Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills

Christopher L Dutton,Stephen K Hamilton,David M Post,Amanda L Subalusky,Emma J Rosi

doi:10.1038/s41467-018-04391-6

Christopher L Dutton, Stephen K Hamilton + Show 3 more

Open Access

https://doi.org/10.1038/s41467-018-04391-6

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Abstract

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia. Hypoxia is often attributed to anthropogenic pollution and is not common in unpolluted rivers. Here we show that organic matter loading from hippopotami causes the repeated occurrence of hypoxia in the Mara River, East Africa. We documented 49 high flow events over 3 years that caused dissolved oxygen decreases, including 13 events resulting in hypoxia, and 9 fish kills over 5 years. Evidence from experiments and modeling demonstrates a strong mechanistic link between the flushing of hippo pools and decreased dissolved oxygen in the river. This phenomenon may have been more widespread throughout Africa before hippopotamus populations were severely reduced. Frequent hypoxia may be a natural part of tropical river ecosystem function, particularly in rivers impacted by large wildlife.

Highlights

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia
In 49 out of the 55 flushing flows we documented over 3 years, dissolved oxygen (DO) concentrations at the New Mara Bridge (NMB), downstream of all hippo pools in our study region, decreased by 0.04 to 5.5 mg L−1 over a range of discharges (Fig. 1a, b; Supplementary Fig. 1)
It is possible that there were more fish kills than we were able to document because the Mara River flows through a remote area, and we have observed that scavengers rapidly consume the fish carcasses associated with fish kills

Summary

Introduction

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia. Evidence from experiments and modeling demonstrates a strong mechanistic link between the flushing of hippo pools and decreased dissolved oxygen in the river This phenomenon may have been more widespread throughout Africa before hippopotamus populations were severely reduced. Numerous wildlife species transport substantial amounts of nutrients and organic matter from terrestrial into aquatic ecosystems[18,19,20,21,22,23] These resource subsidies can have strong effects on recipient ecosystem function[2,24,25]. Large increases in discharge (>2× above calculated baseflow conditions, defined hereafter as a flushing flow) often result in dissolved oxygen (DO) decreases, sometimes to hypoxic levels, in the river channel

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