Skip to page content
You are here: Home > Stories > Tackling Tomahawk's troubled waters

Tackling Tomahawk's troubled waters

Nature lovers in Dunedin are investigating why the once-beautiful Tomahawk lagoon's health is declining and how they might be able to revive it.

Toxic algae in the lagoon

Toxic algae. Dead fish. Dangerous levels of bacteria. These are just some of the problems found in Tomahawk lagoon and locals in Ocean Grove, Otago, are trying to pin down the cause.

Wildlife fans, kids, families and scientists have been working together to routinely measure the lagoon's cleanliness, temperature and acidity, as well as keeping an eye on the plants and animals who call the lagoon home.

“We’re building a database first and foremost, because we don't have this yet,” says Andrew Innes, a retired science teacher who is leading the project through his local initiative Ecotago.

“Then we are going to turn this knowledge into a story so that others like the council and DOC can understand what is happening.”

Community members checking the samples they have just taken from the lagoon

Once the investigators have gathered a few years' worth of information, then they should be able to spot some long-term patterns or trends.

“The lagoon is really close to my school so it's good to be able to learn more about it in this way.” says Kyall, 13, from Tahuna Normal Intermediate School.

However, in the two years that locals have been monitoring Tomahawk lagoon, they are still finding it tricky to put their finger on what the source of its problems are, because its health seems to change from month to month.

Dead fish

“So far, we are seeing some distinct patterns linked to the seasons and when there’s heavy rainfall,” Andrew says. “For example, we had a significant fish die-off in the lower lagoon and in the stream feeding the upper lagoon, and this also happened at a similar date last year.

“One explanation could be how the land around its water catchment is being used; or this could just be how this lagoon will naturally behave if we don’t take any action to help it. What should it look like without the human impact?”

The locals have no plans on stopping their research until they find answers to this question.

One part of their research project involves using nets to sample wildlife from the sides and bottom of the lagoon and analysing them with the aid of freshwater scientist Lena Schallenberg. This gives an idea of how many different types of invertebrates such as bugs and worms live in the water; the bigger the number and variety, the healthier the lagoon.

“I really like looking at the animals in the water and finding out about how they live in their habitats and how they're so different from humans,” says Lucy, 12, from Tahuna Normal Intermediate School.

Trio of girls looking at bugs in the waterKyall looking down a microscope at bugs in the water

Other locals and scientists look at how turbid (cloudy) the water is, and whether there is there is too much bacteria in the water that would make it unsafe for activies such as swimming.

"This reads how dirty the water is," 10-year-old Ben explains, in reference to the turbidimeter he is using. "You put the water in the glass jar then put it in the machine. The bigger the number, the more dirty it is."

Checking how dirty the water is

Another group also checks the concentrations of phosphates and nitrates in the water at the lab with scientists Jonathan Kim and Murray Vickers. When the concentrations are too high, algae can start overgrowing and use up too much oxygen in the water, which native plants and animals need to flourish.

To check these concentrations, locals and scientists first add specific chemicals to a sample of the water to reveal the phosphates in it as a blue colour, and then different chemicals to a different sample to show nitrates as pink. The group then use a finely-tuned ‘colour reader’ (photometer) that converts the strengths of colour into numbers to calculate the precise phosphate and nitrate concentrations.

“We really like seeing how science is done in the real world. And actually doing it! It's really meaningful and has a real purpose, which is not like what we've learnt at school,” say En, 16, Ben, 17, and Boen, 16, from John McGlashan College.

Student creating pink nitrate solutions

The project findings are already becoming a wake-up call to others in the community.  

At the end of 2017, the team presented a ‘report card’ to locals, council members and scientists at a community symposium. The report card shows a summary of whether the measurements taken in five specific areas of the lagoon get a ‘pass’ or ‘fail’ compared to the water quality guidelines. 

The lagoon failed on almost all counts. The phosphate and nitrate levels were too high at all five sites, there was too much algae at most sites, the water was too turbid, and it had unsafe levels of E.coli bacteria at three sites.

Measuring the lagoon's depth

In response to these findings, Otago Regional Council have just met with locals to discuss reviving Tomahawk Lagoon and to work out a plan together. The Council representatives asked the community, ‘what would you like the lagoon to be like in 30 years?’

The answer, Andrew says, is to identify and fix the cause of these problems and make the lagoon completely healthy again – but the aim is also to create quiet walkways so that more people can continue to enjoy the natural features of the lagoon once it has been revitalised.

Visit the Tomahawk Lagoon research project website

About the project

Ecotago logoThis project is run by Andrew Innes at Ecotago with support from the Otago Participatory Science Platform and in collaboration with Tahuna Normal Intermediate School, Bayfield High School, John McGlashan College and Otago Girl’s High School along with DOC, Otago Regional Council, Healthy Harbour Watchers and the University of Otago.

Scientists and locals collaborating around a table

Participatory Science Platform

The Participatory Science Platform supports collaborative projects that bring together communities and scientists or technologists on research investigating a locally-important question or problem.

Find out more

View all stories