Estimating terrestrial contribution to stream invertebrates and periphyton using a gradient‐based mixing model for δ 13 C

Summary 1.  This paper outlines a gradient‐based model that can be used for isotopic signature source partitioning, even if source signatures are not distinct, as long as their spatial gradients differ. A model of this type is applied to the partitioning of autochthonous vs. allochthonous contribution to stream invertebrate δ 13 C signatures, which has often been confounded by overlap in source signatures. 2.  δ 13 C signatures of inorganic carbon and most autochthonous production exhibit pronounced gradients along rivers, being depleted relative to terrestrial signatures in upstream reaches, and enriched downstream. Terrestrial detritus, by contrast, exhibits no gradient. Thus terrestrial food consumption reduces downstream signature slopes in proportion to the amount of terrestrial food consumed. 3.  The gradient‐based mixing model produces estimates of the proportion of terrestrial consumption ( p T ) from signature slopes of consumers; p T estimates for invertebrate primary consumers were: herbivore/grazers (0·15) 1, indicating selective assimilation of the autochthonous component from the biofilms.