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

Abstract

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 (TPGlu, based on glutamic acid and phenylalanine nitrogen isotope ratios) and both metazoan and protistan diets (TPAla, based on alanine and phenylalanine nitrogen isotope ratios). Both TPGlu and TPAla 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 TPGlu and TPAla, suggesting little contribution from protistan diets. Two copepod species (P. parvus s. l. and Acartia omorii) exhibited TPGlu values of approximately 2.0 but their TPAla 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 TPGlu (2.4–3.1) and TPAla (2.7–3.4), suggesting that protistan trophic transfers enhance TPAla (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. © 2024 Elsevier Ltd