Deep trouble

Underwater invasions can be the last straw for lakes under pressure (11 minute read)

For New Zealanders of a particular age, late forties to early fifties, the name Wilberforce conjures a sense of dread. Even now, as I write the word, I can see images from the TV series of Under the Mountain, where the Wilberforces were giant shape-shifting slugs who wanted to turn the Earth into a mud planet – believe me, it makes sense in the context of the story. Based on the novel of the same name by Maurice Gee, at the time I thought the 1981 TV series was utterly gripping, even if it was a little too frightening for me (not half as terrifying as Day of the Triffids though).

If you feel like a nostalgia trip, the whole series is available online, the first part here and the second part here. But if you don’t know it, I’d recommend the book instead. It’s still in print and available as an ebook, and it’s aged much better.

I was thinking about the story because I’ve been thinking about lakes. The lake I knew best as a child was Lake Pupuke, on Auckland’s North Shore, and Under the Mountain was set on the shore of that lake. I have a lot of different memories of it – its appearance in Under the Mountain, the theatre at the historic Pumphouse right beside the water, the ever-present black swans with their bright red beaks, my father sailing his Laser, a racing yacht that was usually sailed single-handed, although he took me out on occasion. And the waterweed. The waterweed was particularly memorable.

Pupuke always had waterweed. From the edge of the shore to about three metres out, there was a solid mass of waterweed around almost the whole lake. The leaves were like long straps, a couple of centimetres wide, all tangled and twisted on the surface of the water. I was warned that people had drowned after becoming caught in it, so it wasn’t just a nuisance, it was deadly. Pupuke’s waterweed frightened me almost as much as the Wilberforces.

Pupuke waterweed (image credit: Peter Newfield)

I never knew it at the time, but Pupuke’s waterweed problem was unusual in New Zealand. Some of the invasive waterweeds in New Zealand are widespread. But the species with the tangled strap-like leaves is relatively uncommon. For nearly 100 years, Pupuke was one of the only known sites.

As I’ve looked at issues with lakes and rivers in New Zealand, one area I haven’t covered in any detail is the one I actually know best – invasive species. Although I wrote about the catfish and weeds in the Rotorua lakes, and the work that Te Arawa Lakes Trust is doing to fight those problems, there’s a lot more to it. It’s an area I worked on myself, half a lifetime ago, when I was living in Nelson. At the time, I was helped by scientists from the National Institute for Water and Atmospheric Research (NIWA), so I decided to contact them to help bring my knowledge up-to-date.

At a conference last year, I talked with Dr Deborah Hofstra, who leads NIWA’s freshwater biosecurity programme. Later, I spoke with her via Zoom. It turned out that she also remembers Lake Pupuke from childhood, but it was in her university years that she developed an interest in waterweeds.  

“After I did my B.Sc., there was an opportunity to do a Masters that was looking at native aquatic plants. That was really when my natural curiosity about the world around me and science joined-up in the aquatic space. I was looking at genetic variation in native aquatic plants and I visited a whole bunch of Northland lakes and also went down to Waikaremoana and Waikareiti. Then after doing my Masters, I got into invasive aquatic plants for my PhD and looked at the weed hydrilla.”

If you’ve ever had a goldfish, you’ve probably encountered a plant known as “oxygen weed”¹. There are a number of related plants known by this name, four of which have been introduced to New Zealand. The weeds that I wrote about in the Rotorua lakes a few weeks ago are types of oxygen weed. They cause serious problems in our waterways, forming dense growths which crowd out native species, block access for swimmers and boats, and clog irrigation channels. They are also a serious problem for hydro-electricity production.

Overseas, hydrilla is the most troublesome of all the oxygen weeds, but in New Zealand it was only known from a few lakes in the Hawkes Bay. Experts were watching it and worrying, but it seemed as if nothing much happened for years.

Deborah agrees. “It felt like a bit of a hot potato in terms of who was going to do anything. There was still some debate about whether or not it was the same thing here as overseas and whether it would be as bad here. There was a lot of conversation around how bad it would be in New Zealand, which led to my PhD research topic, investigating the invasion biology of that species. How bad could it be compared with some of the weedy species we already have? Would it be worse?

“We did a bunch of studies that looked at growing hydrilla beside other waterweeds. It could absolutely hold its own and then some. The other challenge with hydrilla is that it has long-lived tubers, which the other plants don’t have. The tubers would always give hydrilla the advantage and that’s the challenge when it comes to its eradication – the time for which an eradication response programme needs to be in place to be sure that you’ve got it all.”

Deborah’s PhD and subsequent research contributed the science to an eradication effort, led by the Ministry for Primary Industries, which began in 2008 and is still underway. The reason it has taken so long is that hydrilla can regrow from long-lived tubers buried in the sediment at the bottom of a lake. There needs to be a way to control plants quickly when they resprout, before they can re-establish. But how can that be done when they are growing on the bottom of a lake?

