Something in the water

What is turning our lakes and rivers toxic? (10 minute read)

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The water of the Waikanae River is so clear that I can see every grain of gravel and sand on the riverbed. It flows by me, the smooth surface visible through a gap in the grasses which line the bank. Standing still, I can hear the river’s music of water riffling over rocks. I want to plunge my hand into its cool, clear flow. I want to submerge myself, refreshing my body in the chilly stream. I know how it will feel against my skin, and I take a moment to savour the sensations in my mind. There is something about running water, it is life-giving, restoring, invigorating, mesmerising.

The sight of the water swells my soul, but my head isn’t so easily swayed. Nailed to the bridge there’s a warning about toxic algae, something which blooms in many of our rivers during late summer. The sign says it can make people sick and kill dogs, so I know it’s dangerous. It’s a reminder to me that all is not well in our rivers. Just because the water looks pure, it doesn’t mean a river is healthy.

To scientists, toxic algae are not algae at all, but a type of bacteria known as cyanobacteria. We seldom hear much about them, but they are common around us and are remarkably adaptable. In the ocean, they are an important part of the group known as phytoplankton. In fresh waters, they can be found clinging to rocks, floating in the water or even drifting as scums on the surface. They are common in thermal springs, such as those found near Rotorua. But they aren’t found just in wet areas. In deserts, cyanobacteria grow as fragile crusts on the soil surface, performing a crucial role in holding the soil together. Some are even found in Antarctica, in pools of meltwater, in the soils of the Dry Valleys and even growing inside rocks.

So, cyanobacteria are diverse and widespread, but if that is the case, why don’t we hear about poisoning more often? Is it a particular type which is dangerous? Is there something about the water in our rivers that creates the problem?

These questions, it turns out, aren’t simple to answer. But, as I continue my reading, there’s a name which keeps coming up: Dr Susie Wood, at the Cawthron Institute in Nelson. So, I get in touch and she very kindly agrees to talk to me about her research, which began when she was a student and realised that there was little known about toxic cyanobacteria in New Zealand.

From Wood, I learn that there’s more to cyanobacteria than I’d ever imagined. “If we were to go back 3 billion years,” she tells me, “they are the organisms that oxygenated earth’s atmosphere. In a way we owe our existence to cyanobacteria.”

I had no idea cyanobacteria were so old. Algae, such as seaweed or the green slime you might see in a pond, is a comparative newcomer at just over 1.5 billion years old. Jellyfish, one of the earliest types of complex animals, date back only around 500 million years. The first plants appeared on land at around the same time.

Wood makes the point that cyanobacteria are an important part of many environments. “Many of them are not harmful or undesirable at all, in fact they’re really important parts of the food web.” But, under certain conditions, cyanobacteria can be a real problem. “When we get certain environmental conditions, some species will form blooms. Many of them produce toxins which are harmful to humans and animals that come in contact with them.”

When Wood talks about “blooms”, she doesn’t mean a bloom like a flower. A bloom in cyanobacteria means rapid or excessive growth. In a lake, a cyanobacterial bloom can turn the water an unnatural shade of milky green, as seen in this photo of Lake Moawhitu in Marlborough. In a river, it can blanket the gravel in thick black or dark brown mats.

Lake Moawhitu (image credit: Susie Wood)

Even if cyanobacteria didn’t produce toxins, blooms would be a problem. Although they produce oxygen, when they grow to excess, they can actually suck oxygen from the water. This happens for several reasons. Partly, it’s because cyanobacteria (and plants) only produce oxygen during the day, when there’s sunlight available. At night, they use up the oxygen they produce. It’s also because they block light from reaching plants under the water, prompting them to die off and decompose, a process which uses up oxygen. The bloom itself can start to die off and decompose, using up oxygen too. Without oxygen, fish and other aquatic animals die. These kinds of die-offs are sometimes seen in lakes around New Zealand. A quick internet search shows up a number of examples, including one in Te Awamutu in 2021.

But some cyanobacteria produce toxins, meaning they are a danger to more than just fish. Livestock drinking the water are at risk. Wood tells me about a lake I can remember from my time living in Christchurch, Te Waiwera or Lake Forsyth. “Back in the 70s there were reports of stock dying that had access to the lakes there. Te Wairewa has had a long history of cyanobacterial blooms, with a particularly toxic species found in the lake.” Although Wood mentions that there’s work underway to try and manage the problem in Te Waiwera, warnings about toxic blooms there have been reported as recently as February of this year. There was a report from 2016 of 30 sheep being killed by drinking water from the lake.

Humans are at risk from the toxins too. Most people aren’t likely to drink such obviously polluted water, but skin contact can be harmful, so swimming or boating in an affected lake is a risk. And, Wood tells me, the toxins can accumulate and can be present in fish from affected lakes. “It’s not too much of a health risk if you gut the fish and you wash it down and you’re not eating it every day. If you were eating those fish every day, then it’s a health risk but most people don’t do that. We worked out if you eat fish from Rotoehu (Rotorua) you had to eat about 200 grammes a day for extended periods for there to be a health risk.”

