Reconstructing δ13C isoscapes of phytoplankton production in a coastal upwelling system with amino acid isotope values of littoral mussels

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 δ 13CEAA patterns remain unchanged up food chains. To test the potential for δ13CAA values to identify C source and resource flow in complex littoral ecosystems, we measured bulk and δ13CAA 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). δ13CAA patterns were highly consistent in all mussels, and δ13CEAA 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 δ13CEAA '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 δ13CEAA of filter feeders in coastal regions. A correction derived from literature data for phytoplankton δ 13CEAA values yielded reasonable δ13C values for coastal production, supporting this idea. Together, our results suggest δ13CEAA as a new approach for constructing baseline δ13C isoscapes and may have important implications for ecosystem studies in both modern and paleo-environments. © Inter-Research 2014.