Conservationists want major bays and estuaries along the Great Barrier Reef tested for contaminants after a five-year study found “alarming” levels of some chemicals in unhealthy turtles on the reef.

Scientists working on the research have also recommended expanded monitoring of turtle-population health on the Great Barrier Reef “as an indicator of the health of the reef itself”.

The research was launched after a mass stranding of green turtles in 2012, when more than 100 green turtles washed up dead or dying in Upstart Bay, south of Townsville.

In one of the largest turtle research projects to date, scientists screened water, sediment, food, and the blood and shells of turtles for a wide range of elements.

They conducted tests on populations in two coastal locations – Upstart Bay and Cleveland Bay – and compared the results to samples from the Howick group of islands, a “comparatively pristine site” 500km north of Townsville.

WWF Australia has published the final report on the project.

In the coastal locations, turtles were found to have elevated levels of metals such as cobalt, antimony and manganese in their blood and food and showed signs of poorer health.

In Upstart Bay turtles, cobalt levels were as much as 25 times higher than in the remote populations, and greater than previously recorded for any vertebrate species.

Cobalt is an essential metal for animal and human health but in high levels can become toxic.

Scientists grew green-turtle skin cells in a lab so the impact of chemicals could be tested without harming the animals.

Cesar Villa, an adjunct research fellow at Griffith University, said further research into how the levels become elevated in the first place is still needed, but possible sources include run-off from industrial activity, such as mining.

“It’s something that needs to be investigated,” he said.

He said further research was also needed to determine how high levels of certain chemicals could get before they became a problem for turtles.

In some of the unhealthy animals tested, cobalt levels were as high as 800 micrograms per litre of blood.

“For the normal ranges, they were much lower, between seven and 33 migrograms per litre,” Villa said.

The research does not conclude definitively that the high levels of contaminants were the cause of the deaths in 2012, but Villa said the results showed there was “no excuse not to do large-scale, non-target screening” on the reef for a wide range of contaminants.

The first turtles tested in the study were captured nearly two years after the first reports of mass turtle stranding and deaths.

“These were the survivors that were still struggling along, so we didn’t know when it started and how bad that first batch [could] have been.”

Christine Madden Hof, the WWF Australia marine species project manager, said the ratio of cobalt found in turtles’ blood and shell was “consistent with the theory that there was acute cobalt exposure at Upstart Bay in 2012”.

She said the methods developed in the research project could be used in further studies as a cost-effective way to detect poor water quality and chemical contamination early.

“If we found all these contaminants in just two coastal locations what’s happening in the reef’s other major bays and estuaries?” she said.

“What effects are chemicals having on other wildlife in these waterways?

“We have shown turtles are good indicators of reef health because they absorb chemicals in their environment.”