The answer may surprise you – use fish, specifically a type of fish called grass carp. At first glance, this might not sound like a great idea. What if the fish eat other plants? What if they escape and become invasive themselves? It’s true that grass carp do eat other plants, but in the lakes which were invaded by hydrilla, native plants were already severely restricted by the hydrilla which had taken over their habitat. Grass carp also have very specific breeding requirements, which make them unlikely to breed here, something which was carefully tested in containment decades earlier by fisheries scientists before they were used. If they escape from those lakes, they aren’t going to become a pest because they would get washed out to sea. But escape is still a problem, Deborah tells me.

“For them to be effective in terms of weed control, you need to be able to contain them. You can think of them like sheep, if you can’t keep your gate closed your sheep will get out of the pen and they won’t be eating the grass. The grass carp are similar in that way, you have to be able to contain them in the lake. But in the recent past we know there have been some high water events where the overflow barrier in that lake will have been topped by water, providing a mechanism for the fish to be able to leave.

“It was six years since we’d seen hydrilla in that lake and longer in the other lakes. The native plants do have seed banks and we had seen wonderful native plant recovery once the hydrilla was gone. It’s really at that stage where it is that last 5-10%, where you know there’s still going to be a lot of effort to get that last bit, but it is very close. And of course now there’s been a cyclone, with a massive amount of turbidity in the lake, and high water that will have facilitated the fish leaving the lake.”

Even without hydrilla, though, other submerged waterweeds are a problem in New Zealand. Deborah tells me about the three worst of them. “There’s lagarosiphon which has moved its way down the country, it’s in several lakes in the South Island. Hornwort, where the plan is to ensure that it just stays only in the North Island² because of its massive impacts, and egeria is the other one. A tricky thing with invasive species under the water in lakes is that often people don’t see the impacts until it is almost too late. In other words, when people start to encounter these weeds, it’s usually because there’s some drift material that has accumulated on the shore and it’s smelly or unsightly, or some of it gets caught up in their outboard motor.

“But by the time that’s happening, the impacts under the water are already quite significant. They are space invaders essentially, invading the space where the native plants already existed. Native plants generally occupy the shallow water or the littoral zone within a lake, but these invasive species are more competitive, they grow bigger, they grow faster, they grow more densely and so they overtake those spaces, that is, the habitat of the native plants, and of course when those spaces get overtaken, then all of the other creatures, so not just the plants but the fauna [the animals] that were living on those native plants, their habitat changes too

“That can mean things like, at water depths from about one to five metres, it can be totally full of invasive waterweeds. And the sediment underneath those weeds becomes anoxic [lacking oxygen]. The kākahi [freshwater mussel] that would have been present when there was a less dense native flora, also become displaced. It’s the same in terms of the smaller creatures, like the invertebrates that are on some of the plants. They might be on the top and outside edge of an invasive weed bed but again, because it gets so dense in the middle of a weed bed, the habitat is quite different to the comparatively more open growth-form of the native plants. So the changes are quite significant in terms of species displacement.”

Waterweeds, though, aren’t the only problem. Many of our waterways also have introduced fish species. In fact, Deborah tells me, they might be our most damaging freshwater pests. “Species like koi carp, when they feed they bioturbate, or turn over sediments. Even in their spawning behaviour they’re really scuffing up and disturbing the sediment. And you’ve already got more sediment going into aquatic ecosystems from catchment land use change, for example. This creates shading for submerged plants, either shading because of sediment that’s in the water or there’s shading because of algal blooms in the water. The algae are being fuelled by the nutrients that are coming in and so that puts pressures on the plants too.

“There’s a cumulative effect of these added pressures or added stressors on aquatic ecosystems. When some of the invasive species arrive, that can be the last straw leading to ecosystem collapse or degradation. But even without these added pressures, invasive species can create change all on their own. We describe them as transformer species, because they will transform the environment.

“You end up with a situation where the pressures become so great that the conditions where native flora and fauna can thrive aren’t there anymore. It’s like a pendulum that’s gone from one side all the way over to the other. It doesn’t take long before you have this situation where a lake can ‘flip’ and then it goes from being in that clearer water state which has its native plants, through to a degraded state which is generally high in nutrients, algal dominated and turbid water and the challenge of course is how to flip it back.”

This brings me back to what is happening in Lake Pupuke. It’s got several different types of waterweed. It’s got fish such as koi carp. Over the last twenty years, the remaining native plants have been declining. At the same time, blooms of toxic algae have become more frequent. Is there anything which can be done?

There’s an important lesson I learned from my own work on managing invasive plants. The point is it’s not just about controlling the invasive species. If it was, it wouldn’t be such a difficult problem. Killing things is comparatively easy. Killing some things but not others is more difficult. But when it comes to restoring lakes, it’s even more difficult than that. We haven’t only changed the balance of species in our lakes. In some cases, the physical structure of the environment has been changed. It’s not like taking an area of pasture, planting a whole lot of native trees and waiting for them to grow. It’s much, much more complicated.

The scale of the challenge is huge – something I hadn’t appreciated until I spoke with Deborah. So I’m going to split this article in two and will devote next week to looking at what it might take to restore our lakes.

Windfoiler on Lake Pupuke (image credit: Peter Newfield)

1. The name is misleading and unhelpful, because any submerged plant, including native species, will release oxygen into the water.

2. I’ll tell you more about this one day. It did reach the South Island and I played a part in eradicating it from there.