The disappearing forest

Why is seaweed in trouble - and why does it matter? (12 minute read)

image credit: Melanie Newfield

A piece of my family history lies in the middle of Wellington Harbour. During World War One, Matiu-Somes Island was used as an internment camp for men deemed to be ‘enemy aliens'. Most were German citizens, but there were also a number of citizens of the Austro-Hungarian Empire. Among that group was my great-grandfather, Marian Devcich. He had no great sympathy for the Austro-Hungarians, who had occupied his birthplace, Dalmatia, for the previous century. But his citizenship made him vulnerable. He was reported to the authorities as ‘unpatriotic’ and was arrested on his farm at Hikutaia, near Thames. From there, he was taken to Matiu-Somes, where he remained for seven months.

Matiu had originally been a strategic pā and place of refuge for local Māori. Around 1840, it was purchased under questionable circumstances by the New Zealand Company and renamed Somes Island. For a while it was leased, then it was classified as a reserve, but it was mostly unoccupied for a number of years. Then, in 1872, the ship ‘England’ arrived in Wellington with smallpox on board. Matiu-Somes was used to confine the passengers and crew until the disease ran its course. Apart from its interlude as a prison camp, Matiu-Somes remained a human quarantine station until the 1920s, and was then an animal quarantine station until the 1990s.

These days Matiu/ Somes tells a story about restoration and recovery. Rats were eradicated in the 1980s. The New Zealand Geographic Board confirmed the name Matiu-Somes in acknowledgement that it was important to both Māori and Pākehā history. It was handed back to a collective of local iwi in a Treaty settlement, and is now managed jointly with the Department of Conservation, as a reserve which is accessible to all via a regular ferry service. Rare birds, reptiles and insects have been reintroduced. The trees are being replanted too, and in a hundred years or so much of the island will look very much as it did before humans arrived. The forest will form a tightly-enmeshed canopy, in defence against the relentless wind. Seabirds and tuatara will shelter in burrows beneath. Where conditions are too harsh for forest, the mis-named mountain flax will dominate.

Mountain flax is one of nature’s survivors. It grows from one end of the country to the other, from the coast to the mountains – which is why the English name is a misnomer. I prefer the Māori name for mountain flax, wharariki. It has a music which calls to mind the constant movement of the leaves in the wind. Wharariki grows in places that humans and other mammals find hard to reach and have little use for, like cliffs and rock stacks. It tolerates the dry and the wet, the hot and the cold, the salt-laden wind. As global, and local, temperatures rise, wharariki is unlikely to be troubled.

But there’s another story about what is happening with the environment of Matiu-Somes, and it’s far less positive. Despite my interest in the island, I knew nothing about it until very recently. But then, a few months ago, I heard that the kelp forest around Matiu-Somes had disappeared, and that the disappearance had happened within the last few decades. I realised that there was another story which needed to be told.

Kelp is a type of seaweed, however neither term is precise to a biologist, so please forgive me for the diversion into seaweed classification. There are a number of different types of seaweed, but the most conspicuous are those known as ‘brown algae’. If you’ve noticed seaweed on a beach, it was probably a type of brown algae. New Zealand has around 1600 species, of which I can identify only two, which is why I brought in some experts for this article – more on them in a moment. ‘Kelp’ refers to a couple of hundred species of brown algae which grow attached to rocks and can grow very large. The largest species, giant kelp, which is found in cooler waters around the world including New Zealand, can reach 50 metres long. Kelp is an important group because it forms underwater forests which support numerous marine species. Kelp forests are among the most diverse marine environments.

So, onto the experts. One is Chris Cornwall from Victoria University, who spoke to me about coral and climate change a few months ago. He also happens to study seaweed. In fact, he got into coral research from his work on seaweed, and I can see how much he cares about it. The other is Zoe Studd, cofounder and Executive Director of the Mountains to Sea Wellington Trust, an environmental group which is connecting people with nature around Wellington. Zoe is passionate about making sure that people can connect with undersea environments as well as those on land. One of Zoe’s projects is called ‘Love Rimurimu’ – rimurimu being the Māori word for seaweed – and it’s that project I wanted to talk with her about.

Wellington Harbour once supported great kelp forests – but we don’t know this from science. Chris tells me that “there’s no really good baseline because none of the scientists have been very interested in the harbour. We know where the kelp forests were in 1990 because there was a survey, but not before that”. Instead, I find out from Zoe, who has drawn her knowledge from fragments outside the scientific record. “You see pictures – Nancy Adams was this amazing seaweed expert and she took photos of these drifts of seaweed after a storm and they’re metres deep. That’s how much seaweed there was getting ripped out and dropped on the coastline. We’d never see that now. And we know from old names and pūrākau [Māori sayings], we know from observations when people first came in and out of the harbour, we know from people who say ‘I used to have to cut a path through the seaweed to get my boat out here’. But it’s hard to quantify.”

It’s still possible to see magnificent kelp forest in Wellington, Zoe says. “Taputeranga Marine Reserve [on Wellington’s south coast] has the most incredible seaweed forest. The south coast is amazing. It’s really nice to take people snorkelling and show them the healthy forest.”

The kelp forests on the south coast may be healthy, but inside the harbour is a different story. The forest has been devastated, and this isn’t something which happened in the distant past. A lot of the kelp forest recorded in 1990 has now gone, and it’s still happening. Chris tells me: “last year we went out to survey a bed that was present the year before and it had disappeared.”

