YSM Issue 91.3

ecology
NEWS
HIPPOPOTAMI AS ENGINEERS
Organic matter affects water characteristics
BY TIFFANY LIAO
IMAGE COURTESY OF CHRISTOPHER DUTTON
Hippos defecating in a pool that will eventually become a “hippo
pool” and drop DO levels in the river downstream.
Hippos may not be able to do math or solve complicated
physics equations, but it turns out that they play a major role
as engineers in the environment. A joint team of researchers
from Yale University and the Cary Institute for Ecosystem
Studies have focused on the downstream effects of organic
matter produced by hippopotami on the Mara River for the
past four years. They found that the hippopotami waste results
in drops in the dissolved oxygen concentration levels in
rivers, resulting in mass killings of the fish in the streams and
creating a significant impact on the ecosystem.
The researchers observed the Mara river over the span of
three years. During this time, there were 55 flushing flows,
events where hippo pools (pools full of hippo waste) are
washed downstream, decreasing dissolved oxygen (DO)
concentrations and potentially killing the fish in the stream.
Out of the 55 flushing flows, 49 had DO concentration
drops by 0.04-5.5 miligrams per milliliter (mg/L).
“Most organisms need a certain amount of oxygen and
when that oxygen concentration falls below 30% or generally
2mg/L, then those organisms can’t survive because metabolism
shuts down,” said David Post, Yale Professor of Ecology
and Evolutionary Biology and a researcher on the project.
The typical DO levels in a river is around six to eight
mg/L. The lowest drop of 5.5 mg/L left the DO concentration
at around 0.5-2.5 mg/L, which indicates a fish kill.
Next, the team of researchers investigated a possible correlation
between hippo waste and the DO drops. The project occurred
in three stages: small scale microcosm experiments, experimental
streams, and whole ecosystem simulation. In the
first stage, the scientists put hippo waste in bottles of water and
measured dissolved oxygen over time. They observed that DO
content decreased overtime. However, since the bottles of water
were only representative of still water, the team decided to make
experimental streams as they were more accurate to a moving
river, allowing for reiterations and cycling of the water. In the
experimental stream, there was a drop in DO levels and then a
recovery, which occurred 8-12 hours into the experiment.
In the last portion of the study, the team constructed a dam
and flushed a hippo pool downstream. Similar to the earlier
experiments, DO levels at the beginning dropped significantly,
but rose again in a matter of hours. To Post, most surprising
part was the fact that the results were so consistent.
“It was quite surprising and reassuring that the results all tell
the same story. It is not often that this happens in science.”
What shocked Dutton the most was the frequency of these
flushing flows and the speed of the recovery. “If you didn’t measure
the DO levels like we did throughout long periods of time,
you wouldn’t even know this happened because in the span of
12 hours, the river is back to normal and the fish killed would all
be eaten by scavengers so no trace would be left,” said Dutton.
The extent of the effects of flushing events have only been
researched recently, but the repercussions on the environment
are extensive. The Mara is an important area for tourism
and sustaining the wildlife of the river is important. If
these flushing flows significantly shift the food chain by killing
the fish population, both the wildlife and the economy
could suffer. At the same time, fish kills could actually benefit
the rest of the ecosystem by providing food for scavengers.
“The nutrients of the river could be put back into the
ecosystem on land as the scavengers eat and defecate.
This life and death process is happening everywhere and
it is a fascinating system,” said Post.
Just as humans deposit waste into streams and rivers, degrading
our ecosystem, animal waste can affect wildlife and habitats.
However, this does not mean the waste humans produce is good
for the environment. Post emphasizes that humans create longterm
oxygen depletion whereas animals like hippos only create
periodic oxygen depletion. Essentially, human waste and animal
waste have effects on different time scales. So humans, don’t use
this as an excuse the next time you drop trash into the ocean.
PHOTOGRAPHY BY MICHELLE BARSUKOV
Yale researcher David Post holds a syringe system made up of light
meters for sample collection.
www.yalescientific.org
October 2018
Yale Scientific Magazine
11