Like flooding, heatwaves or drought, toxic algae blooms are yet another marker of a changing climate.
Cyanobacteria is ancient and a natural part of the environment, but when the conditions are right — or wrong — an imbalance is created.
Toxic algae blooms form, and regional councils issue alerts and information on how to recognise the danger.
In lakes and ponds, it looks like pea soup, and in rivers, it looks like black-brown tar on rocks or mats on the riverside.
University of Otago department of botany associate professor Tina Summerfield says the toxic blooms that have in recent summers posed a threat to dogs, livestock and even humans are likely to stay and, if New Zealand follows global trends, could increase.
"I think it would be really hard to get rid of cyanobacterial blooms," Prof Summerfield says.
"All of the literature, global analysis, says ... this isn’t controversial at all, says that it’s predicted that the occurrence and persistence of cyanobacterial blooms will increase."
It gets difficult when talking about trends in specific regions, or an increasing number of toxic blooms here year on year — it gets too "granular", she says.
The number of toxic algae blooms will vary year to year as it depends on the weather, the amount of rainfall and the temperature.
"You would expect that New Zealand would follow the global trends — I think that’s about as bold as I am prepared to be."
However, climate change, increased agricultural runoff and urban development are some of the factors behind the global trend, she says.
Cyanobacteria have evolved over time to fit a variety of niches.
Given the right conditions, they harvest light energy and in the context of blooms, they use that light energy to increase their biomass and produce toxins, she says.
"I think it’s a big challenge for the councils, and potentially a big problem."
Otago Regional Council lakes scientist Hugo Borges says the number of toxic algae blooms has increased compared with decades ago, but now people’s awareness is up also.
Cyanobacteria occur naturally in the environment, but in controlled, or low levels.
There has been research into algal blooms that shows how the environment changed after European settlement, he says.
Because of development such as farming activities and urban growth, more nutrients are entering waterways, which feeds the algae.
When there are more nutrients, the naturally occurring cyanobacteria thrive and outcompete other algae.
"With warmer weather and the addition of nutrients to the water bodies, then consequently you start seeing algal blooms everywhere."
But adding to the appearance of an increasing trend was that 20 years ago, people did not know much about them, he says.
"You can see a trend of increasing toxic blooms.
"However, we are increasing our monitoring capabilities as well.
"There are more reports on it, there is more knowledge."
Notably, in Otago, the places that usually have algal blooms are still "pretty much" the same.
"We don’t see these trends of increase of cyanobacteria specifically ... maybe we can say that in some areas there’s a trend of increased chlorophyll a, which is a proxy for algae."
But not all algae is toxic. An extensive search for cyanobacteria in lakes where there have never been reported blooms, will probably result in finding algae that are potentially toxic — in very low amounts, Mr Borges says.
"It’s part of the ecology.
"They’re a naturally occurring organism in water bodies."
With cyanobacteria, that’s not the concern: rather than prevent the entry of the cyanobacteria into other water bodies, the focus is to prevent the conditions that might allow them to get out of control.
While there have been several reports of animal deaths, there was only one report of a death of a human linked to toxic algae that dated back to the 1990s, he says.
Nevertheless, there is the potential, which is "very alarming".
"What happens in a bloom, it’s an imbalance, right?
"Something that was there, completely in balance with other organisms, now took over the whole environment.
"But also for us humans, they’re toxic, they’re lethal, they can actually kill people and they can kill animals," he says.
Environment Southland surface water senior scientist Katie Blakemore says, in Southland, river flows have been low and stable this summer so far and water temperatures have warmed, providing conditions where blooms are likely to form.
The council reviews its monitoring of water resources regularly.
However, in relation to toxic algae, a key focus is educating the public on what to look out for and where they can find further information (council websites).
"Toxic algae can form blooms when environmental conditions are favourable, and it is entirely possible that toxic algae blooms could occur at sites where we haven’t recorded them before."
Q&A
The Otago Daily Times asked Otago Regional Council water quality senior scientist Rachel Ozanne about toxic algae.
It seems there have been more reports of toxic algae this year than usual. Is that true?
In the ORC’s regular lake sampling locations, there have not been more reports of toxic algae than usual so far. We have set up warning notices this season at Lake Waihola and Butchers Dam for short periods. The situation was similar last year. We have detected no toxic algae this season in Lake Hayes, Pinders Pond, Tomahawk Lagoon and Falls Dam so far. In rivers, permanent warning/information signs are present for the Silverstream, Manuherekia near Ophir and the Waianakarua River near SH1. This year the public notified ORC of toxic algae in Hanley’s Farm Creek and the Clutha River near Roxburgh.
How many alerts has the council issued over the summer period?
So far this year, Lake Waihola has had two incidences of toxic algae blooms. Butchers Dam has had one bloom, and in rivers, toxic algae has been found in Hanley’s Farm Creek and the Clutha at Roxburgh.
Is the council able to say how many were issued at this time last year, or the year before?
At this time last year, Butchers Dam had one algae bloom, and like this year, pre-emptive warning signs were in place on rivers known as hotspots for toxic algae
Can the council say how many toxic algae alerts were issued over the course of the entire summer over the last couple of years and what we’re on track for?
In 2021 Lake Waihola had toxic algae alerts for two weeks and Butchers Dam for one week. Tomahawk Lagoon and Falls Dam had no alerts, and we did not monitor Pinders Pond. In 2021 our monitored lakes had no toxic algae blooms (Lake Waihola, Falls Dam, Tomahawk Lagoon). We did not monitor Butchers Dam and Pinders Pond in 2021.
If there has been more toxic algae identified in the region’s waterways, what’s going on?
Toxic algae blooms are more common during the summer months. Low rainfall, warm temperatures, the right level of nutrients and more sunlight create an environment where it can thrive.
