Pollution has long been blamed for the deaths of fish in the world’s waterways.
But now researchers have identified a more natural culprit: Poop. Specifically, hippopotamus poop.
Scientists studied Africa’s Mara River, which cuts through Maasai Mara National Reserve of Kenya, to come to the somewhat surprising conclusion.
The reserve is home to more than 4,000 hippos.
And those hippos produce a lot of dung.
“Together, the Mara’s resident hippos add about 8,500 kg – or 9.3 tons – of partially digested plant material into the river each day,” study co-author Emma Rosi, a Freshwater Ecologist at the Cary Institute of Ecosystem Studies, said in a statement. “We were interested in how this massive influx of organic matter and nutrients influenced aquatic life.”
As it turns out, all that feces is stealing oxygen from the local aquatic ecosystem in a process known as hypoxia, which is normally attributed to human-caused pollution and is generally not common in unpolluted rivers.
And that is killing fish.
The study, published in the journal Nature, is titled rather blandly, “Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills.”
The researchers, who were also from Yale University and Michigan State University, documented 49 high flow events over three years that caused dissolved oxygen decreases. Those included13 events resulting in hypoxia, and 9 fish kills over 5 years.
This “phenomenon” may have been more widespread throughout Africa before hippopotamus populations started to shrink, the authors found.
And it might be a natural part of ecosystem function, particularly in rivers impacted by large wildlife, according to the study.
“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,” lead author Christopher Dutton of Yale University said in a statement, “This system offers a window into the past and illustrates how ecosystems might have functioned before human impacts.”
Photos Amanda Subalusky, Christopher Dutton