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Spatial variation and interspecies differences in the trophic position (TP) of copepods were investigated using nitrogen isotope ratios of amino acids. In the summer of 2021, coastal waters and the Changjiang diluted water generated clear seawater temperature and salinity fronts in the South Sea of Korea. Paracalanus parvus s. l. was a dominant species in the copepod community, and the second dominant species differed among inshore, intermediate, and offshore sites. The TP of each copepod species was estimated in two ways, considering only metazoan diets (TP_Glu, based on glutamic acid and phenylalanine nitrogen isotope ratios) and both metazoan and protistan diets (TP_Ala, based on alanine and phenylalanine nitrogen isotope ratios). Both TP_Glu and TP_Ala indicated trophic variability among copepod species and the contribution of protistan diets as a food source in the study area. Calanus sinicus …