Taxon-specific variation in δ13C and δ15N of subfossil invertebrate remains: Insights into historical trophodynamics in lake food-webs

Carbon and nitrogen stable isotope ratios of sub-fossil invertebrate remains are potentially powerful indicators of nutrient flux, habitat-specific resource utilization, and trophic interactions in lentic food webs, but are rarely estimated for multiple species within lakes. Here we examined historical time series of delta C-13 and delta N-15 in remains of individual invertebrate taxa representing pelagic, littoral and benthic habitats during the 20th century in five boreal lakes of central Canada. We applied a novel statistical approach based on Generalized Additive Models (GAMs) to quantify the differences in centennial means and trends (i) between invertebrate remains and sedimentary organic matter (SOM), and (ii) among different taxa within each lake to evaluate the coherence of isotope signals during the 20th century. Differences in mean delta C-13 and delta N-15 were usually significant (p < 0.05) between SOM and invertebrate taxa, and among individual taxa, reflecting selective feeding by invertebrates and differences in trophic position within food webs. In contrast, patterns of historical variance in isotope values varied among lakes with few consistent differences between long-term isotopic trends of SOM and invertebrate remains. In particular, SOM and invertebrate isotopic trends were similar in relatively dystrophic lakes, likely due to the importance of terrestrial carbon in both SOM and invertebrate diets. However, significant SOM-invertebrate trend differences were observed for both delta C-13 and delta N-15 in relatively clear-water lakes, possibly reflecting temporal variation in diets or tissue fractionation. Comparisons of historical trends in isotope values among taxa revealed few consistent patterns, likely indicating uncoupled carbon and nitrogen fluxes through invertebrate consumers with different habitat specializations or feeding modes. Together, our findings suggest that evaluation of taxon-specific delta C-13 and delta N-15 can provide valuable insights into historical tropho-dynamics in lake food webs.