The hippopotamus conveyor belt: vectors of carbon and nutrients from terrestrial grasslands to aquatic systems in sub‐Saharan Africa

Abstract

Summary Hippopotami can play a significant role as ecosystem engineers and may play an important role as carbon and nutrient vectors from savanna grasslands to aquatic systems. We coupled the results of a feeding study of captive hippopotami, faeces leaching/mineralisation experiments, hippopotamus consumption estimates and the stoichiometry of savanna grasses to calculate excretion and egestion rates of hippopotami. We then used time budgets and population estimates to calculate nutrient loading by hippopotami in the Mara River, Kenya. In captivity, hippopotami consumed 11.0 g dry matter (DM) kg hippopotamus−1 day−1 (110.6 C; 6.8 N; 1.0 P) and egested 5.3 g DM kg−1 day−1. They excreted or egested 2.72 g C, 0.28 g N and 0.04 g P kg hippopotamus−1 day−1, and urine comprised 12% of C, 70% of N and 33% of P of total excretion and egestion. By integrating data from previously published work on hippopotamus digestion with the data we collected in the field, we estimated an average hippopotamus in the Mara River would excrete or egest 1.93–3.58 g DM, 0.78–1.47 g C, 0.13–0.19 g N and 0.01–0.02 g P kg hippopotamus−1 day−1, and that half of this excretion/egestion would enter the river. The hippopotamus population increased by 1500% inside the Maasai Mara National Reserve, Kenya, between 1959 and 2006. We estimate that hippopotami egest 36 200 kg faeces day−1 into the Mara River (wet mass). Daily loading into the river by excretion and egestion equals 8563 kg DM, 3499 kg C, 492 kg N and 48 kg P, which is equivalent to 670% of CPOM, 15% of DOC, 27% of TN and 29% of TP of loading from the upstream catchment. This research provides the first estimates for hippopotamus inputs to rivers that include both excretion and egestion and provides evidence that hippopotami are important resource vectors in sub‐Saharan African rivers, even when compared to other sources of carbon and nutrients.