Checking the health of our lakes - from space

Scientists are developing a new way to keep a constant eye on the state of our lakes. Photo / File

Scientists are set to test the health of New Zealand's lakes – from space.

With summer again turning public attention to the condition of our under-pressure freshwater estate, a new study is focusing on the persisting problem of blue-green algal blooms.

Reports of these often-toxic blooms, which float on the surface of lakes, have become common each season: and health warnings were put in place at Rotorua's Lake Okaro and Lake Rotoehu even before summer began.

Yet, at present, just 0.5 per cent of New Zealand's lakes are monitored for blue-green algae, or cyanobacteria.

"We are concerned that we are missing a lot of lakes where cyanobacteria may exceed current guidelines," said Professor Ian Hawes, a freshwater ecologist at the University of Waikato.

"This is because there is good evidence that intensive agriculture, and the increased run-off of the nutrients that support cyanobacterial growth, plays an important role in promoting toxic blooms, and models indicate that there are many more lowland lakes where catchment land use suggests vulnerability to potentially toxic blooms than are currently monitored."

But keeping a constant check on lakes is expensive.

Councils have to collect samples and then transport them to laboratories, where they are subject to costly tests to quantify the presence of potentially toxic algae.

In a new million-dollar project, a team of scientists led by Hawes will develop a new way to continually watch for blooms, by using satellite imagery that picked up and measured the give-away colour of cyanobacteria.

Scientists had already used a similar approach to assess the dominant colour of New Zealand lakes, and had also begun to measure overall concentrations of the sediment and algae within them.

"This is the next step," Hawes said.

"What we are really looking for is a way to monitor the status of a large number of New Zealand lakes – we hope more than 1000 – using freely available remote sensing images."

Hawes said recent modelling of vulnerability to blooms has involved extrapolating water quality measurements from a few sites around the country to other lakes with similar degrees of intensification around the country.

"It's an attractive approach, but depends on the assumption that lakes or rivers with similar catchments and land use will have similar water quality," he said.

"Such an approach is so much stronger if we can provide a validation data set."

The main issue was that all models have errors, and while they offered a good overall picture, when it came to specific lakes, they can be wrong.

"We want to help us to get over that problem," he said.

"It is also important to remember that we are trying to improve water quality across the country."

Having a method of looking at lakes as they changed is critical to evaluating the success of interventions.

Hawes pointed out that the study wouldn't be quite as simple as teaching a computer model how to colour-code.

The colour of 1486 lakes, determined from satellite images taken in August 2016, and expressed as an intensification of the colour as perceived by the human eye. Image / Supplied

New Zealand was home to a wide range of lake types and forms of cyanobacteria.

On top of that, our lakes had varying appearances – from peat-stained waters in the West Coast, to clear dune lakes in the north.

"Each of these have a natural background colour, and we need to be able to differentiate the cyanobacterial colour signal against this," Hawes said.

"So we are taking things in a series of steps."

First, the team would develop an optical classification, based on factors like geography and the impacts of surrounding vegetation and satellite measurements, enabling them to identify the type of background colour of lakes without visiting them all.

At present, just 0.5 per cent of New Zealand's lakes are monitored for blue-green algae, or cyanobacteria. Photo / File

The next part would use the colour of lake water, measured with boat and drone-based instruments, to infer cyanobacteria and other algae, and finally to use those data to develop satellite-based measurements of cyanobacteria.

"It's a slightly risky endeavour, but we are confident that we will be able to get to the point where we can use drone-based measurements to determine cyanobacteria," Hawes said.

"This is because the instruments that we can fly on the drone capture colour with high resolution – or lots of colours - and small pixel size, of the area of lake surface measured."

Current generations of satellites resolved colour into fewer bands, with a bigger pixel size, which would pose a challenge.

"But then again, the one thing that we can be sure of is that there will be a generation of satellites that will be deployed that will have better and better wavelength and area resolution. This technology is part of the future and we need to be ready to make the most of it"

What came out of the project, Hawes hoped, would be a tool able to provide surveillance of dangerous water quality, right across New Zealand.

"It will show us the lakes that are most affected by catchment activities, and prioritise these for more intensive investigation," he said.

"And we hope that this tool will be used to document the improvement in lake water quality over time as policies rolled out by government, councils and land owners begin to reverse the decline."