Study shows oxygen levels dropping in rivers, lakes and streams

(CN) — Inland waters like rivers, lakes, streams and reservoirs need oxygen to survive, just like we do, but oxygen levels have dropped dramatically since 1900, researchers warn.

The reason? Human behavior.

That’s according to researchers of the Netherlands’ Utrecht University in theirstudy, published Friday in the journal Science Advances.

“More farming, more wastewater, more dams, and a warmer climate — they all change how our freshwater ecosystems function,” said one of the paper’s lead authors, Junjie Wang, in a written statement.

Co-author Jack Middelburg, added, “We found that the main causes lay in these direct human activities. First, it turns out that nutrient input through, for example, over-fertilization, is a major driver of this acceleration. Secondly, the longer travel time of freshwater to the sea through the construction of dams and reservoirs has proven to be just as important.”

The results of this oxygen depletion are already being felt around the globe, in the form of dying fish, disrupted food chains and poor water quality.

Wang, Middelburg and their colleagues have constructed a model for what they call the “global inland-water oxygen cycle.”

According to Middelburg, the model offers “the most complete possible understanding of this cycle on a large scale, so that one can see oxygen related problems coming, get to know the causes, and hopefully intervene in time.”

The model tracks the oxygen “turnover” — how much oxygen is produced and consumed — in inland waters since 1900. The researchers found that the rivers, lakes and streams are producing and consuming more oxygen than they were a century ago, but the rise in the consumption has far outpaced production.

Meaning that inland waters are now consuming much more oxygen than they are producing, making them a “sink for atmospheric oxygen,” according to researchers in their paper.

“The present-day oxygen uptake from the atmosphere is similar to oxygen production in inland waters,” the researchers write. “However, this was not always the case during the evaluated 110-year span, especially in the period before intensified and expanded human impacts. In the 1900s, oxygen uptake from the atmosphere was four times as much as inland-water oxygen production because oxygen production was largely limited by nutrient availability.”

Even though the surface areas of inland waters take up only about 0.2% of the ocean’s total area, the researchers estimate that the inland waters are responsible for “about half the quantity of net oxygen degasssing from the ocean to the atmosphere.”

“Even though these waters cover just a tiny fraction of Earth’s surface, they now remove nearly 1 billion tons of oxygen from the atmosphere each year — overall half of what the entire ocean emits back,” said Middelburg, in a written statement.

Scientists construct models to predict earth’s oxygen levels, as well as levels of predicted global warming. Until now, these models haven’t been taking into account the changes in the oxygen turnover in inland waters. The researchers argue that should change.

“‘We can’t ignore inland waters in global climate and oxygen budgets anymore,” said Wang. “They’re changing faster than we thought, and they’re crucial pieces of the Earth system puzzle.”

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