Hippos and the Mara River

[African bird call sound] The Mara Serengeti supports
millions of wildlife. This includes 1.2 million
wildebeest a couple hundred thousand Zebras. Elephants.
It’s a really spectacular display of wildlife
that you don’t find anywhere else in the world. The Mara
River is considered the life blood of the Serengeti because
it is the only permanent source of water for wildlife during the
dry periods of the year. Back in 2008 we began to notice
something really strange happening in the Mara River
where every once in a while after a flood came
through all the fish would die in a section of the river.
That kind of peaked our interest to what may be causing these big
fish kill events. It wasn’t until about 2010… in 2011 when we actually put
a water quality sonde in the river and we saw these
really strange drops and dissolved oxygen
that would occur during the flood pulse and rebound
within a period of about eight hours Among those wildlife are four
thousand hippos that spend every day in the river but they spend
their nights in the grasslands eating grass. And
when they return to the river we estimate that they excrete eight
to nine thousand kilograms of organic matter every day into
the Mara River. So we use a number of small
bottle experiments where we tested the effect of adding
hippo feces and adding hippo pool water to just normal river
water and how it depletes the dissolved oxygen. We then scaled
up from the bottles to an experimental stream array
that we’ve built in the field. So the stream array has about 60
litres of water in it and a small paddlewheel
pushes the water around it so it simulates what a river
experiences with oxygen exchange with the environment. We added
hippo feces and hippo pool water to those streams to try
to really understand how the oxygen is depleted and the
mechanisms involved. After the experimental stream
array, we again try to upscale what we’re doing and we
experimentally flushed a hippo pool. We found a pool that never
had hippos in it, we built a small dam upstream of it,
let the water back up over several days. We then breached
the dam. [voice – There it goes. Sound of running water] And let that flood move through
the pool to catalogue what’s happening to a normal pool when
a flood moves through it. We then did that experiment
again but we added over 16,000 litres of water from the bottom
of a real hippo pool into that experimental pool. We built the
dam again and then we flushed it [voice – 3-2-1… sound of rushing water] to then see experimentally
what’s happening downstream with these hippo pools when we
do flush them. So all of these measurements
and methods we employed showed us that organic matter
as it decomposes was robbing the river of its oxygen.
Our studies of the Mara river suggest that large wildlife
and unregulated rivers can also have low oxygen events like we
see where there is human inputs of sewage or other agricultural
waste. So our studies on the Mara River are giving us
a picture of what rivers may have looked like historically
when they were unregulated and when there were large
populations of wildlife interacting with the river. [Sound- African bird call ]

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