Regional and interannual productivity of biogenic components and planktonic foraminiferal fluxes in the northwestern Pacific Basin

Abstract

Time-series sediment trap experiments at subtropical (WCT-1) and subarctic (WCT-2) stations in the northwestern Pacific indicate seasonal, latitudinal and depth variations in total particulate, biogenic and foraminiferal fluxes. At the subtropical station, the average total mass flux was 19.4 mg m −2 day −1 in the shallow trap (1060 m) and 21.5–26.1 mg m −2 day −1 in the deep trap (3930 m) during the sampling period. At subarctic station, these values were 91.5–176.9 mg m −2 day −1 in the shallow and 68.6–112.3 mg m −2 day −1 in the deep trap. We recognized 12 and 15 planktonic foraminiferal species at Station WCT-1 and Station WCT-2, respectively. The planktonic foraminiferal flux and species turnover are related to seasonal and interannual changes in source water and water column conditions at both stations. At Station WCT-1, the highest flux was recorded during the summer, with a peak in mid to late June associated with similar flux patterns of the dominant species, Globigerinoides ruber and Globigerinita glutinata. The total flux of foraminiferal tests at the shallow and deep traps is similar in numbers and magnitude. At Station WCT-2, the peaks of total flux of foraminiferal tests at the two trap depths differ in number, and their magnitude in the deep trap is almost half of that in the shallow trap. A distinctive seasonal pattern occurred in the shallow and the deep trap, with a peak in total foraminiferal flux in mid June to mid July. Globigerina quinqueloba, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei dominate the planktonic population throughout the year. Subtropical Station WCT-1 was characterized by low total foraminiferal fluxes and low total mass flux, which is dominated by calcium carbonate and depleted in opal, whereas high foraminiferal fluxes and a high total mass flux dominated by high biogenic opal, and less calcium carbonate and organic matter characterize subarctic Station WCT-2. The foraminiferal carbonate that reaches the seafloor accounts for an average 20–27% and 22–23% of the total calcium carbonate at Station WCT-1 and Station WCT-2, respectively. The primary reason for the difference in flux at both stations thus lies in the different contributions of siliceous and calcareous planktonic assemblages. The seasonal variation in biogenic particulate flux at both stations implies that temporal changes in biological productivity are governed by large-scale seasonal climatic variability and local hydrography.