Twenty-year long time-series fluxes of calcareous nannoplankton in the Bering Sea and central subarctic Pacific Ocean, 1990-2009

Two sediment traps set since 1990 and continued till 2010 in the Bering Sea and subarctic Pacific, allowing the long-term observation of particle fluxes. One trap was located at Station AB (57.5N, 177W) in the Bering Sea and the other at Station SA (49N, 174W), in the subarctic Pacific Ocean. Calcareous nanno fluxes were investigated for the 20 years. A significant shift in calcareous nanno's fluxes is observed around 2000. The dominant taxa at both sites were Coccolith pelagicus and Emiliniania huxleyi. At Station AB the mean total fluxes during the 20-year period was: 2.09x10^10 coccos m2 d-1; and 2.03x10^10 at Station SA. In addition, we proceeded with enumeration of discrete placoliths as well. Placolith counts of the two dominant taxa were converted into equivalent number of coccos. The calcareous nanno's fluxes show periodic occurrences with increases of twice a year. The first is during June, and the second is during October-November. The common traits of both stations are that, the major in June is C. pelagicus and Oct-Nov is E. huxleyi. The highest fluxes of the year occurred during Oct-Nov, conforming with the annual flux maxima of CaCO3 (Takahashi et al., 2000). The correlation results between nannos and CaCO3 mg m-2 d-1 indicate 0.67 at Station AB; and 0.61 at Station SA. Analogous flux changes of spring and fall are also seen in diatoms, planktonic foraminifers, and total mass (Takahashi et al., 2000, 2002, 2007; Asahi and Takahashi, 2007; Asahi et al., 2007; Onodera and Takahashi, 2009). The calcareous nanno's fluxes indicate a significant change around 2000 at both stations based on the performed T-test: the mean fluxes of the 20-years is used as a standard; and the flux difference between the values from each year and the mean is compared. Differences are accounted only for sometimes during 1990-2000 whereas frequent differences are encountered during 2000-2009. In addition, the calcareous nanno's fluxes decreased around 2000 at both stations. This result is also based on the binary conversion values. Furthermore, it appears that total flux levels of the number of coccos and placoliths are more or less the same during the 20 years period without significant increase or decrease. Such a steady trend is at an expense of the decrease of one of the two dominant taxa. Namely, the long term flux trend of C. pelagicus is a steady decrease during the twenty-years at Station AB whereas that of E. huxleyi is an increase. While combined cocco's fluxes of the dominant taxa did not change significantly during the 20 years CaCO3 mass contributed by the combined taxa must have decreased substantially since the mass of the former is nearly two orders of magnitude greater than the latter. The results at Station SA, however, show an opposite of what we saw at Station AB. The calcareous nanno's fluxes at both stations will be evaluated against diatom fluxes, SST, salinity, and climate index such as the El Niño Southern Oscillation.