The Great Barrier Reef World Heritage Area, off the northeast coast of Australia, is “one of the richest and most diverse natural ecosystems on Earth.” Thousands of species of mollusks, more than 1,500 species of fish, and 400 species of coral live within its waters.
There’s something else there, too – herbicides, washed into the ocean via rivers draining agricultural areas.
Herbicides can breakdown in nature (UV rays can degrade the molecules, or microbes can consume them), but it’s unclear how long that process might take under the conditions that occur around the Great Barrier Reef – many of the studies conducted in the past included unrealistic conditions, like unusually low temperatures or levels of herbicides 500 times higher than what researchers typically find there. A group of scientists working in Queensland, Australia, sought to detail the timing of herbicide persistence in seawater from the Great Barrier Reef lagoon kept under more natural conditions; they recently published the results of their study in the journal PLoS ONE.
Working with samples of water scooped up from off the coast of Queensland, the researchers added several different herbicides, then stored the flasks under different combinations of conditions: some in the dark, some under partially lit conditions, some at 77 degrees Fahrenheit (the average temperature of seawater around the Great Barrier Reef over the course of a year), and some at 88 degrees Fahrenheit (the summertime high temperature of seawater near the shore in part of the Great Barrier Reef lagoon). They took samples from the flasks every few weeks for a year to measure the herbicide concentrations, allowing them to estimate how quickly the herbicides broke down.
The estimated half-life of each herbicide – the time it takes for half of the initial amount of the chemical to degrade – ranged from about 150 days to more than 5,000 days. The light and temperature conditions the flasks were stored under didn’t lead to consistent patterns in how the herbicides broke down.
The scientists also added a chemical that stops microbial activity to some of the flasks, and no herbicide degradation occurred under those conditions, suggesting that it was the microbial community present in the seawater collected from the Great Barrier Reef lagoon that broke down the herbicides.
“Chronic exposure of [the Great Barrier Reef] and catchment biota to . . . herbicides (from microbial communities to macrophytes) remains largely unstudied,” the researchers note, “and should be a future focus for research and risk assessment.”