The explanatory variables and final GLM descriptions were selected using the corrected Akaike information criterion (AIC), which may be used to assess the likelihood of the model. The lsmean values and SEs based on the AIC-selected GLMs were used to visualize the influence of the explanatory variables on the δ13CPOM or δ15NPOM. ANOVA was used to test the effects of the lsmean values of the categorical variables (the class, season, and depth when they remained).