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 δ¹⁵N of benthic organic material (BOM) in littoral surficial sediment. δ¹⁵N_LB as a function of δ¹⁵N_BOM was significant across 51 lakes with an R² of 0.84, supporting our hypothesis. In a subset of six lakes, the mean TP_LB did not differ significantly when calculated using baselines of δ¹⁵N in BOM, mussels (UN), gastropods (SN), or from a stable isotope mixing model. In a subset of 26 lakes, mean TP_LB calculated from δ¹⁵N_BOM and δ¹⁵N_UN baselines were not significantly different, but TP_LB calculated using δ¹⁵N_BOM was not significantly correlated with TP_LB calculated using δ¹⁵N_UN. Although δ¹⁵N_BOM should not be used interchangeably with δ¹⁵N_UN as a baseline for estimating TP_LB, δ¹⁵N_BOM 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.