Vertical Distribution of Living Planktonic Foraminifera in the Northwestern North Pacific

Planktonic foraminifera reflect the upper ocean environments in terms of water temperature, salinity, food supply and so on. Fossil foraminiferal shells thus provide paleo-surface ocean information. A better understanding of the living planktonic foraminiferal ecology and the relationship between foraminifera and oceanographic conditions are important for more detailed reconstruction of paleo environments. Since detailed information on vertical foraminiferal distribution was not available in the northwestern North Pacific, we conducted the plankton tow in the upper 200 m for seven depth intervals (0-20 m, 20-40 m, 40-60 m, 60-80m, 80-120 m, 120-160 m, 160-200 m) at three sites (Site A (36N, 142E), Site B (41.5N, 142E) and Site b (41N, 143E)). We also measured hydrographic conditions and primary production with CTD and FRRF at the same time. The plankton tow yielded 20 foraminiferal species (> 125 um) at Site A and 8 species at both sites B and b. Globigerina bulloides (av. 19.7 %), Globigerinoides ruber (av. 18.9 %), Neogloboquadrina pachyderma (including Neogloboquadrina incompta) (av. 18.9 %), Neogloboquadrina dutertrei (av. 14.9 %), and Globigerinoides sacculifer (av. 6.7 %) were predominant at the southern site (Site A), and N. pachyderma (av. 49.2 and 37.5 %), Globigerina quinqueloba (av. 39.7 and 56.7 %), and G. bulloides (av. 6.4 and 4.7%) were abundant at northern sites (Sites B and b). The average standing stock of living foraminifera (0-200 m) were 24, 18, and 30 shells m-3 at Site A, Site B and Site b, respectively. The standing stocks at all three sites were closely correlated with the chlorophyll a concentrations except for the upper most surface layer at Site A, perhaps because major species in this study are mostly herbivores. The profiles of vertical temperature at northern sites (Site B and b) were very similar. However, the standing stock profiles at both sites showed different patterns depending on chlorophyll concentration. It suggests that foraminiferal population is more related by chlorophyll concentrations than by temperature in this study. On the other hand, standing stocks of surface layer (0-40 m) at the southern site (Site A) were poorly correlated to chlorophyll concentrations and temperature, and well correlated to light intensity. It can be attributed to predominance of endosymbiont-bearing species which may obtain nutrition from their symbiont photosynthesis. The hosting symbiont species, G. ruber and G. sacculifer and facultative symbiont species, N. dutertrei, Globigerinita glutinata, Pulleniatina obliquiloculata and Globorotalia menardii have high standing stocks of 45 shells m-3 at surface layer (0-20 m), mean value of 19 shells m-3 (0-80 m). These species were abundant only in surface layer and diminised rapidly with depth. Predominance of endosymbiont-bearing species is the reason behind high standing stock of surface layer. Vertical distribution of foraminiferal abundance depended more on light intensity and photosynthetic activity of symbiont than on chlorophyll concentration and temperature because of predominance of endosymbiont-bearing species.