Diel Changes in Vertical Distribution and Feeding Conditions of the Chaetognath Parasagitta elegans (Verill) in the Subarctic Pacific in Summer

Diel changes in vertical distribution and feeding conditions of the chaetognath Parasagitta elegans (Verill) were observed in three regions of the subarctic North Pacific in the summer of 1997. Samples were collected by repeated vertical hauls with a Vertical Multiple Plankton Sampler (VMPS) for 15–45 hours by demarcating the 0–500 m water column into four sampling layers. Integrated abundance through the entire water column and the proportion of juveniles were higher in the Bering Sea than the western and eastern subarctic Pacific. Juveniles always inhabited the surface layer in the western subarctic Pacific and Bering Sea, but they inhabited the underlying layer in the eastern subarctic Pacific. Stages I–III concentrated into the upper 150 m in the western subarctic Pacific but were distributed widely from 20–300 m in the Bering Sea. Among them, Stages II and III migrated rather synchronously over a wide vertical range in the eastern subarctic Pacific. The feeding rate of P. elegans was calculated to be 0.18 prey/chaetognath/day in the western subarctic Pacific, 0.27 prey/chaetognath/day in the Bering Sea and 0.07 prey/chaetognath/day in the eastern subarctic Pacific.     

An in situ iron-enrichment experiment in the western subarctic Pacific (SEEDS): Introduction and summary

To test the iron hypothesis in the subarctic Pacific Ocean, an in situ iron-enrichment experiment (SEEDS) was performed in the western subarctic gyre in July–August 2001. About 350 kg of iron (as acidic iron sulfate) and 0.48 mol of the inert chemical tracer sulfur hexafluoride were introduced into a 10-m deep surface mixed layer over an 80 km² area. This single iron infusion raised dissolved iron levels to 2.9 nM initially. Dissolved iron concentrations rapidly decreased after the infusion, but levels remained close to 0.15 nM even at the end of the 14-day experimental period. During SEEDS there were iron-mediated increases in chlorophyll a concentrations (up to 20 μg l⁻¹), primary production rates, biomass and photosynthetic energy conversion efficiency relative to waters outside the iron-enriched patch. The rapid and very high accumulation of phytoplankton biomass in response to the iron addition appeared to be partly attributable to shallow mixed-layer depth and moderate water temperature in the western subarctic Pacific. However, the main reason was a floristic shift to fast-growing centric diatom Chaetoceros debilis, unlike the previous iron-enrichment experiments in the equatorial Pacific and the Southern Ocean, in both of which iron stimulated the growth of pennate diatoms. The iron-mediated blooming of diatoms resulted in a marked consumption of macronutrients and drawdown of pCO₂. Biological and physiological measurements indicate that phytoplankton growth in the patch became both light- and iron-limited, making phytoplankton biomass relatively constant after day 9. The increase in microzooplankton grazing rate after day 9 also influenced the net growth rate of phytoplankton. There was no significant increase in the export flux of carbon to depth during the 14-day occupation of the experimental site. The export flux between day 4 and day 13 was estimated to be only 13% of the integrated primary production in the iron-enriched patch. The major part of the carbon fixed by the diatom bloom remained in the surface mixed layer as biogenic particulate matter. Our findings support the hypothesis that iron limits phytoplankton growth and biomass in a ‘bottom up’ manner in this area, but the fate of algal carbon remains unknown.     

