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A diver swims through a kelp forest in Rakiura, Stewart Island. Photo: Chris Hepburn

A new wellness trend in the big strange world is called "forest bathing" or "shirin-yoku". The idea is that people can improve their mental and physical health by spending time under a green canopy surrounded by big trees. Most of us find that time spent in the bush does make us feel better. But what about forests that actually are bathing — underwater?

Divers know that on rocky coasts, just beyond low tide, there are stunning forests up to 20 metres tall. Among the tall straight stems, in dim blue light, brown fronds wave to and fro, and fish dart about. The rocky forest floor is dappled with moving shadows, covered with a mosaic of small seaweeds, molluscs and kina. These underwater cathedrals are called kelp forests, after the giant seaweeds that form them.

Kelp is the common name for a group of big brown seaweeds, found in cool and cold coastal waters all over the world. They are very tough but flexible, and often have air pockets or hollow bladders that float them up to the light. Unlike trees, kelps can grow very fast. Bladder kelp (rimurimu) is the fastest-growing organism in the world, lengthening as much as 60cm on a good day.

Technically, of course, they aren’t plants: their “roots” don’t gather water or nutrients and are really just for holding on — so they are called holdfasts. The “leaves” do turn sunlight into energy, but otherwise don’t act like plant leaves, so we call them blades. And the strong, buoyant “stems” are called stipes, lifting the blades high up into the sunlight.

In the same way that trees provide a forest habitat for smaller plants and animals, kelps create an ecosystem under water, offering places to hide and shelter. Kelps are too tough to eat for some herbivores; their main role in the food chain is in the form of kelp flakes — little bits of the blades that break off and float away. You can buy processed kelp flakes as a seasoning, but for small marine life, they are the stuff of life.

In a big storm, kelps may come adrift. As they travel the high seas, floating seaweed rafts can carry hitch-hikers, animals which might colonise new lands. Researchers studying the genetic patterns in bull kelp (rimurapa) around the Southern Ocean have been able to learn about times in the past when the world was much colder and sea ice extended much further.

Marine science MSc student Maddy Glover has been interviewing local experts in Otago to see how kelp forests on our coastline have been changing. Her work, like other studies around the world, shows decline everywhere. It is especially concerning that we have lost a huge kelp forest, stretching from Taieri to Brighton, since the 1950s. Loss of kelp forest cover is likely to be related to declining stocks of crayfish, paua and reef fish.

It is our activities on land that most threaten kelp forests — fertilisers, pesticides, herbicides and sediments all run off into the sea, and none of them are good for kelp. Actively cleaning up our rivers and streams will help our coastal marine ecosystems recover as well.

Global warming, too, is a real threat to these cold-water forests. Gradually warming ocean temperatures may see kelp forests slowly shift southwards. But there are also sudden declines: after a strong marine heatwave in early 2018, bull kelp became locally extinct in and around Lyttelton Harbour; its place was taken by an invasive Asian kelp.

Overfishing and the loss of predators can mean that herbivores take over, and young kelps get mowed down by hungry vegetarians.

When we realise that our activities are endangering a whole ecosystem on land, we can either regulate our activities (stop cutting down trees), or choose to save a fragment of the forest (as a national park). Underwater forests may be invisible to most of us, but they deserve saving as well. Marine reserves can act as underwater national parks, but not if the water coming off the land is dark with pollution.

It’s time to clean up our act. A recent MBIE grant is supporting a team based at the University of Otago looking at ways to reseed and restore kelp forests. The rocky shore that kelps love is still underfoot; with a little help they could grow back to form the epic bathing forests that once were there.

Otago’s kelp

There are two main kinds of giant kelp found in New Zealand:

Bladder kelp - Macrocystis pyrifera.  Huge, tens of metres tall. Long stipes carry long flat blades, each with an air bladder at the base. Found from Wellington south.

Bull kelp - Durvillaea species, rimurapa.  Large, up to 10m tall, but with many fronds, some of which are an inflated honeycomb full of air pockets to keep it floating.  Prefer exposed rocky coasts in cold water. While D. antarctica is found all over New Zealand, and D. willana is found from Wellington south, the broader, flatter D. poha is found just in the deep South.

You might also see three smaller kelps, about 1m tall:

Common kelp - Ecklonia radiata. The blades are divided into lobes, held off the surface by a thick stipe.

Lessonia variegata blades look like a pile of narrow straps.

Asian kelp - Undaria pinnatifida invaded New Zealand in the 1980s. It is the edible seaweed wakame, usually found on concrete or wooden piles in harbours or ports. Blades have a rib running down the middle along their length.

Keen on learning more about brown seaweeds?  Download "Beautiful Browns" from http://www.niwa.co.nz/coasts-and-oceans/marine-identification-guides-and...

Kelp chips

Did you know you probably ate some kelp today?  Its main product, algin, is found in toothpaste, salad dressings, ice cream, cosmetics, ink, paper, paints, medicines ...

We are familiar with seaweed in China and Japan cuisines, but it also has a role in traditional Maori gathering and cooking. Bull kelp blades inflated into a bag or poha were used to transport and preserve titi. 

You can cook fish and shellfish in a kelp pouch, wrap baking fish in kelp fronds, fry kelp chips, and dry kelp flakes to sprinkle on your food. Kelp is rich in iodine, iron and vitamins; you don’t need much to get the benefits.

Abby Smith is a professor of marine science at the University of Otago.