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 13 C values of essential amino acids (EAAs) are particularly useful as recorders of primary production, because animals cannot synthesize EAAs de novo, so diagnostic biosynthetic 13 C EAA patterns remain unchanged up food chains. To test the potential for 13 C AA values to identify C source and resource flow in complex littoral ecosystems, we measured bulk and 13 C 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 13 C value for the entire region clustered around a relatively constant value (-15.7 0.9 ), with no latitudinal trend (R 2 = 0.022). 13 C AA patterns were highly consistent in all mussels, and 13 C EAA patterns closely matched expectations for marine phytoplankton, supporting the hypothesis that consumer EAA isotope values are unchanged from primary production. While bulk 13 C values were ambiguous in terms of source, application of a multivariate 13 C EAA 'fingerprinting' approach clearly indicated microalgae as the major source for all mussel EAAs. We hypothesize that integrated 13 C values of coastal phytoplankton production can therefore be directly derived from 13 C EAA of filter feeders in coastal regions. A correction derived from literature data for phytoplankton 13 C EAA values yielded reasonable 13 C values for coastal production, supporting this idea. Together, our results suggest 13 C EAA as a new approach for constructing baseline 13 C isoscapes and may have important implications for ecosystem studies in both modern and paleo-environments.