17 Oct 2021

Satellites used to diagnose health of Bay of Plenty lakes

8:32 pm on 17 October 2021

Satellite images from the European Space Agency are being used to diagnose the health of Bay of Plenty lakes with researchers using images from space to hunt out algal blooms and help manage them.

The algal bloom at Lake Rotorua on 23 October.

The algal bloom at Lake Rotorua on 23 October. Photo: Trish Bailey / Supplied

The health of lakes across the country is often monitored by water samples taken at specific points from the lakes being tested.

But as Dr Moritz Lehmann, from the University of Waikato and Xerra Earth Observation, has found, its not always accurate.

"It's a great smaple, it has a lot of information but we have appreciated for a long time that lakes have variables within themselves.

"And so you may not sample that algal bloom that may even be toxic or you may oversample some of the higher concentrations.

"James Dare our collaborator from the Bay of Plenty Regional Council has looked at some of the samples that he's been taking from the shore, and compared them to patterns that we see on satellite images and they're obviously related. But they show that some of those shore samples, don't show the bloom at all. Where in the bigger picture. The lake's completely green with algae."

Satellite images of a Rotorua Lake showing algal blooms.

Satellite images of a Rotorua Lake showing algal blooms. Photo: Supplied /Waikato University

The blooms are seen from space as patches and swirls of green and brown colours. Satellite images can support the ground management of the lakes by sending council staff to the perfect locations for water sampling.

"Using satellite imagery, we can pinpoint exactly where the blooms are and how they are spreading over time. This helps to ensure we get the best water samples, building a full picture of how healthy our lakes are, or not.

"Blooms are a natural phenomenon and they've always occurred. So these are bacteria that photosynthesize, but they can be toxic. That can be toxic to animals, and in aquatic life, and also to humans.

"Dogs that would go swimming or people that would go swimming, sometimes even breathing in spray from the lake could cause reactions."

With satellites flying over New Zealand constantly and access to the images from the European Space Agency provided free, the methods developed on the Rotorua lakes have potential to help manage the health of all New Zealand's lakes and lakes worldwide.

This project was started by Eike Schütt, a student enrolled at the University of Kiel in Germany who wanted data to support his coursework.

"The Rotorua lakes boast some of the best water quality monitoring data sets in the world, so I sent Eike a decade of data from the Bay of Plenty Regional Council,"Lehmann said.

With the support of Dr Martin Hieronymi and Dr Hajo Krasemann (Helmholtz-Zentrum Hereon, Germany), Eike calibrated an algorithm to detect algae in the Rotorua lakes with better accuracy than before. The team then wanted to demonstrate how satellites can be used in routine monitoring of lake health, so they teamed up with James Dare, environmental scientist at the Bay of Plenty Regional Council.

Over time the satellite maps show chlorophyll a concentration, (chlorophyll a is an indicator of algal biomass) and the team was able to describe these patches using statistics, Lehmann said.

"This allows lake managers to understand how algae is distributed through the lakes without having to visit them and provides information on where the best monitoring sites are to test water quality."

The collaborative work through this study and a recently awarded MBIE Smart Ideas project will support further developments of the methodology.