Stable hydrogen and oxygen isotopes in aquatic food webs are tracers of diet and provenance

The stable hydrogen and oxygen isotope (2H and 18O) composition of animal tissues are well-established tracers for terrestrial migration ecology and wildlife forensics. However, the behaviour of these isotopes in aquatic ecosystems and their potential as tracers of diet and provenance are complicated because of inputs from ambient H2O and diet. We conducted controlled aquaria-based experiments to quantify the mechanisms that drive the H and O isotopic flow within and among aquatic species. The isotopic composition of water and diet of two aquatic species (Chironomus tentans and Poecilia reticulata), representing two trophic levels, was varied in six isothermal treatments. Both species were raised from juvenile to adult to ensure that tissues were in isotopic equilibrium with their dietary and environmental conditions (ambient water, food, dissolved oxygen). We measured water, dissolved O2, diet, tissue protein and lipids for 2H or 18O. The flows of H and O isotopes for tissue formation in aquatic organisms were parameterized using a steady-state multi-pool mass-balance model. The ambient H2O contribution to tissue protein H and O isotopes in both species was significant (3050% for 2H and >80% for 18O). An apparent trophic effect for 2H and isotopic discrimination between water and protein for 18O (c. 15 parts per thousand) were identified. Our isotopic data and model revealed potential applications and cautions in using 2H and 18O measurements for ecological studies in aquatic food webs. Tissue 2H values may be a complementary trophic tracer in aquatic food webs, but only when the main controlling mechanisms are properly accounted for (i.e., H isotopic exchange with water during protein synthesis and influence of metabolic water). Measurements of 18O, on the other hand, reflect that of water and so can be used for predicting isotopic assignment to origin of aquatic organisms as there is no complicating trophic effect, but more 18O field data and improved analytical precision may be required to better establish the strengths to ecological applications.