Over the last few months, the Murray-Darling Basin has been reeling from fish kills, most notably a mass die-off in the Menindee Lakes, near Broken Hill in western New South Wales, after water quality plummeted following an algae bloom.

Algae are vital organisms, producing the bulk of the planet’s oxygen and supporting marine and aquatic food webs. In normal conditions, microscopic algae float to the surface to photosynthesise before again sinking. But unusual conditions can cause massive blooms, which form a visible layer of scum on bodies of water. Some algae are toxic, and in large quantities can poison the water. However, they need not be toxic to unleash devastation. 

Blooms are complicated phenomena, caused by changes in temperature, water nutrients and many other factors. In Menindee, heatwaves stimulated the blooms, which collapsed after a drop in temperature. 

Collapses like this cause a big problem. The sudden presence of millions of unicellular corpses sends decomposers – organisms that break down organic matter in dead plants and animals – into overdrive. They consume the algae, stripping dissolved oxygen from the water.

This chokes to death anything that needs oxygen. In slow-moving waters, the deoxygenated water turns the river lethal, like a wind that starves your lungs.

With climate change increasing global temperatures and decades-old scientific warnings of increased blooms, the obvious culprit for these ecological disasters is carbon pollution. However, while global warming is a contributor, more is happening in the basin. 

Seventy gigalitres of water were released from Menindee Lakes in 2017 to benefit downstream agriculturalists. The Wentworth-Broken Hill pipeline was also approved because, despite a shrinking population, the rivers have been damaged to the point that it is no longer viable for Broken Hill to draw water from the lakes. 

Upstream, the Murray-Darling Basin Plan was used to subsidise big agribusiness. Water secured with public funds was siphoned off by irrigators after the New South Wales government relaxed environmental regulations, denying downstream areas like Menindee the water they need to stay viable. 

There has been a systemic failure of regulators. Prioritising the interests of those who extract wealth from the basin over the interests of the ecosystems and the communities that once thrived there has left the river system vulnerable to worsening droughts. There is not enough water to provide a buffer for extreme conditions.

Taken with the federal government’s removal of a section of a UN report that concluded the Murray-Darling Basin Plan increased total water removal, a pattern of knowing malpractice emerges. 

What happened at Menindee is emblematic of a larger problem. If the anger at this catastrophe needs a target, look no further than the unsustainable system of agricultural production undermining some of the country’s key river systems.