Hippos in the Mara River, Kenya; Photo: Chris Dutton
Death by poop – it sounds like a horrible way to go, but that is precisely what happens to the resident fish in East Africa’s Mara River. The river flows through the Maasai Mara National Reserve of Kenya and the Serengeti National Park of Tanzania. In the Kenyan portion of the river there are more than a whopping 4000 hippopotami spread throughout 171 hippo pools in the main channel and the associated tributaries.
As one can imagine this leads to a lot of poop and it is estimated that the hippos release over 8500 kg (9.3 tons) of organic matter into this ecosystem DAILY. Typical to other rivers in the region, the Mara River’s channel is deeply cut. This is significant because during times of high discharge levels the water remains within the channel rather than dissipating over the flood plains. As a result, during certain times it can lead to heavily reduced dissolved oxygen (DO) and even hypoxic conditions in the channel. Work on this ecosystem has recently been published in Nature Communications and asks all the burning questions we have a hippo poop.
One of the researchers Emma Rosi, an ecologist at the Cary Institute of Ecosystem Studies, says "We were interested in how this massive influx of organic matter and nutrients influenced aquatic life."
As hippos are so dangerous to humans the team used remote controlled boats fitted with sensors to gather data from the hippo pools. At night the hippos move to land to feed on grasses and during the day rest and seek refuge within the rivers. When they defecate, all that nutrient-rich poop descends to the river bed where it is decomposed. Bacteria have the task of this dirty job and as they decompose the faeces they also consume oxygen in the water, effectively making it hypoxic. Not only that, the microbial activity also produces potentially toxic chemicals like carbon dioxide, methane, ammonium, and hydrogen sulfide.
The hippos themselves are fine and happy in their poop filled hypoxic pools, however when river water is high due to rainfall, the anoxic waters are flushed into fish habitats. The sudden influx of water with reduced oxygen causes a state of temporary hypoxia and effectively causes the fish to suffocate. Led by ecologist Christopher Dutton of Yale University, the team documented 55 incidents where the river flows increased two-fold over three years. From the total, 49 events had measured decreases in DO ranging from 0.04 to 5.5 mg L-1. Thirteen of the flushing events had DO concentrations lower than 2 mg L-1, which is the concentration defined as hypoxic in terms of the stress to the fishes, and lasted for several hours. Four mass fish kills were documented over the course of this study and a further five kills were reported prior to the initiation of water quality sampling in 2013.
The authors are quick to note that this is a natural process. High flushing events due to rainfall clean out the river and carcasses provide a food source for scavenging terrestrial and aquatic species that live in the ecosystem, like crocodiles and birds. Changes to river morphology, hydrology and hippo populations over time likely influence the frequency and severity of the low DO events. It is no doubt that the hippos have a significant effect on the river, however is it not clear if anthropogenic factors directly exacerbate theses events.
For this fascinating work see the full text here:
https://www.nature.com/articles/s41467-018-04391-6
Dutton CL, Subalusky AL, Hamilton SK, Rosi EJ, Post DM (2018) Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills. Nature Communications. 9. Article number: 1951 doi:10.1038/s41467-018-04391-6
Hippo thats probably pooping during this photo. Photo: Tony Heald
