How do fish breathe underwater? It's the kind of question children ask. They get their oxygen in the water, we tell them - but sadly, that's not always true.
The world's oceans are currently undergoing a dangerous transformation. Scientists have detected vast 'dead zones' around the world, where hypoxic water lacks the necessary oxygen for marine life to survive.
This metamorphosis – which has increased more than tenfold since the 1950s – was one of the key mechanisms behind the deadliest mass extinction in history. And now, new research reveals, this suffocating phenomenon is not exclusive to the sea, but is also emerging in freshwater urban streams in the United States.
"We were surprised to find these dead zones are happening in our own backyards, not just in rivers and coastal waters downstream of major point sources of nutrient pollution," explains ecosystem ecologist Joanna Blaszczak from Duke University.
In their new study, Blaszczak and her team monitored dissolved (gaseous) oxygen concentrations in six urban headwater streams in North Carolina over a period of 18 months, also measuring light levels, water chemistry, and stream flow.
What they found is that the impact of human activity on the natural environment around rivers and streams affects the 'flow regime' of these water courses – thanks to erosion processes lessening flow in times of low flow, leading to increased water stagnancy and pooling.
"Streams draining developed areas are subject to intense, erosive storm flows when roads and stormwater pipes rapidly route runoff into streams during storms, without allowing the water to infiltrate into the soil," Blaszczak says.
"We found that erosion caused by these intense flows changed the shape of some stream channels to such an extent that water essentially stopped flowing in them during late summer."
When water stops flowing like this, a stream can effectively become like a series of connected pools of mostly motionless water. That stagnancy comes at a cost: an accumulation of nutrient runoff and organic matter, including nitrogen from leaking sewer pipes, fertiliser, and pet waste.
This heady mix may not seem like healthy fare for stream-dwellers - and it's not. In half the streams studied, chronically degraded conditions during baseflow periods led to low oxygen concentrations being recorded.
In one such case, the team observed dead fish following a hypoxic period. The dead zones are also a threat to marine life that we can't see so easily: bacteria and other small organisms that live in these ecosystems, and which depend on a ready source of dissolved oxygen in the water to stay alive.
"We found that growth rates of algae that support stream food webs was slower in streams with more frequent intense storm flows," Blaszczak says.
"Together with the occurrence of hypoxia, this paints a bleak and stressful picture for freshwater organisms that are trying to survive in these urban streams."
It's too early to say whether these kinds of results – sourced from just six streams in North Carolina – would be replicated in larger rivers running throughout the US. But the researchers say it's more than likely.
As the study points out, the features of the streams studied here are not unique, and previous research has already uncovered varying oxygen levels in waterways in places like Illinois and Kansas.
In other words, if we're not careful with how we affect these natural ecosystems around us, we could see dead zones proliferate throughout them – just like they already do in our oceans.
If that happens, we'll have bigger problems than figuring out what to tell our kids when they ask us how fish breathe.
"Hypoxia is not commonly assumed to occur in streams and rivers because of stream flow, which typically moves water fast enough to prevent the drawdown of dissolved oxygen by bacteria to hypoxic levels," Blaszczak says.
"However, dam building and other human alterations that stop the flow of water make these freshwater ecosystems particularly vulnerable to hypoxia with negative implications for biodiversity, especially in rivers already burdened with high nutrient pollution."
The findings are reported in Limnology & Oceanography.