Trophic variability of inter- and intra-copepod species in the South Sea of Korea during summer

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 showed a similar herbivorous TP of 2.0 in both TP_Glu and TP_Ala, suggesting little contribution from protistan diets. Two copepod species (P. parvus s. l. and Acartia omorii) exhibited TP_Glu values of approximately 2.0 but their TP_Ala values increased from 0.1 to 0.5, indicating mixed diets of both primary producers and protists. The other three copepods (Pseudocalanus sp., Oithona similis, and O. atlantica) showed a wide range in TP_Glu (2.4–3.1) and TP_Ala (2.7–3.4), suggesting that protistan trophic transfers enhance TP_Ala (by up to 0.5) in omnivorous copepods. We found a spatial variation in the TPs of copepods among water masses by various controlling factors including surface seawater temperature, salinity, and size-fractionated Chl-a. Our findings support that the TP values could be potential indicative of interspecies variability, providing useful information on the composition of the planktonic food web.