Use of Stable Isotopes in Benthic Organic Material as a Baseline for Estimating Fish Trophic Positions in Lakes.

Estimating the trophic position of predators in food webs using stable isotopes requires establishing a 'baseline'; typically served by bivalves or snails. However, the frequent absence of such organisms in lakes leaves researchers in a difficult situation. Here we test the hypothesis that trophic position (TP) of largemouth bass (LB) in lakes can be estimated from δN of benthic organic material (BOM) in littoral surficial sediment. δN as a function of δN was significant across 51 lakes with an R of 0.84, supporting our hypothesis. In a subset of six lakes, the mean TP did not differ significantly when calculated using baselines of δN in BOM, mussels (UN), gastropods (SN), or from a stable isotope mixing model. In a subset of 26 lakes, mean TP calculated from δN and δN baselines were not significantly different, but TP calculated using δN was not significantly correlated with TP calculated using δN. Although δN should not be used interchangeably with δN as a baseline for estimating TP, δN can be useful as a separate baseline where estimates of fish TP are needed in all study lakes given that mussels are frequently absent and BOM is always available for collection.