Understanding the 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, known as a key species in the Yellow Sea should use the YSBCM as a refuge to survive through the hot summer (>25°C in surface) because their high limited tolerance is 15°C. However, no solid evidence wasreported yet to prove it. Therefore, we investigated their horizontal and vertical distribution and feeding ecology during spring (YSBCM was not formed) vs. summer (YSBCM was well developed) in 2010. In spring, eggs predominated in all stations, but the early larval stage (nauplii) and gravid females (≥2cm TL) were concentrated in the coastal region with an active diel vertical migration. In summer, the abundance of larvae (calyptopis and furcilia stage) with a few number of eggs increased in the coastal region. Juveniles and small adults (≈1cm TL) were concentrated in the YSBCM of the central region. 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.