Diel vertical migration of the copepod Calanus sinicus before and during formation of the Yellow Sea Cold Bottom Water in the Yellow Sea

Abstract

To understand the effects of the Yellow Sea Cold Bottom Water (YSCBW) on the diel vertical migration (DVM) of the copepod Calanus sinicus, we surveyed vertical distribution of C. sinicus at a fixed station in the Yellow Sea before (spring) and during (summer) formation of the YSCBW. Cold water (<10°C) was observed in the bottom layer when the water column was thermally stratified in summer, but the water column was thermally well-mixed in spring 2010. Samples were collected from five different layers at 3-h intervals using an opening-closing net. Adult females (1–155 ind./m³) showed a clear normal DVM pattern throughout the entire water column in spring, whereas adult males did not migrate. DVM of copepodite V (CV) individuals was not clear, but the maximum abundance of CI-CIV occurred consistently in the upper 10–20 m layer, where there was a high concentration of chlorophyll-a (Chl-a) (0.49–1.19 µg/L). In summer, weak DVM was limited to cold waters beneath the thermocline for adult females (<30 ind./m³), but not for adult males. The maximum abundance of CI-CIV also occurred consistently in the subsurface layer (20–40 m) together with high concentrations of Chl-a (0.81–2.36 µg/L). CV individuals (1–272 ind./m³) moved slightly upward nocturnally to the near-surface layer (10–20 m), where the average temperature was 25.74°C, but they were not found in the surface layer (0–10m; 28.31°C). These results indicate that the existence of the YSBCW affected food availability at depth and the vertical temperature distribution, leading to variation in the amplitude and shape of stage-specific vertical distributions (CI to adults) in C. sinicus before and during the formation of cold waters in the Yellow Sea during the study period.