Fifty-two million years ago lakes and rivers turned into 'toxic soup - and warning signs similar to those that occurred then are now appearing today.
The end-Permian mass extinction event – the worst in Earth’s history – saw toxic microbial blooms seething in rivers, delaying the recovery of animal life by millions of years, fossil evidence has revealed.
Scientists at University of Connecticut said the toxic soup could also be found in the fossil records of other related mass extinctions in Earth's history.
Today, freshwater microbial blooms have been on the rise, the researchers warned.
Professor Tracy Frank said: "We're seeing more and more toxic algae blooms in lakes and in shallow marine environments that's related to increases in temperature and changes in plant communities which are leading to increases in nutrient contributions to freshwater environments.
Watch: Sea Otters Threatened by Toxic Algae Blooms
"So, a lot of parallels to today. The volcanism was a source of CO2 in the past but we know that the rate of CO2 input that was seen back then was similar to the rate of CO2 increases we're seeing today because of anthropogenic effects.
"We can get a sense of how much climate has changed in the past, what the extremes are, how fast it can change, what the causes of climate change are and that gives us a nice backdrop for understanding what’s happening today."
According to this year’s report by the Intergovernmental Panel on Climate Change (IPCC), the influence of humans on the changing climate is "unequivocal", creating conditions that favour the spread of these warmth-loving microbes.
In combination with an influx of nutrients from water pollution, mostly from agriculture and deforestation, this has led to a sharp increase in toxic blooms.
This has resulted in mass fish die-offs, severe human and livestock health effects, and an annual cost measurable in billions of dollars.
The three main ingredients for the toxic soup are accelerated greenhouse gas emissions, high temperatures, and abundant nutrients.
During the end-Permian mass extinction, the volcanic eruptions provided the first two, while sudden deforestation caused the third.
When the trees were wiped out, the soils bled into the rivers and lakes, providing all the nutrients that the microbes would need.
In a healthy ecosystem, microscopic algae and cyanobacteria provide oxygen to aquatic animals as a waste product of their photosynthesis.
But when their numbers get out of control, these microbes deplete free oxygen, and even release toxins into the water.
By studying the fossil, sediment and chemical records of rocks near Sydney in Australia, the researchers discovered that several pulses of bloom events had occurred soon after the first volcanic rumblings of the end-Permian mass extinction.
The freshwater systems then seethed with algae and bacteria, delaying the recovery of animals for perhaps millions of years.
Frank said: "We are trying to understand what conditions these plants were living in, for instance were they lake deposits versus river deposits? Then what can we determine details about the salinity and temperatures of the waters, those details come from the geochemistry."
"The end-Permian is one of the best places to look for parallels with what’s happening now," said geologist Chris Fielding from University of Connecticut.
"The scary thing is we are used to thinking in terms of timescales of years, maybe tens of years, if we get really adventurous. The end-Permian mass extinction event took four million years to recover from. That’s sobering.
"The other big parallel is that the increase in temperature at the end of the Permian coincided with massive increases in forest fires. One of the things that that destroyed whole ecosystems was fire, and we’re seeing that right now in places like California.
"One wonders what the longer-term consequences of events like that as they are becoming more and more widespread."
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