The following are three hypotheses for the ambiguous relationship between δ15NPOM and nitrate concentration: (1) our dataset; and (2) the variety of nitrogen sources. First, our observations were mainly conducted in the summer, during which time the nitrate was depleted at the surface, and the nitrate concentration in 182 of the 494 samples was not detectable (< 0.01 µM). The δ15NPOM in the nitrate-depleted waters varied greatly (mean ± SD: 2.8 ± 1.2‰, n = 182). This δ15NPOM variation in nitrate-depleted water may have rendered the relationship between δ15NPOM and nitrate concentration unclear, rendering the association statistically insignificant. When the GLM approach was conducted for subsamples with detectable nitrate (>0.01 µM), the nitrate concentration remained as the explanatory variable in the least-AIC model; the coefficient was negative but not significant (ANOVA, p = 0.12). As a result, the imbalanced dataset was not rejected; nonetheless, it was not the primary cause of the ambiguous relationship between δ15NPOM and nitrate concentration.