Isotopic discrimination of ¹⁵N / ¹⁴N of amino acids among the calanoid copepod Acartia steueri and its food items, eggs, and fecal pellets

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Stable nitrogen isotopic composition (δ¹⁵N) of glutamic acid and phenylalanine in a laboratory-cultured calanoid copepod Acartia steueri and in its food items (the diatom Thalassiosira weissflogii), eggs, and fecal pellets was determined to examine the isotopic discrimination of ¹⁵N / ¹⁴N of amino acids among these samples associated with a single grazing process. Glutamic acid has a wide variation in the δ¹⁵N value among the bodies (+ 8.8‰ for 10 days with feeding and + 8.9‰ for the following one day without feeding), eggs (+ 8.8‰), and fecal pellets (+ 3.3‰) of A. steueri and its food items (+ 0.3‰), whereas phenylalanine falls in a narrow range of the values from − 3.7‰ to − 3.1‰, 1σ = 0.3, for all samples. Based on these δ¹⁵N values, the trophic position (TP_{Glu / Phe}) was characterized as 2.1 for bodies and eggs, 1.5 for fecal pellets, and 1.1 for the food items. These TP_{Glu / Phe} values for A. steueri and its food items are consistent with the expected trophic positions (2.0 and 1.0, respectively) of these samples. To our knowledge, this is the first published data for the TP_{Glu / Phe} value of fecal pellets, which is 0.4 units higher than that of the food items. Because fecal pellets supply basal food resources for many organisms living in aphotic zones, our data will provide a better understanding of their trophic positions.