In the wake of the enormous fires that have razed huge swathes of drought-stricken Australia, scientists fear that when rains eventually fall, they will wash charred debris into rivers, dams, and the ocean, killing wildlife and even tainting the drinking supplies of major cities, such as Sydney.

For many weeks, ash, soot, and blackened gum tree leaves have collected along the shorelines of Sydney’s beaches, clogging the waves and lapping in the tide. Originating in fires blazing in forested areas to the west, the debris has been carried on the breeze along with the pungent bushfire smoke that blanketed Australia’s largest city for much of December.

But what has carried on the wind is just a taste of the huge quantities of debris that are likely to wash into rivers once there are heavy downpours. As of press time, more than 26 million acres—an area bigger than Portugal—have now burned, mostly in the continent’s southeast. That includes areas of land known as catchments—also called watersheds—where rainfall begins its earthly journey into specific rivers, lakes, and dams.

It is an ecological disaster unprecedented in Australia’s history, and that spells trouble for drinking water supplies, coastal ecosystems, and the freshwater rivers that support iconic Australian wildlife, such as the platypus. (Australia’s flying foxes are already dying en mass due to the extreme heat.)

“Disturbance on this scale is almost certainly going to impact biodiversity. I’m remarkably worried about the effects on freshwater ecosystems,” says Ross Thompson, a freshwater ecologist at the University of Canberra.

“Given the severity of the fires, pretty much everything in burned landscapes is gone, so one of the big issues is that when we do get some rain, a lot of that ash and crap, nothing is going to stop it running in our catchments,” adds Ricky Spencer, a conservationist and ecologist at the University of Western Sydney in New South Wales (NSW).

Algal blooms and fish kills

One of the biggest worries is that the sudden influx of nutrients into waterways may lead to “blackwater events” caused by blooms of blue-green algae, also called cyanobacteria. Oxygen can subsequently drop to dangerously low levels for other aquatic life, potentially leading to mass fish kills and affecting species such as freshwater turtles.

Blooms caused by agricultural run-off and the ongoing drought led to the death of millions of fish—mostly golden perch and Murray cod, which are important recreational Australian fish species—in the rivers of western NSW last summer. Algal blooms were likely this year anyway in drought-afflicted waterways, but Spencer fears they will be exacerbated by the bushfires.

“These blooms could be ongoing until we start to see some growth along the river systems, or have a really good flush of rain,” he says.

Some research has also shown that fire mobilizes mercury in runoff, adds Katherine Dafforn, an environmental scientist at both Macquarie University in Sydney and the Sydney Institute of Marine Science.

“Higher concentrations of mercury have been found in fish from lakes in burned catchments compared to reference catchments,” she says, with potential impacts for human health if fish at the top of the food chain are eaten.

Algal blooms in dams could lead to significant problems for the water supply of major cities, such as Sydney and Melbourne. Warragamba Dam, to the west of Sydney, provides drinking water for 3.7 million people. But vast bushfires in and around its catchment area have so far burned more than 148,000 acres of forest.

Forests have burned across “80 to 90 percent of the significant parts of the catchment,” says Stuart Khan, an engineer at the University of New South Wales (UNSW) in Sydney who studies contaminants in water treatment processes.

Tainted water

A bushfire in the catchment in 2006 was the likely cause of an algal bloom that lasted several months in the dam in 2007, and the current bushfires are far more extensive.

Algal blooms in the drinking supply are problematic for several reasons. The resultant deoxygenation can cause fish kills, but it also makes iron and manganese soluble, which can give water a poor taste, odor, and color. Cyanobacteria can also produce chemicals that give water a musty or earthy flavor.

In rare cases, blooms produce dangerous cyanotoxins, which “will require a lot of attention to find out and be looking closely at what’s growing in these reservoirs,” adds Khan. While he thinks public health problems are unlikely in Sydney, he does worry about the challenge of keeping treatment plants running.

“We are not accustomed to having massive amounts of cyanobacteria and ash coming through our plants … and the treatment processes might be slowed down significantly,” he says.

With the current drought, Sydney’s water supplies are already overstretched, necessitating the use of a desalination plant. If the rate of production of treated water from Warragamba slows, it might lead to temporary but significant shortages and severe drought restrictions. Forested areas that make up water catchments have been incinerated throughout southeast Australia, and the fire crisis is likely to continue for several months, so it’s possible the drinking supply of many urban centers across the southeast could be affected by the end of the summer.

Platypus feeling the heat

The bushfires will likely have further serious effects on Australia’s river systems, which could exacerbate the dangers to the duck-billed platypus. Loss of shady vegetation around the small streams and pools it inhabits may raise water temperatures to levels it finds intolerable—typically anything above 83 degrees Fahrenheit.

“Platypus are weird little creatures, in that they don’t have any way of getting rid of heat. Their only response to warm water is to just sit in their burrow and hope it goes away,” says Thompson. During the terrible Millennium Drought 20 years ago, many platypuses simply retreated into their burrows and died there, he adds.

This year’s drought is even worse, with the Australian Bureau of Meteorology last week reporting 2019 was the both the driest and hottest year in 120 years of record taking. It’s possible the platypus could become locally extinct in parts of the north and west of its eastern Australian range, something that Thompson’s research suggested was already on the cards with climate change.

“It’s becoming a death by a thousand cuts,” he says. “The Millennium Drought knocked back a lot of our freshwater biota. They’ve never quite recovered in many cases, and now we have these big landscape-scale fires… . It’s a real concern, and we could certainly lose species.”

Ocean watch

Eventually the ash, soot, and sediment that runs into the rivers will wash out to sea, but the effects of bushfires on marine life have been little studied and are much less certain.

“The scale of these fires are unprecedented, and the images we are seeing of ash- and soot-laden waves along our beaches indicate that there is a very high density in the water column, so that is likely to have localized effects in those areas,” says Emma Johnston, a marine ecologist at UNSW.

Detritus doesn’t necessarily have to be toxic to cause problems, as tiny particles can clog the gills of fish and the foraging apparatus of filter feeders such as mussels, sponges, and corals.

While few studies have looked at the impact of charcoal and ash on marine organisms and possible toxins generated by bushfires, it’s certainly possible they could be detrimental, adds Johnston, and the influx of nutrients into the ocean may also cause algal blooms there.

Thankfully, Australia’s Great Barrier Reef has not yet been affected, as the focus of the fires have so far been further south, and the ocean currents carries water in a southerly direction, away from the reef.

Slow recovery

While very little is yet known about the effects of soot and ash on marine life, there could be an array of impacts if the levels of exposure are high enough, says Andrew Negri at the Australian Institute of Marine Science in Townsville, Queensland, who has studied the negative effects of coal dust from mining on the Great Barrier Reef.

For instance, shading limits the light that can reach marine plants, hindering growth. Or particles might contain metals, sulfur, and chemicals called polycyclic aromatic hydrocarbons that can dissolve in seawater and become toxic to marine species.

However great the damage to Australia’s marine and freshwater ecosystems eventually is, it may be decades before it can be rectified.

“The striking thing is just how long the effects of these fires persist,” says Thompson. “The time it takes these systems to recover, particularly in terms of providing a water supply, is in the order of a decade. That’s important because both Sydney and Melbourne’s catchments have been reasonably badly affected by recent fire events.”