Importance of the bottom cold-water mass as an over-summering refuge for Euphausia pacifica in the Yellow Sea

Abstract

The bottom cold water mass (YSBCM: ≤ 10°C) is a unique physical feature of the Yellow Sea. It forms through winter cooling and mixing and it is persistently observed in the deep central region during summer. Euphausia pacifica (hereafter simply called krill) is one of key species in the Yellow Sea because of the highest biomass among zooplankton and major prey for important fish species (anchovy, sardine, etc.). Some-how, they maintain their population in the Yellow Sea even though in the summer, when the surface water temperature goes up to 28°C, which is much higher than their high tolerance temperature (15°C). They should have developed the strategies (migra-tion, diapauses, lowering metabolism, etc) to survive through these extreme conditions (high temperature and low food availability). However, no solid evidence was reported yet to prove it. Therefore, in this study, we investigated their behavioral (i.e., diel verti-cal migration; DVM) and physiological shifts (i.e., diet, energy) from spring (whole wa-ter columns were well mixed) to summer (YSBCM was well developed) in order to un-derstand how krill survival through the hot summer, particularly what the role of YSBCM for their summer survival.