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Anecdotes of Winter Rivers devoid of Oxygen, by Frederick Schueler

In 1982 I taught freshwater ecology for a sabbaticalling professor at the University of Ottawa (U of O). Having collected frogs all across Canada, I thought I was perfectly qualified, but at the U of O freshwater ecology was  limnology, a deepwater sport, more akin to oceanography than it was to dipnetting frogs from roadside ditches. I stayed about a week ahead of the students in Robert G. Wetzel’s Limnology,[1] which was a great textbook, but with a significant tadpole in its lemonade: Wetzel affirmed that, under their ice cover, streams remained well oxygenated through the winter.

The problem with this was that in the winter before I’d been studying Leopard Frogs hibernating in Kemptville Creek, cutting holes in the ice and setting nets in the current to catch frogs and fish that were swept downstream. It had turned out that thousands of  fish and frogs had died, and in the winter of 1982-83 I borrowed an oxygen meter from the course to confirm that it was the lack of oxygen in the water which was killing them.

It was easy to reason that this anoxia occurred when solid ice kept the air from the water. It was also easy to infer that the difference between Kemptville Creek, with a gradient of 14 cm per kilometre, and New York and Wisconsin, where most of Wetzel’s material seemed to be sourced, was that the topographic relief in those hilly States allowed their babbling brooks more access to the air  than our Beaver-flattened creek obtained under 40 cm of ice liberally buried in snow. In the years since we’ve had various confirmations that in areas of low topographic relief, and increasingly to the north where winters are longer, anoxia, and the resulting deaths of gill- or skin-breathing animals, is widespread.

Throughout the winter, Mudpuppy Night in Oxford Mills is our weekly visit to Kemptville Creek below the dam at Oxford Mills.[2]  In some winters we experience the organic and hydrogen sulphide “cat farts” odor of anoxia, when murky brown oxygen-depleted water comes over the spillways, obliging the Mudpuppies to retreat into the rocky creek downstream, where the water has picked up some oxygen. There are no Mudpuppies above the dam.

In the polar-vortex winter of 2013-14, episodes of anoxia began in December, causing a kill of frogs below the dam that were seen to be eaten by Mudpuppies in the following weeks. Large numbers of ‘puppies were seen until the high water of a mid-January thaw, then through February, and into March, the oxygen was on and off, with many nights when no Mudpuppies were seen.

While anoxia was just an inconvenience to Mudpuppies, when spring finally came the usual mass movements of Leopard Frogs across the roads from hibernating to breeding sites above the dam just did not occur. We went out again and again without seeing any frogs on the road and heard no Leopard Frog choruses. Through the later spring there was not much calling by other aquatic-hibernating frogs, and through the summer very few frogs of water-hibernating species were on the roads, leaving us to conclude that there had been massive winter mortality along with, or caused by, the anoxia.

The classic cause of anoxia is organic pollution from sewage. Oxygenation is regarded as “good water quality,” and the conventional conservationist’s role is to prevent  it. Winter anoxia happens while academics are in their classrooms, government employees are in their offices, and the water is under ice, making it harder to study than something that happens in the summer. Natural anoxia is an important factor in understanding our rivers: it will often determine which species live in low-gradient rivers, amplify the biological breaks caused by dams, and cause wide swings in abundance of species depending on the character of winters.

How can you tell if and when your stream is anoxic? There’s the ‘cat farts’ smell – an organic enhancement of whiffs of the hydrogen sulphide released by reducing conditions – the water is often cloudy with humic acids that have been loosened by the change in water chemistry, and dead animals, such as frogs and fish, are a good sign that anoxia has occurred. Meters for measuring oxygen directly are cheaper every year, and can sometimes be borrowed from office-bound institutions which wouldn’t be using them during the winter.

One of the precautionary strategies of river conservation is to know things that will make a potential exploiter’s job more difficult, and “episodes of hibernal  anoxia” is one of these, because many disturbances can be surmised to enhance it, and the proponent of exploitation is unlikely to have a glib reply. To really assess “the effects of  [a potential disturbance] on patterns of occurrence of hibernal anoxia,” a proponent would need to spend several winters measuring under-ice oxygen levels, and by that time the political climate might have changed, or the proponent might have run out of money.

Contact Fred Schueler at


[1]    Wetzel, Robert G. 1975. Limnology. 743 pp. Thomson Learning & W.B. Saunders (Philadelphia, London, Toronto).
[2] – this page includes links to field notes from all Mudpuppy Nights