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, 9 (1), 1951

Organic Matter Loading by Hippopotami Causes Subsidy Overload Resulting in Downstream Hypoxia and Fish Kills

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Organic Matter Loading by Hippopotami Causes Subsidy Overload Resulting in Downstream Hypoxia and Fish Kills

Christopher L Dutton et al. Nat Commun.

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.

Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Observations of flushing flows, hypoxic events, fish kills, and hippo pools in the Mara River. a Dissolved oxygen (blue) and discharge (black) for a 3-month subset of data from November 2014 through January 2015. Red indicates discharge >2 times the calculated baseflow. b Dissolved oxygen at the beginning and lowest value for each flushing flow from the 3-year record. c Dead fish from a fish kill observed on December 10, 2013 (image credit: Christopher Dutton). d Robotic boat surveying a hippo pool (image credit: Amanda Subalusky). e 3-D interpolation of the conductivity within pools lacking or containing high densities of hippopotami
Fig. 2
Fig. 2
Oxygen consumption by river water after addition of either hippopotamus feces or hippo pool water (HPW). a Dissolved oxygen for the three treatments in the microcosm experiment (reference, hippopotamus feces, and HPW). Errors bars reflect standard deviation. b Dissolved oxygen over 2.5 days for the 6 experimental stream channels with variable amounts of hippopotamus feces added (dashed line indicates time of addition). c Dissolved oxygen over 2.5 days for the experimental stream channels with variable amounts of HPW added (dashed line indicates time of addition). d The linear relationship between maximum drop in dissolved oxygen and fraction of added HPW is shown as a solid black line with points corresponding to the colors in c. 95% confidence intervals shown in gray. The dashed line represents the drop in dissolved oxygen expected for simple mixing of anoxic water with oxic (air-equilibrated) water, without accounting for reaeration, and is thus the minimum possible drop
Fig. 3
Fig. 3
Experimental flush of a reference and treatment hippo pool. a A temporary dam was built upstream of the pool (image credit: Amanda Subalusky). b Map of the pool and sonde/sample location. c Dissolved oxygen (DO) and biochemical oxygen demand (BOD) for the whole-ecosystem manipulation immediately after breaching the dam in the reference and treatment flows. d Depth, DO, BOD, and Rhodamine tracer (RWT) immediately after the breach of the dam in the treatment flow

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