Understanding the importance of the Yellow Sea Bottom Cold Water Mass (≤10˚C) as an over-summering refuge for Euphausia pacifica in the Yellow Sea

Abstract

The Yellow Sea Bottom Cold Water Mass (YSBCM: ≤10˚C) is a unique physical feature of the Yellow Sea, which forms through winter cooling and mixing and persistently observed in the deep central region during summer. Although it has been recognized the important role of YSBCM on physical and biological processes in Yellow Sea, a few studies have been conducted to understand its role. Particularly, it provides a refuge (i.e. over-summering sites) for some organisms (i.e. Euphausia pacifica) to survive through the hot summer (>20˚C in surface). Therefore, various researches on E. pacifica, such as their feeding ecology, live experiments, and their spatial and seasonal distribution using net sampling with acoustics, are being conducted to better understand the role of the YSBCM on E. pacifica. According to the preliminary results, the high abundance of E. pacifica’s egg was found during the spring before YSBCM was formed but none in the summer. However, their lipid content (about 10% of dry weight) closely linked with their living strategy was not significantly changed between seasons. According to detailed profiles of the dietary lipid biomarkers and gut content analysis, E. pacifica adults mainly fed on diatoms/dinoflagellates during the spring phytoplankton bloom, while they fed on microzooplankton such as protozoa during the summer. These results suggest that E. pacifica may not only utilize the YSBCM as an over-summering site but also switch their food sources to survive through the hot summer.