It's not just pollution that can harm oxygen levels in rivers. Another culprit: hippo poop.

A study lead by researchers at Yale University found hippos pooping in the Mara River in Africa are killing off the fish.

In the Mara River, about 4,000 hippos leave behind more than nine tons of dung each day as they wade in shallow pools of water to keep cool. Researchers learned this after monitoring the water chemistry and flow downstream of hippo pools over a three-year period.

During wet seasons with intense rains, the poop will flush out of pools, but when it's dry, the feces builds up and can create a lack of oxygen called hypoxia.

When the water from those pools flushes into the river, it can cause massive drops in oxygen.

"This sudden pulse of deoxygenated water can cause temporary hypoxia and fish kills," said Emma Rosi, a co-author on the study and ecologist with Cary Institute of Ecosystem Studies.

Scientists found the hippo excrement not only kills off oxygen levels, but leaves behind chemicals such as ammonium and sulfide, which is also harmful to fish.

"This appears to be a natural process that may have been more common in Sub-Saharan African rivers prior to the extirpation of hippos from much of their historical range,” said Christopher Dutton, the study’s lead author from Yale's Department of Ecology and Evolutionary Biology, in a statement.

The Mara River of East Africa flows through two noted areas: Kenya's Maasai Mara National Reserve and Tanzania's Serengeti National Park.

Their findings were published on May 16 in the journal Nature Communications.

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