Calculation of trophic position and δ15NBulk of each sample
Trophic position (TP) of samples with δ15NAAs data were estimated based on differences in δ15N between glutamic acid and phenylalanine as follows (Chikaraishi et al. 2009):
, (2)
where δ15NGlu and δ15NPhe represent the δ15N values of glutamic acid and phenylalanine, respectively. We also calculated δ15NBase by using the estimated TPGlu-Phe and the trophic discrimination factor of phenylalanine (0.4‰) as follows:
. (3)
We then performed linear regression analysis to explain variations in the δ15NBase of each species as a function of δ15NBulk as follows:
, (4)
where a and b were derived from the liner regression equation. In cases where the correlation between δ15NBase and δ15NBulk in each equation was high (i.e. r2 > 0.8; see Fig. S2), the regression equations were used to calculate δ15NBase values of copepod samples for which δ15NAAs was not measured.