Short-term changes in population structure and vertical distribution of mesopelagic copepods during the spring phytoplankton bloom in the Oyashio region

To evaluate the responses to the spring phytoplankton bloom, short-term changes in population structure and vertical distribution of mesopelagic copepods (Gaetanus simplex, Gaidius variabilis, Pleuromamma scutullata, Paraeuchaeta elongata, P. birostrata, Heterorhabdus tanneri and Heterostylites major) were studied in the Oyashio region. Samples were collected with a 60 μm mesh VMPS from 9 strata between 0 and 1000 m both day and night on five occasions during March–April 2007. All the species except Heterorhabdidae species performed reproduction during the spring phytoplankton bloom, while no recruitment to copepodid stages was detected because the newly born individuals were eggs or nauplii. The shallower-living species, G. simplex, P. scutullata and P. elongata had nocturnal ascent diel vertical migration (DVM). While suspension feeding copepods cease DVM after 11 April (P. scutullata) or 23 April (G. simplex), carnivorous P. elongata continued DVM over the study period. Since the gut contents of G. simplex showed a nocturnal increment even in the period of no DVM (23 and 29 April), they might be feeding at depth without DVM. Thus, the cessation of DVM in mesopelagic suspension feeding copepods would be induced by the increase of sinking particles (e.g. food for suspension feeders) during the spring phytoplankton bloom.