Hippo excrement is being blamed for fish mortality in Kenya's Mara River.

According to a new survey of the hippo population and river health in the Maasai Mara National Reserve, the mammals excrete 9.3 tons of waste into the Mara River each day.

Researchers suggest the massive influx of waste can trigger fish kills as the half-digested plant material decomposes and robs sizable swaths of the river of oxygen. As microbes break down the excrement, ammonium and sulfide are produced -- toxins harmful to fish and other aquatic life.

At night, hippos graze in the savanna. During the heat of the afternoon, they lounge in the river.

"During dry periods, oxygen-poor water accumulates in hippo pools," Emma Rosi, a freshwater ecologist at the Cary Institute of Ecosystem Studies, said in a news release. "Periodic intense rains eventually flush the water downstream. This sudden pulse of deoxygenated water can cause temporary hypoxia and fish kills."

Over three years, scientists monitored water chemistry downstream from 171 hippo pools. During that time, scientists observed 55 flushing events, when rains doubled the river's volume. Of the 55, 49 depressed oxygen levels. Of the 49, 13 plunged levels low enough to kill fish.

Lab experiments allowed researchers to replicate the biochemical processes happening in hippo pools and confirm the causes of low-oxygen water. Flushing events not only carry low-oxygen water downstream, they also churn up sediment rich with hippo waste, which continues to decompose and deplete oxygen further down the river.

"In the Mara River system, flushing flows are important for cleaning hippo waste out of pools, but the accumulated toxic chemicals and deoxygenated water have severe impacts on aquatic life downstream," said researcher David Post of Yale University.

While the new findings -- published this week in the journal Nature Communications -- may seem like a negative, the dead fish offered scavenging opportunities for birds and crocodiles.

"There's this idea that pristine rivers are not supposed to have dissolved oxygen crashes, but we think this is because generations of scientists have studied places that no longer have intact large wildlife populations, whereas the Mara River is unique because it does," Yale researcher Christopher Dutton said. "This system offers a window into the past and illustrates how ecosystems might have functioned before human impacts."

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