Assessing the Utility of Hydrogen, Carbon and Nitrogen Stable Isotopes in Estimating Consumer Allochthony in Two Shallow Eutrophic Lakes

Hydrogen stable isotopes (δ(2)H) have recently been used to complement δ(13)C and δ(15)N in food web studies due to their potentially greater power to separate sources of organic matter in aquatic food webs. However, uncertainties remain regarding the use of δ(2)H, since little is known about the potential variation in the amount of exchangeable hydrogen (H(ex)) among common sample materials or the patterns of δ(2)H when entire food webs are considered. We assessed differences in H(ex) among the typical sample materials in freshwater studies and used δ(2)H, δ(13)C and δ(15)N to compare their effectiveness in tracing allochthonous matter in food webs of two small temperate lakes. Our results showed higher average amounts of H(ex) in animal tissues (27% in fish and macroinvertebrates, 19% in zooplankton) compared to most plant material (15% in terrestrial plants and 8% in seston/periphyton), with the exception of aquatic vascular plants (23%, referred to as macrophytes). The amount of H(ex) correlated strongly with sample lipid content (inferred from C:N ratios) in fish and zooplankton samples. Overall, the three isotopes provided good separation of sources (seston, periphyton, macrophytes and allochthonous organic matter), particularly the δ(2)H followed by δ(13)C. Aquatic macrophytes revealed unexpectedly high δ(2)H values, having more elevated δ(2)H values than terrestrial organic matter with direct implications for estimating consumer allochthony. Organic matter from macrophytes significantly contributed to the food webs in both lakes highlighting the need to include macrophytes as a potential source when using stable isotopes to estimate trophic structures and contributions from allochthonous sources.