 10 years. Bull kelp forests are highly productive systems, contributing vast quantities of organic matter and nutrients to coastal food chains.
Four species of bull kelp are found around New Zealand, and the most common, Durvillaea antarctica, also grows around the subantarctic islands and southern coasts of South America. In northern New Zealand it grows only on very exposed headlands and becomes more common in the cooler waters south of Cook Strait. Fronds of Durvillaea antarctica have an internal honeycomb-like tissue, full of air, which keeps the blade buoyant. The form of the frond differs according to conditions: the more exposed the site, the more divided the fronds.
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 kelp is unwelcome in New Zealand as it can change the structure and composition of native marine communities.