274 — Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills
For years, researchers blamed the cases of extreme oxygen-concentration reduction, hypoxia, of the water in the Mara River of East Africa on human activity: Injecting sewage streams directly into rivers is known to reduce the dissolved oxygen (DO) from the water.
Hippopotami poop a lot.
And because they’re large animals, they’re restricted to deep pools of water in which to wallow.
It turns out that according to the mathematical models put forth by the researchers in this work, the sudden flooding and “flushing” of “hippopotamus pools” — essentially, the flooding of these very poopy pools of water — is likely a cause of numerous cases of fish die-off and the dramatic reduction in DO levels.
So hippos poop enough that when the pool’s water is eventually washed downstream, it…. kills stuff.
This is due to the tendency for pools to stratify: Low DO water tends to sink, and conduct electricity better, which makes it easier to measure using a robotic hippopotamus poop detector-boat. Hippo wallowing behavior keeps the flowing water aerated, but during these flushing events, the settled, deeper strata mix with the water flowing downstream.
This isn’t necessarily a bad thing, as scavengers take advantage of the fish carcasses after these fish kill events. (Remember that they are suffocated, not poisoned.) And these flushing events are likely important for maintaining the natural biodiversity of the rivers. The takeaway is that the relationship of these events — and the relationship of hippo poop — is more complicated than it seems at the surface, so to speak.