Population dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific

We characterized the community composition of phytoplankton in the western subarctic Pacific from the pre-bloom to the decline phase of the spring bloom with special reference to decreases in the silicic acid concentration in surface waters as an index for diatom bloom development. Furthermore, responses of heterotrophic bacteria and viruses to the spring bloom were also concomitantly investigated. Under pre-bloom conditions when nutrients were abundant but the surface mixed layer depth was relatively deep, chlorophyll (Chl) a concentrations were consistently low and green algae (chlorophytes and prasinophytes), cryptophytes, and diatoms were predominant in the phytoplankton assemblages as estimated by algal pigment signatures. Together with the shallowing of the mixed layer depth and the decrease in silicic acid concentration, diatoms bloomed remarkably in the Oyashio region, though the magnitude of the bloom in the Kuroshio-Oyashio transition (hereafter Transition) region was relatively small. A total of 77 diatom species were identified, with the bloom-forming diatoms mainly consisting of Thalassiosira, Chaetoceros, and Fragilariopsis species. It has become evident that the carotenoid fucoxanthin can serve as a strong indicator of the diatom carbon biomass during the spring diatom bloom. Differences in the species richness of diatoms among stations generally enabled us to separate the Oyashio bloom stations from the Transition and the Oyashio pre-bloom stations. Relatively high values of the Shannon-Wiener index for the diatom species were also maintained during the Oyashio bloom, indicating that a wide variety of species then shared dominance. In the decline phase of the Oyashio bloom when surface nutrient concentrations decreased, senescent diatom cells increased, as inferred from the levels of chlorophyllide a. Although the cell density of heterotrophic bacteria changed little with the development of the diatom bloom, viral abundance increased toward the end of the bloom, suggesting an increased likelihood of mortality among organisms including diatoms resulting from viral infection. This is the first report on the microbial trophodynamics, including viruses, during the spring diatom bloom in the western subarctic Pacific.     

High-West and Low-East Distribution Patterns of Chlorophyll a, Primary Productivity and Diatoms in the Subarctic North Pacific Surface Waters, Midwinter 1996

We have determined chlorophyll a (Chla) concentration, primary productivity, cell density and species composition of diatoms, and the number of microzooplankton at the surface in the subarctic North Pacific in January 1996. The wet weight of copepods obtained by vertical tows from 150 m to the surface was also measured during the cruise. Chla concentration and primary productivity tended to be higher in the region west of 180°, the western subarctic North Pacific (WSNP), than east of 180°, the eastern subarctic North Pacific (ESNP). The same results were observed for the total diatom cell densities and for the genera Thalassiosira and Denticulopsis. Significant linear relationships were observed between the Thalassiosira cell density and Chla concentration and primary productivity, indicating that Thalassiosira contributes to the high-WSNP and low-ESNP distribution patterns of Chla concentration and primary productivity. Moreover, naked ciliate abundance tended to be lower in the WSNP than in the ESNP, whereas copepod biomass showed an inverse trend. Significantly negative Spearman rank correlations were found between the Thalassiosira cell density and the number of naked ciliates and between the number of naked ciliates and the wet weight of copepods. These results indicate that copepod grazing indirectly controls Thalassiosira cell density via predation on the naked ciliates. We conclude that the high copepod biomass in the WSNP is a factor controlling the high-WSNP and low-ESNP Thalassiosira abundance and hence Chla concentration and primary productivity patterns.     

A 9-year time-series of planktonic foraminifer fluxes and environmental change in the Bering sea and the central subarctic Pacific Ocean, 1990–1999

A 9-year study of planktonic foraminifer fluxes was conducted in the Bering Sea (Station AB) and in the central subarctic Pacific (Station SA). Results clearly reflected variations of the water mass characteristics in the upper layers. The 9-year means of total foraminifer fluxes were the same (1400 shells m⁻² d⁻¹) at both stations. However, total foraminifer flux at Station AB tended to show its primary maximum during fall (October–December) and its secondary maximum in spring (April–June), whereas the primary maximum appeared in spring and the secondary maximum in fall at Station SA. Seasonal variation was more apparent at hemipelagic Station AB than at pelagic Station SA. Planktonic foraminifers found at both stations were of six species: Neogloboquadrina pachyderma, Globigerina umbilicata, Globigerinita glutinata, Globigerina quinqueloba, Globorotalia wilesi, and Orbulina universa. The foraminifer assemblages at the two stations reflected the temperature difference in the surface waters. The variable %G. umbilicata tended to be high in the warm surface waters during the summers. The temporal and geographical variation of %G. quinqueloba indicated that this taxon prefers regions with relatively low diatom fluxes. A notable appearance of O. universa occurred in 1997 at Station SA. During this period, other measured biogenic particle fluxes, such as those of diatoms, were low. This unusual 1997 event may be a reflection of global climatic change that happened to be observed in the central subarctic Pacific Ocean.     

