Naonobu Shiga

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.

Interannual and decadal variability in zooplankton communities of the southeast Bering Sea shelf

The southeastern Bering Sea shelf ecosystem is an important fishing ground for fin- and shellfish, and is the summer foraging grounds for many planktivorous seabirds and marine mammals. In 1997 and 1998, Northern Hemisphere climate anomalies affected the physical and biological environment of the southeastern Bering Sea shelf. The resulting anomalous conditions provided a valuable opportunity to examine how longer-term climate change might affect this productive ecosystem. We compared historical and recent zooplankton biomass and species composition data for the southeastern Bering Sea shelf to examine whether or not there was a response to the atmosphere–ocean–ice anomalies of 1997 and 1998. Summer zooplankton biomass (1954–1994) over the southeastern shelf did not exhibit a decline as previously reported for oceanic stations. In addition, zooplankton biomass in 1997 and 1998 was not appreciably different from other years in the time series. Spring concentrations of numerically abundant copepods (Acartia spp., Calanus marshallae, and Pseudocalanus spp.), however, were significantly higher during 1994–1998 than 1980–1981; spring concentrations of Metridia pacifica and Neocalanus spp. were not consistently different between the two time periods. Neocalanus spp. was the only taxon to have consistent differences in stage composition between the two time periods—CV copepodites were much more prevalent in May of the 1990s than early 1980s. Since relatively high zooplankton concentrations were observed prior to 1997, we do not attribute the high concentrations observed in the summers of 1997 and 1998 directly to the acute climate anomalies. With the present data it is not possible to distinguish between increased production (control from below) and decreased predation (control from above) to explain the recent increase in concentrations of the species examined.

Variation of size-fractionated Fe concentrations and Fe(III) hydroxide solubilities during a spring phytoplankton bloom in Funka Bay (Japan)

Abstract Vertical distributions of size-fractionated Fe concentrations ( 0.22-μm fractions) and Fe(III) hydroxide solubilities were studied during a spring phytoplankton bloom (February to April, 1995) in Funka Bay, Japan. “Soluble Fe” (

Temporal and spatial variability of coccolithophore blooms in the eastern Bering Sea, 1998-2001

Abstract In the late summer of 1997, aquamarine waters, resulting from a massive bloom of coccolithophore algae, covered most of the continental shelf of the eastern Bering Sea. This was the first such event recorded in the area, but since then, coccolithophore blooms have been common. The objectives of this study were first to determine the threshold value of the Sea Wide Field-of-view Sensor (SeaWiFS) coccolithophore algorithm using a comparison of satellite and ship-based optical observations, and second to analyze temporal and spatial variability using SeaWiFS images of the coccolithophore blooms in the Bering Sea from 1998 to 2001. In late July 1998 and 2000, we made bio-optical measurements of the coccolithophore blooms, which were composed of Emiliania huxleyi. In situ measurements of water leaving radiance (nLw) at characteristic wavelengths yielded values that differed from the standard NASA coccolithophore values. We therefore defined new threshold values for the mask and applied it to a time series of SeaWiFS images from the Bering Sea. Generally, the coccolithophore bloom began in February each year, as melting began along the edge of the sea ice, and then expanded northward. The bloom reached its peak in April, and then from May through August the area covered by the bloom dwindled in size. However, its area increased again in September. Throughout the study period, the blooms were located at depths of 20 to 100 m. There was annual and seasonal variability in the area affected by the coccolithophore bloom. There were large blooms in 1998 and 2000, whereas in 1999 and 2001 the area affected was smaller. These differences might be related to sea surface temperature (SST), since the largest blooms occurred in the warmest years.

Feeding habits of stone flounder Platichthys bicoloratus larvae in Mutsu Bay, Japan

To clarify the feeding strategy of pelagic larvae of stone flounder in Mutsu Bay, the dietary composition and pray size was investigated from February to April during 1989–1999. Diets were compared with the numerical and volumetric compositions and frequency of occurrence of each prey species. Mensuration formulae were applied to estimate individual prey volume in the diet, while the length of planktonic species was measured from net and water samples. Prey shapes were assumed as sphere, cylinder, ellipsoid, pyramid, two elliptical cones, or a combination of ellipsoid and cylinder. Prey-size range increased as the larvae grew. Preflexion larvae fed mainly on copepod nauplii. Flexion and postflexion larvae ingested primarily appendicularians, with a suggestion that these larvae might depend on some parts of the microbial food web. Low frequencies of flexion and postflexion larvae with empty guts (1.7 and 1.4%, respectively) might be derived from feeding on slow-swimming appendicularians. From a size comparison between ‘house’-like organ length and trunk length of the appendicularian Oikopleura sp., almost all house-like organs with trunks in the larval diet were nonexpanded ‘house rudiments’, not expanded ‘houses’. Thus, stone flounder larvae may not chew the houses, but swallow the house rudiments with trunks.