Handy seaweed facts


South coast Wellington rockpool

Next time you’re asked a seaweed question in a pub quiz you will thank me for this post.

But don’t get too carried away with you’re praise of my knowledge because the world of seaweed is complex with thousands of species.  Below is an overview of a few things I have picked up during the project. If you really want to get the full story I recommend reading some of the books from my previous post.


“Seaweed” is not a weed, and isn’t exclusively found in the sea.
The name “weed” implies seaweed is a fast growing nuisance plant. Although it is relatively fast growing and can be a nuisance to us at times, seaweeds are actually classified as algae – photosynthetic organisms which, with the exception of the green species, is only remotely related to land plants.

Although they are most prolific in salt water, they can be found in both marine and brackish environments like estuaries. A few have even been found inland-locked fresh water far from the sea.

Seaweed comes in three colours
Brown, green and red. From a creative person’s perspective this description is a bit simplistic because there definitely some that are coloured blue, purple, yellow, black and white but scientists group them into brown, green and red because, colour aside, the three groups differ from one another in profound ways – their pigments, reproduction, life histories and evolutionary backgrounds.

Often white on seaweed is a sign of stressed seaweed, as I found out by keeping it out of it’s normal environment too long.

Seaweeds don’t have roots, flowers, fruits or cones
Although they often look like plants, the different parts of seaweed do not have the same specialist roles that the leaves, roots, trunks and stems of plants do. Many parts of a seaweed can perform the tasks of nutrient absorption and photosynthesis to some degree. They have no special cells for the transport of water or nutrients.

The root-like structures that connect the algae to the seabed are called holdfasts. Roots on land plants draw nutrients from the ground but most of the outer layer of a seaweed can absorb nutrients from the water. Some algae lives free-floating with no permanent attachment at all.

Some algae have evolved floats and long stems to place parts of it closer to the light allowing it to survive in deeper water. The deepest recorded seaweed in New Zealand has been found in the deep clear waters of the Kermadec region at 200m.

They come in all sizes.
There are about 900 known species of seaweed in the New Zealand region with many more likely to be discovered. Due to our unique geographic location many are endemic.

They vary in form and size considerably, from the microscopic to forests of massive kelp as tall as giant redwood trees. Some can grow up to half a metre a day.

In general terms the warmer the water, the smaller they tend to grow, as I discovered when I tried to find seaweed in Wellington that also grows in the Kermadec region and found the only one in common was really quite tiny. Conversely Dr Wendy Nelson from Niwa said the kelp in Antarctica can grow to truly impressive size.

Although some occur throughout mainland New Zealand there are distinctive northern and southern species. In particular those found from the northeastern  North Island to Bay of Plenty differ markedly from that growing south of Kaikoroura.

Seaweed is important
It has a vital role in the environment. Many fish and invertebrates find refuge and habitat in algal beds. Kelps in particular are known to be very productive in terms of the carbon dioxide they fix.

Although some seaweeds are eaten directly by herbivorous fishes, or by invertebrate grazers such as pāua and kina, much of the productivity of the seaweeds enters the food chain indirectly. The mucilages and, slimes and films produced by algae is ideal food for bacteria, fungi and various protozoans, which in turn are eaten by filter feeders. Fragments of seaweed broken free by the sea swell break down to be consumed by a wide range of organisms. Beach-cast seaweeds may be annoying when you sunbathe but they provide valuable nutrients for coastal food chains as many gardeners will testify to.

We use seaweed for much more than fertilizers or soil conditioners. They are used in a wide range of products including scientific agar gels, ice cream, cosmetics, medicines, fabric dyes, and of course food – in Japan for instance, it constitutes up to 10% of the average diet.

UK based company Seaweed & Co is a good read for more on the commercial use of algae


Some of my seaweed storage and water transport madness

Keeping seaweed happy while photographing it
Seaweed varies in it’s robustness and ability to stand up to different conditions. Some can survive quite a while out of the water or even short bursts in fresh water but in general terms it is best to keep it in seawater as much as possible. Changing the water daily provides vital nutrients and I used a ‘bubble stone’ from a fish tank to stop the water becoming stagnant.

During the day all seaweed requires light and carbon dioxide to photosynthesize (producing oxygen). They also respire day and night producing carbon dioxide.

To play it safe I kept them out of direct sun to stop them getting ‘burnt’. Some lasted for many days while others deteriorated after a day or so.

Seaweeds by David Thomas. ISBN: 0-565-09175-1
NZ Seaweeds an illustrated guide by Dr Wendy Nelson. ISBN: 978-0-9876688-1-3

Thanks to Wendy Nelson for her conversation, time and patience in supporting this project.

More on Project Weed >>