Seasonal Response of Planktonic Foraminifera to Surface Ocean Condition: Sediment Trap Results from the Central North Pacific Ocean

The fluxes of planktonic foraminifera (calcareous shell producing zooplankton) were examined in order to clarify temporal and regional variations in production in the upper ocean in relation to hydrographic conditions. Three time-series sediment traps were deployed in the central North Pacific along 175°E for about one year, beginning in June 1993. Trap sites were located in the subarctic, the transition, and the subtropical water masses, from north to south. The southernmost site was under the influence of the transition zone in January to May. Both temporal and regional fluxes of planktonic foraminifera showed large variations during the experiment. In the subarctic water mass, high total foraminiferal fluxes (TFFs) and high organic matter fluxes (OMFs) were observed during summer to fall, suggesting that food availability is the most important factor for the production of planktonic foraminifera. Furthermore, low TFFs during winter were ascribed to low food availability and low temperatures. The OMFs and TFFs correlated well and increased rapidly after the disruption of the seasonal thermocline during winter, peaking in late February to early March in the transition zone. In the subtropical water mass, both OMFs and TFFs remained low due to lower productivity under oligotrophic conditions. In general, TFFs show a positive correlation with OMFs during the trap experiment, suggesting that food availability is one of the factors controlling the production of planktonic foraminifera in the central North Pacific. Relatively low TFFs during summer to fall in the subtropical water mass may be caused by the thermal structure of the upper ocean. Low SST possibly reduces the production of foraminifera during winter in the subarctic region.