Thursday, June 7, 2007 - 11:30 AM
379

Hydrogen isotopes outperform carbon in discriminating between aquatic and terrestrial energy sources

Jane C. Marks, PhD1, Eugènia Martí2, Daniel Von Schiller2, Miquel Ribot2, Paula Fonolla2, and Richard Doucett1. (1) Department of Biological Sciences, Northern Arizona University, PO Box 5640, Flagstaff, AZ 86011, (2) Limnology Group, Centre d'Estudis Avançats de Blanes (CSIC), Accés a la Cala Sant Francesc 14, Blanes (Girona), 17300, Spain

Comparing how functional feeding groups use algal versus terrestrial energy sources is essential for testing fundamental theories in stream ecology including the river continuum concept. Recent studies have relied on stable isotopes of carbon to calculate the percent of algae versus leaf litter in animal diets.  Carbon isotopes however have limited utility where isotope signatures of algae and detritus overlap.  Here we demonstrate the utility of hydrogen isotopes for discriminating between terrestrial and aquatic energy sources by comparing C versus H isotopes in 14 streams in a Mediterranean catchment. In each stream we collected leaf litter, fine benthic organic matter (fbom) and algae.  Algal samples were separated into four categories: biofilm (rockscrapes), green filaments, cyanobacteria mats.  The average absolute difference in isotope values between plants and algae was significantly larger for hydrogen (63.5 ppm) than carbon isotopes (4.2 ppm).  Variation based on replicate samples of a food source within a river was similar for both elements.  Hydrogen isotopes definitively separated algae from leaves in every river.  In contrast, in 9 rivers overlap in carbon signals between algae and detritus prevented clear results.  Hydrogen is most useful in systems where there are multiple types of algae with different carbon isotope signatures. 

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