Seasonal changes in energy sources and trophic positions of E. pacifica and C. sinicus in the central-eastern Yellow Sea: Insights from bulk and CSIA-AAs

Abstract

To compare the seasonal changes in energy sources and trophic positions of Euphausia pacifica and Calanus sinicus, crucial food sources for fishery resources, zooplankton samples were collected from August 2020, May and October 2021, and March 2022 in the central-eastern Yellow Sea. Then bulk carbon and nitrogen stable isotope ratios (δ13C and δ15N) and δ15N of amino acids in both zooplankton species were determined. Although there were variations by season and/or station, the bulk δ15N values of the krill E. pacifica and the copepod C. sinicus were relatively similar. In contrast, their bulk δ13C values showed distinguishing differences in periods of high phytoplankton biomass. Upon examining the trophic position (TP) calculated through the analysis of δ15N in amino acids, it was observed that in season and/or station with low phytoplankton biomass, TPGlu values were similar (~2.3) between the two species. However, when phytoplankton biomass was high, the value of E. pacifica decreased to around 2, while for C. sinicus, it exceeded 2.4 at certain stations. Meanwhile, a relatively large difference in TPAla was observed between the two species, with TPAla values being higher compared to TPGlu. This suggests that although variations in importance by season and/or station, both species are supplied with energy through the microbial food web. Considering their role as crucial food sources for higher trophic level consumers, these findings demonstrate the significance of energy transfer via the microbial food web to fisheries resources in the central-eastern Yellow Sea.