Vertical distribution, community structure, and active carbon flux of two macrozooplankton taxa: Amphipods and euphausiids in the summer of the western North Pacific

This study conducted diel vertical migration and active migration flux estimation of macrozooplanktonic amphipods and euphausiids at 0-250 m water column of the three stations in the western North Pacific during summer. For amphipods, 25 species belonging to 17 genera were identified. Their standing stock was 60-574 ind. m-2 during the daytime and 35-5,228 ind. m-2 at night-time. For euphausiids, 19 species belonging to 7 genera were identified. The standing stock of euphausiids was 80-382 ind. m-2 and 286-2,156 ind. m-2 during the day and at night, respectively. Feeding impacts during the night were estimated to be 0.19-11.76 mg C m-2 day-1 (amphipods) and 5.12-16.42 mg C m-2 day-1 (euphausiids). Respiration during the daytime accounted for 0.08-6.38 mg C m-2 day-1 (amphipods) and 2.33-7.70 mg C m-2 day-1 (euphausiids). The feeding impact and respiratory flux by active migration of microzooplankton taxa were estimated as 8.34-28.18 mg C m-2 day-1 and 3.15-14.08 mg C m-2 day-1, respectively. These values corresponded to 3.7-12.4% primary production (feeding impact) and 2.4-10.7% of sinking passive flux from the euphotic zone (respiratory flux). Thus, this study emphasizes the importance of active flux by microzooplankton for vertical material flux in the western North Pacific.