Quantifying terrestrial carbon in freshwater food webs using amino acid isotope analysis: Case study with an endemic cavefish

Flow of terrestrial carbon though aquatic ecosystems (allochthony) is an important but underestimated component of the global carbon cycle. A lack of clear consensus about the importance of allochthonous (terrestrial) organic carbon is sometimes attributed to uncertainties associated with conventional 'bulk' isotope data, the most widely used ecological tracer. Amino acid-specific isotope analysis is an emerging research method promising to address existing limitations of bulk C and N isotope analyses. We tested the efficacy of amino acid delta C-13 data as a generalizable measure of allochthony by analysing an aggregated dataset (n = 168) of primary and secondary data of carbon sources from disparate geographical locations across the globe. We found the delta C-13 fingerprints amino acids to be consistently distinct between allochthonous (terrestrial) and autochthonous (aquatic) carbon sources. We also found that our approach is most effective when we use only essential amino acid tracers (i.e. isoleucine, leucine, phenylalanine, threonine and valine). Predictive trends in delta C-13 fingerprints appear to be largely compatible across studies and/or laboratories. As a case study, we used this approach to quantify the contribution of terrestrial carbon to an endemic cavefish, Cryptotora thamicola, and found that its biomass was comprised largely of autochthonous carbon (similar to 75%).