Abstract: Stable carbon and nitrogen isotope values (δ¹³C and δ¹⁵N, respectively) of zooplankton can reflect environmental changes in marine and are commonly used to establish a stable isotopic baseline. In this study, copepods were collected from the northwest Pacific Ocean in March (spring) and September (autumn) of 2019. We investigated the impacts of morphological characteristics of copepods and environmental factors on stable isotopic values. The results showed that the morphological characteristics (length, area, and volume) of copepods were significantly higher in spring than that in autumn (P<0.05), and that the δ¹⁵N values were significantly positively correlated with morphological characteristics in both seasons. For δ¹³C values, only copepods in spring had a significant positive correlation with morphological characteristics, probably due to the similar distances between sampling locations and Japan islands in autumn with similar carbon sources. Results of the generalized additive model (GAM) showed that the seasonal variations of δ¹³C and δ¹⁵N values in copepods were mainly correlated to chlorophyll-a concentration and water temperature at 50 m depth. These findings provide basic information for further studies of stable isotope ecology of marine organisms in the northwest Pacific Ocean.

Key words: northwest Pacific Ocean, zooplankton, stable isotope, morphological characteristics, environmental factor