I ask Wood about water birds like ducks and geese, because I’ve heard about them dying in some polluted lakes, but that’s an area where the science is still uncertain. “You do see a lot of dead water fowl around Te Wairewa which are probably related to the toxins, but we’ve never conclusively proved it. Overseas, there have been cases. In America, bald eagles have been dying and they eventually have traced that back to cyanobacteria which grow associated with aquatic plants.”[1]

Rivers also have problems with blooms of cyanobacteria, but the situation is slightly different. In rivers, the cyanobacteria cling to the rocks on the bottom of the rivers, sometimes forming thick mats which break off and wash up on the banks. This is another area that Wood has worked on. “In 2005 we had a spate of dog deaths at the Hutt River in Lower Hutt, and that was the start of a whole lot of work on the cyanobacteria that grow on the bottom of rivers. When that happened, we thought it was an isolated incident, but actually every summer since then we’ve had another river that’s got problems. Often it’s found by a dog that eats the mats and it’s a pretty horrific death. For a 20 kg dog as little as a teaspoonful can be fatal for them. It’s a neurotoxin (affects the nervous system) and it’s up there with some of the most toxic compounds in the world. The reason so many dogs die is they’re attracted to the smell when it washes up on the shore. Whereas fortunately humans are unlikely to eat the brown, musty mats. But we always worry because children playing at the edge of rivers could pick it up and put it in their mouths.”

The picture is more complicated than just an overgrowth of cyanobacteria, though. “Within the species we have toxic and non-toxic strains and they live together. Sometimes you’ll get a completely non-toxic mat, sometimes you’ll get a completely toxic mat and sometimes a mixture of both of those.”

The obvious question to ask about the toxins is why the cyanobacteria produce them, but there’s no clear answer to that question. “There’s lots of conflicting reasons out there in the literature,” Wood says. “The common ones are defence against predators that like to eat them, or as a response to environmental stresses. But I think the first explanation is unlikely because these are really ancient toxins. People have done studies which have shown that the toxins evolved well before the predators did. I think they don’t necessarily serve one function; they probably have multiple functions for the cyanobacteria, most likely around regulating stressful situations. But no one has cracked that one yet.”

I find this kind of thing fascinating and could fill many more pages with details about cyanobacteria and their toxins, but there’s something much more important I need to cover. Why is it that our lakes and rivers are experiencing these toxic blooms?

The short answer to this question is “water quality”. Many of our lakes and rivers are in poor condition, and the cyanobacteria are a symptom. But why are our lakes and rivers in such a bad way? Here, Wood’s current research on lake health has some real insights. “The main driver is land use change and land use intensification. Removing all the native forest had some effect, but when we look at our data, around the 1940s, 1950s when they started aerial topdressing and added fertiliser to the land – as soon as we started throwing fertiliser on those landscapes, that just had a huge impact on our freshwater, our lakes in particular.”

This is something I do understand, because I’ve looked at nitrogen and phosphorus fertilisers before. They’ve made a huge contribution to our ability to grow food, but they’ve also polluted our waterways. But then Wood makes a point which has never occurred to me, and makes me realise just how serious the situation is.

“Lakes are really challenging because they receive everything that happens in the land around them and it gets stored in the bottom of the lakes. With a river you can plant the river banks and you can remove the nutrient sources and most of them will be flushed away. But in lakes, the nutrients are stuck at the bottom of the lake and at certain times, particularly in summer, the nutrients from the sediment are released back into the water of the lake. And those nutrients are food for algae and food for cyanobacteria, that’s what they thrive on.”

The problem, then, is that we are flooding our rivers and lakes with excess nutrients, causing the excessive growth of cyanobacteria. But while we can, in theory, fix rivers relatively quickly, lakes are a different problem. “When we talk about lake restoration, it’s not just stopping what’s going into the lake, you’ve also got to try and manage what’s already in the lake. That’s incredibly challenging. It’s a problem in all lakes but particularly in large lakes. We need a huge amount of action and resources, and to start thinking on long time scales.”

As I’ve been writing this article, I’ve realised that there are many more issues around water quality, and I’ve barely broken the surface. Water is one of New Zealand’s most valuable assets, and yet our waterways are in a terrible state. Nearly half of New Zealand’s lakes, Wood tells me, are in a poor or unhealthy condition. So, I will return to the issue of water quality in two weeks, when I will also look at what climate change means for our waterways.

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[1] As an aside to the discussion on cyanobacteria, while I was researching this article I found out that there’s another toxic type of bacteria which has killed thousands of birds in New Zealand this year alone – the type which causes botulism in birds. I will write more about that in a couple of weeks, when I take a closer look at water quality, because I’ve realised that there’s much more to the story than just cyanobacteria.

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