Last updated May 31, 2018 at 1:04 pm
Hippo dung is so potent it can clean out a river, research finds.
Photo: Paulmaz (GFDL)
As if there weren’t already enough self-evident reasons why sharing a river with a mob of hippos is a bad idea, there’s now an extra one – which is very bad news indeed if you happen to have no other option.
A study published in the journal Nature Communications reports that hippos wallowing in Africa’s Mara River fill it with so much poo that they periodically kill fish downstream.
Researchers led by ecologist Christopher Dutton from Yale University in the US studied the environmental consequences arising from the eating and resting habits of some 4000 hippos which call the river home. The Mara stretches across Tanzania and Kenya and flows through the Maasai Mara National Reserve, where most of the hippos live.
Well adapted to the climate of Africa, hippos tend to spend their nights grazing on land, and their days hanging about in the river, escaping the worst of the heat. Relaxing all day in water after several hours of eating, however, brings inevitable consequences.
Hippos, don’t forget, are big – very big – and vegetarian to boot.
“Together, the Mara’s resident hippos add about 8500 kilograms – or 9.3 tonnes – of partially digested plant material into the river each day,” said co-author Emma Rosi. “We were interested in how this massive influx of organic matter and nutrients influenced aquatic life.”
The answer, it turned out, was severely, and unpleasantly.
During the hotter months of the year, water in the Mara evaporates, interrupting the flow and leaving the hippos happily lounging in isolated pools – to which, of course, nine tonnes of poo are added every day.
Then things get a bit toxic
The water thus quickly fouls. The plant matter in the waste decomposes, removing oxygen from the water. Associated microbial activity produces significant quantities of toxic chemicals, such as ammonia and sulfide.
Eventually, and inevitably, rain falls, the river refills, and the fetid contents of the hippo pools are flushed downstream.
Dutton and colleagues spent three years monitoring the chemistry and flow dynamics of 171 hippo pools. Through that time they recorded 55 flushing events – during which the river flowed at twice its normal rate. In 49 of the events the oxygen levels in the water downstream dropped – low enough on 13 occasions to kill fish.
“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 co-author David Post.
Understanding how this happened took considerable ingenuity. Because – as multiple sources testify – getting into the water alongside hippo herds carries with it a very high risk of injury or death, the researchers measured the water chemistry in the wallows using a remote-controlled boat. They also constructed a miniature model habitat by means of incrementally dumping hippo poo into a bathtub.
They concluded that two related phenomena contributed to the fish kills. First, the deoxygenated water flushed from the hippo pools lowers the oxygen levels of water downstream. Secondly, microbial activity occurring in the excreted organic mass continues as it floats along, further chewing up oxygen.
The scientists note, however, that while periodic poo-driven fish massacres might seem like a bad thing, the events resulted in large amounts of free food for scavengers, including crocodiles and several species of bird.
