Keiri Imai

Bering Sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001

Abstract Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3+NO2) rich water (>25 μM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m−3) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy.

Seasonal variability of picophytoplankton and bacteria in the western subarctic Pacific Ocean at station KNOT

Picoplankton community structure in the upper 200 m at Station KNOT in the western subarctic North Pacific was studied by flow cytometric analysis of the time-series samples collected from June 1998 through June 2000. Phototrophic picoplankton, Synechococcus and picoeukaryotes, displayed a clear seasonal cycle in population abundance during the 2-yr period, where chlorophyll a concentration remained relatively low and constant throughout the year, except in spring when the diatom bloom occurred. High picoplankton abundance occurred after the spring diatom bloom when the water column became more stratified. The maximum abundance of Synechococcus occurred in late June to early August, whereas the peak in picoeukaryotes abundance appeared to take place earlier in May and remained high through the summer. Heterotrophic bacteria abundance also displayed a seasonal variation pattern that was closely related to the picoplankton biomass, but not total chlorophyll biomass.

Interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s

Abstract Surface chlorophyll a concentrations were measured at seven stations located at 1° latitude intervals between 52°30′ N and 58°30′ N along longitude 179°30′ W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993–95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179°30′ W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991–2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 μg l−1 in the Bering Sea basin and the values were usually

Chla-specific productivity of picophytoplankton not higher than that of larger phytoplankton off the South Shetland Islands in summer

Abstract Size-fractionated chlorophyll a (Chla)-specific productivity (μgC μgChla−1 h−1) was measured at 11 stations off the northern coast of the South Shetland Islands during summer. The Chla-specific productivity of the 2- to 10 or 10- to 330-μm fraction was highest at 100% and 23% light depths. The Chla-specific productivity of the 2- to 10-μm fraction was generally highest, and that of the <2 or 10- to 330-μm fraction was sometimes highest at 12% and 1% light depths. Temperature was less than 3°C within the euphotic zone at all stations. The hypothesis of Shiomoto et al., according to which Chla-specific productivity of picophytoplankton (<2 μm) is not significantly higher than that of larger phytoplankton (>2 μm) in water colder than 10°C, was supported on condition that light is not limited for larger phytoplankton.