Carbon stable isotopes suggest that hippopotamus-vectored nutrients subsidize aquatic consumers in an East African river

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 ( d 13 C) 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 d 13 C 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 d 13 C 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 d 13 C values, suggesting that higher flows reduced retention and use of H. amphibius subsidies. These data provide preliminary evidence that d 13 C values may be useful, in certain contexts, for quantifying the importance H. amphibius organic matterThe 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 ( d 13 C) 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 d 13 C 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 d 13 C 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 d 13 C values, suggesting that higher flows reduced retention and use of H. amphibius subsidies. These data provide preliminary evidence that d 13 C values may be useful, in certain contexts, for quantifying the importance H. amphibius organic matter