Both Zoe and Chris tell me a similar story – the biggest problem is sediment. This is familiar ground for me, because as I’ve looked at problems with fresh water, deforestation and erosion, the issue of sediment in waterways has come up again and again. Basically, the more we remove forest from the land, the more our soil washes into streams and rivers and eventually ends up in the sea. Once it gets there, it creates a real problem for seaweed, and everything else which depends on that seaweed.

Part of the problem is that sediment reduces the light getting through the water, Chris tells me. “You get seaweeds which can grow to a certain depth, but when you get more sediment in the water, their depth range will shrink and they can only grow in more shallow water. Also, since the sediment blocks the light, they’ll be photosynthesising less and so there’s less food, less habitat, for other species.”

As well as affecting the adult kelp, sediment can also prevent new kelp from growing. Zoe explains: “Very fine particles of sediment act like a bit of sandpaper on the rocks. So really young seaweeds trying to settle, they’ll get scraped off. And thick sediment can bury rocky reefs. A pile of sediment isn’t much use to a seaweed, because it’s got nowhere to settle.”

Both Chris and Zoe tell me that sediment is the main threat to kelp and other seaweeds in Wellington Harbour, but it’s not the only threat. Climate change is a problem too. “Wellington is near the northern most extent of our giant kelp forests,” Zoe says. “As the waters get warmer, it’s likely that our kelp forests aren’t going to be able to survive inside the harbour.”

It isn’t only New Zealand where giant kelp is threatened by climate change either. Chris tells me that its declining on the northern edge of its range in other parts of the world too, such as Tasmania. The last couple of years have been particularly bad, with high sea temperatures around New Zealand linked to La Niña as well as climate change. “It looks like the big marine heatwave we had last year has had flow-on effects to the kelp already in terms of its reproduction. If we keep having these marine heatwaves and temperatures keep warming, then that kelp is going to just disappear because of an inability to reproduce.”

This is where giant kelp contrasts with a species like wharariki. As a species which tolerates a wide range of temperatures around New Zealand, wharariki is ready to cope with climate change. Giant kelp, growing here at its upper temperature limit and already stressed by sediment, will suffer.

But there’s another threat to kelp and other seaweeds, and it’s one I’d never heard of or thought about until I spoke with Zoe and Chris – fishing. It’s obvious that taking too many crayfish or snapper is bad for the population of crayfish and snapper, but it also affects other species. Crayfish and snapper are predators, and they used to be among the most important predators in coastal environments. But both commercial and recreational over-harvest has seen dramatic declines in these species. In the Hauraki Gulf, off Auckland’s east coast, a 2015 study found that crayfish had declined by 76% and snapper by 83% since human arrival. This means that an area which used to have 100 crayfish would now have 24, and an area which once held 100 snapper would have only 17.

By reducing predators like snapper and crayfish, over-fishing has allowed prey species to increase. The most important of these is kina or sea urchin. Kina grazes on seaweeds such as kelp, and as kina numbers have increased, there has been a decline in seaweed. The problem was first noticed by divers in the 1960s, and is now so acute that affected areas have a name – they’re known as ‘kina barrens’. As the name suggests, there’s nothing much there except bare rock and kina. Zoe tells me: “it’s actually a pretty awful thing to see a reef where you’ve got a whole lot of kina and the kina are all starving as well. There’s no roe in them, they’re skinny kina. They’ve got nothing to feed on because anything that does try and grow, they’re grazing it right down. People often say ‘we can remove the kina’, but the kina aren’t the problem, it’s the imbalance in our ecosystem that’s the issue. We’re the top of that food chain, we’re causing it.”

Although I was specifically interested in Wellington Harbour when I was talking to Chris and Zoe, it’s clear that kelp forests, and other types of seaweed, are under threat elsewhere too. Chris particularly mentions the impact that the massive amount of sediment from erosion around the East Cape is having on seaweed and the marine environment there. But sedimentation, overfishing and climate change are worldwide problems. Are seaweeds declining everywhere?

I started looking, and the more I looked, the more I found. A combination of climate change and sea urchin population explosion has seen a major loss of kelp forest in northern California. Tasmania has lost 95% of its kelp forests. The seaweeds of Britain are declining. A unique seaweed forest off the coast of Oman has disappeared. Even farmed seaweed, such as nori in Japan, is suffering.

Should we be worried? Both Zoe and Chris are clear that we should be. Seaweeds, especially ones like giant kelp, are the foundation of our coastal environments. Zoe says: “they’re habitat, they’re a food source, they absorb carbon and release oxygen, they provide shelter and shading.” Chris also notes that while our kelp species are found in other places, many of the species which depend on kelp are unique to New Zealand. “There’s a lot of little red seaweeds that are endemic to New Zealand that live in these kelp forests, and when you remove the kelp, they can’t survive in any other kind of light quality.”

Seaweeds are useful to us too, Zoe points out, and not only because they support other marine species which we value. “They’re an incredible food source that’s been used for eons traditionally and continues to be used. And there’s amazing innovation potential in seaweeds. They have potential for bioplastics or biofuels and amazing biodegradable products. We work with a really cool company that make a seaweed wetsuit lubricant that we use on our wetsuits, to get in and out of our them.”

Is there anything which can be done to halt the decline of seaweed, and especially our giant kelp forests? At one level, the answer is obvious. We need to limit climate change, protect our land from erosion and reduce overfishing. But when it comes to our forests on land, we know we need to do more than halt the decline – we need to restore them. Can we restore kelp forests? Zoe is part of an ambitious programme trying to do precisely that. But, as you’ll see next week, it turned out to be more difficult than she ever expected.