Science Says What? What’s up with dissolved organic carbon (AKA why is my local stream murky?)

Science Says What? What’s up with dissolved organic carbon (AKA why is my local stream murky?)
April 27, 2023 Sharon Oosthoek

Science Says What? is a monthly column written by Great Lakes now contributor Sharon Oosthoek exploring what science can tell us about what’s happening beneath and above the waves of our beloved Great Lakes and their watershed.

If you’ve noticed your local waterway turning a deeper shade of brown over the past few decades, you wouldn’t be alone in remarking on it. Scientists in the Great Lakes region and in other parts of North America and Europe have noticed the same thing.

Trent University aquatic ecologist Maggie Xenopoulos has spent the past 18 years sampling rivers and small lakes throughout Ontario for signs of “browning.” As she explained to me when I tagged along on one of her sampling trips several years ago, rising levels of dissolved organic carbon in the Great Lakes region could be causing waters to darken, kind of like steeping tea. However, the Great Lakes themselves, with their massive volumes of water, appear immune.

Carbon naturally leaches into waterways from surrounding soil as dead plants decompose. Up to a point, that’s a good thing – it blocks UV rays and acts as a kind of SPF for aquatic life. It’s also a basic food source for fish.

But in the past few decades, the leaching process has gone into overdrive and increased browning might turn out to be too much of a good thing. Some researchers worry it could reach a tipping point where the water becomes too brown. They say that could damage aquatic ecosystems, increase the cost of water treatment, and even contribute to global warming.

Browning’s surprising cause

The irony is that browning is partly the result of an environmental success story: the reduction of acid rain.

Acid rain began increasing in the mid-1800s as the Industrial Revolution took off, powered by fossil fuels. Burning these hydrocarbons, especially coal, produces sulphur dioxide and nitrogen oxides, which react with water in the atmosphere to produce acids. By the 1970s, it was apparent that this was damaging trees and aquatic ecosystems, and governments passed laws to clean up smokestacks.

Acid rain began to decrease. But there was an unforeseen consequence. In many temperate and subarctic areas, deposits of sulphur had changed the chemistry of soils, making them “stickier.” This meant most dissolved organic carbon stayed put, and didn’t run off into surrounding rivers and lakes. But as soil sulphur concentrations dropped, this carbon became unstuck.

You might think that browning would slow down once the excess carbon had been flushed out of the soil, but some researchers believe climate change is continuing the effect. Increased growth in vegetation due to greater availability of carbon dioxide, longer growing seasons and heavier rains could all be behind excess carbon flushing into rivers and lakes.

There’s always a caveat. Or two

Curious to know the current state of affairs, I recently called Xenopoulos for an update and she reminded me of a couple important caveats.

One – her study hasn’t been going on long enough to be certain about rising browning.

“Because some years are drier and some are wetter, you really need several decades for the signal to level out, to make a conclusion,” she told me. “Scientists don’t generally start paying attention until they have a data set of 20 to 25 years.”

Two – some scientists believe that dissolved organic carbon concentrations in the water were significantly higher before acid rain, which means any increased browning may just be the ecosystem returning to a natural state.

Natural or not, the reason scientists are paying close attention is that browning could be bad news.

One survey of 168 lakes in Norway found that while initial increases in soil-based carbon were linked to increases in brown trout numbers, continued rises caused the population to steadily drop.

The initial benefits were probably due to carbon’s ability to block UV rays and the fact that when it drains into watercourses it often brings phosphorus and nitrogen too, key nutrients that fuel the growth of organisms at the bottom of the food chain.

However, carbon levels reach a tipping point when the water turns a deep brown, according to research at the Norwegian University of Science and Technology in Trondheim. This prevents sunlight from reaching bottom-dwelling algae and, if the water is dark enough, free-floating plankton. No sunlight means no photosynthesis, and no food at the base of the food web.

The opacity of the water causes another problem: it narrows the oxygenated top layer of a lake – prime fish habitat. Lack of light means photosynthesising phytoplankton can’t grow, so they make less food and less oxygen, so there is less suitable habitat for fish.

How much browning is too much?

No one yet knows what the threshold for this switch from positive to negative effects is, although scientists expect it will be different for each ecosystem. The Norwegian study for example, found that shallow lakes switch at higher carbon loads than deeper lakes, probably because light doesn’t have to travel so far to reach the bottom.

And aquatic life isn’t the only thing to suffer from browning. With less light penetrating the water, phytoplankton die and non-photosynthesizing aquatic bacteria start to dominate. These gorge on a banquet of dissolved organic carbon, producing carbon dioxide as waste, which enters the atmosphere where it can contribute to global warming.

In other words, climate change seems to be increasing browning and browning, in turn, increases climate change.

That’s not all. Rising dissolved organic carbon levels could raise the cost of making water safe to drink. Chlorine – a common disinfectant – reacts with the carbon, leaving toxic by-products. To prevent this happening, iron and aluminium sulphate are added to the water, forcing dissolved organic carbon to clump together and drop to the bottom. Surface water can then be safely treated with chlorine. More will be needed if browning intensifies, and that will be expensive.

So what can we do, given that increasing acid rain isn’t an option? Some scientists say we should restrict development in sensitive watersheds as digging tends to speed the release of soil’s carbon. Others say we need to reduce fishing quotas in freshwater fisheries to avoid crashes in populations. All agree we need to pay closer attention to browning.

“If this continues, we’re going to have dramatically different lakes and rivers,” Xenopoulos told me. “This is going to be a big story.”

Catch more news at Great Lakes Now: 

Science Says What? How 5th-graders counting plants can lead to positive change

Science Says What? Microplastic pollution — how worried should we be?

Featured image: Rainbow trout at the ODNR’s Castalia Fish Hatchery. (Photo Credit: James Proffitt)


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