Abstract
The common hippopotamus, Hippopotamus amphibius, transports millions of tons of organic matter annually from its terrestrial feeding grounds into aquatic habitats. We evaluated whether carbon stable isotopes (δ13C) can be used as tracers for determining whether H. amphibius‐vectored allochthonous material is utilized by aquatic consumers. Two approaches were employed to make this determination: (1) lab‐based feeding trials where omnivorous river fish were fed a H. amphibius dung diet and (2) field sampling of fish and aquatic insects in pools with and without H. amphibius. Lab trials revealed that fish fed exclusively H. amphibius dung exhibited significantly more positive δ13C values than fish not fed dung. Fish and aquatic insects sampled in a river pool used for decades by H. amphibius also exhibited more positive δ13C values at the end of the dry season than fish and insects sampled from an upstream H. amphibius‐free reference pool. Fish sampled in these same pools at the end of the wet season (high flow) showed no significant differences in δ13C values, suggesting that higher flows reduced retention and use of H. amphibius subsidies. These data provide preliminary evidence that δ13C values may be useful, in certain contexts, for quantifying the importance H. amphibius organic matter.
Highlights
Stable isotopes of carbon, nitrogen, and hydrogen are commonly employed to measure how freshwater consumers use terrestrially generated allochthonous organic matter (Finlay 2001, Doucett et al 2007, Finlay and Kendall 2008)
Hippopotamus amphibius surveys A total of 38 visual surveys split evenly between the H. amphibius pool and reference pool were conducted in January and April 2012
Results from laboratory feeding trials and field sampling preliminarily suggest that fish and aquatic invertebrates in Kenya’s Ewaso Ng’iro River make use of organic matter vectored into the river by resident H. amphibius
Summary
Nitrogen, and hydrogen are commonly employed to measure how freshwater consumers use terrestrially generated allochthonous organic matter (Finlay 2001, Doucett et al 2007, Finlay and Kendall 2008). One potentially important but little studied route of terrestrial to aquatic organic matter subsidization in African watersheds may be maintained by the common hippopotamus, Hippopotamus amphibius This herbivorous semiaquatic mega-consumer forages widely on land at night consuming largely terrestrial C4 grasses and some browse (Eltringham 1999, Grey and Harper 2002, Cerling et al 2008). In this study we use carbon stable isotope (d13C) measurements to evaluate whether the organic matter that H. amphibius excrete into riverine ecosystems is utilized by aquatic consumers. Carried out feeding trials of captive river fish fed exclusively H. amphibius dung and conducted field sampling of aquatic vertebrates and invertebrates in parts of a river in central Kenya that did and did not harbor H. amphibius
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