Hippos and the Mara River

Hippos and the Mara River
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    [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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