MEPS 504:59-72 (2014)  -  DOI: https://doi.org/10.3354/meps10746

ABSTRACT: Compound-specific isotope analysis of amino acids (CSI-AA) is increasingly used to decouple trophic isotopic effects from isotopic composition at the base of food webs. The δ¹³C values of essential amino acids (EAAs) are particularly useful as recorders of primary production, because animals cannot synthesize EAAs de novo, so diagnostic biosynthetic δ¹³C_EAA patterns remain unchanged up food chains. To test the potential for δ¹³C_AA values to identify C source and resource flow in complex littoral ecosystems, we measured bulk and δ¹³C_AA values of Mytilus californianus adductor muscle tissue in samples spanning >1700 km of coastline from San Diego, California, to southern Oregon, USA. The average bulk δ¹³C value for the entire region clustered around a relatively constant value (-15.7 ± 0.9‰), with no latitudinal trend (R² = 0.022). δ¹³C_AA patterns were highly consistent in all mussels, and δ¹³C_EAA patterns closely matched expectations for marine phytoplankton, supporting the hypothesis that consumer EAA isotope values are unchanged from primary production. While bulk δ¹³C values were ambiguous in terms of source, application of a multivariate δ¹³C_EAA ‘fingerprinting’ approach clearly indicated microalgae as the major source for all mussel EAAs. We hypothesize that integrated δ¹³C values of coastal phytoplankton production can therefore be directly derived from δ¹³C_EAA of filter feeders in coastal regions. A correction derived from literature data for phytoplankton δ¹³C_EAA values yielded reasonable δ¹³C values for coastal production, supporting this idea. Together, our results suggest δ¹³C_EAA as a new approach for constructing baseline δ¹³C isoscapes and may have important implications for ecosystem studies in both modern and paleo-environments.

KEY WORDS: Mytilus californianus · Carbon isotopes · Compound-specific isotope analysis · California · Isoscape