Seasonal and Interannual Variability in the Distribution of Surface Nutrients and Dissolved Inorganic Carbon in the Northern North Pacific: Influence of El Niño

The seasonal and interannual changes in surface nutrients, dissolved inorganic carbon (DIC) and total alkalinity (TA) were recorded in the North Pacific (30–54°N) from 1995 to 2001. This study focuses on the region north of the subarctic boundary (∼40°N) where there was extensive monthly coverage of surface properties. The nutrient cycles showed large interannual variations in the eastern and western subarctic gyres. In the Alaska Gyre the seasonal depletion of nitrate (ΔNO₃) increased from 8–14 μmol kg⁻¹ in 1995–1999 to 21.5 μmol kg⁻¹ in 2000. In the western subarctic the shifts were similar in amplitude but more frequent. The large ΔNO₃ levels were associated with high silicate depletions, indicating enhanced diatom production. The seasonal DIC:NO₃ drawdown ratios were elevated in the eastern and central subarctic due to calcification. In the western subarctic and the central Bering Sea calcification was significant only during 1997 and/or 1998, two El Ni?o years. Regional C/N stoichiometric molar ratios of 5.7 to 7.0 (>40°N) were determined based on the years with negligible or no calcification. The annual new production (NP_a) based on ΔNO₃ and these C/N ratios showed large interannual variations. NP_a was usually higher in the western than in the eastern subarctic. However, values of 84 gC m⁻²yr⁻¹ were found in the Alaska Gyre in 2000 which is similar to that in the most productive provinces of the northern North Pacific. There were also large increases in NP_a around the Alaska Peninsula in 1997 and 1998. Finally, the net removal of carbon by the biological pump was estimated as 0.72 Gt C yr⁻¹ in the North Pacific (>30°N). This revised version was published online in August 2006 with corrections to the Cover Date.     

Increased Stratification and Decreased Lower Trophic Level Productivity in the Oyashio Region of the North Pacific: A 30-Year Retrospective Study

An analysis of the time series data sets collected from the 1960s to 1990s in the Oyashio Water revealed signs of alteration in the physical, chemical and biological properties of the water column in the western subarctic North Pacific. Wintertime salinity, phosphate concentration and apparent oxygen utilization (AOU) in the subsurface increased linearly over the 30 years. At the same time, salinity and phosphate in the surface mixed layer decreased. An increase in the density gradient in the surface and subsurface suggested that the water column stratification intensified, reducing the vertical exchange of water properties during the period. The Net Community Production (NCP), estimated from the phosphate consumption from February through August, also declined. Water column Chl a was approximately halved and diatoms decreased by one order of magnitude in spring, consistent with the multi-decadal decreasing trend of NCP. Zooplankton biomass was also nearly halved during the same period. In contrast, wintertime Chl a increased by 63% and diatom abundance doubled. Developmental timing became earlier in Neocalanus flemingeri, and spring occurrence of N. plumchrus increased after the 1980s. Reduced vertical water exchange might have limited nutrient supply to the level, decreasing winter-summer NCP for these three decades. It is speculated that, in the meantime, the earlier stabilization of the surface layer might have enhanced wintertime diatom production in the Oyashio's light-limited environment. This condition could allow zooplankton to effectively utilize diatoms from earlier timing, resulting in the apparent early developmental timing and abundance increase. This revised version was published online in July 2006 with corrections to the Cover Date.     

2008年夏季西北冰洋表层水浮游植物群落结构的色素表征及其对不同水团的响应

本文依托2008年夏季中国第三次北极科学考察航次,对西北冰洋海盆区和楚科奇海陆架营养盐及光合色素进行了测定和分析。根据海水理化性质将研究海区分为5个区,并使用CHEMTAX软件（Mackery et al.,1996）讨论了西北冰洋不同海区浮游植物群落组成结构及其与环境因子之间的关系。结果显示在楚科奇海陆架区,太平洋入流显著影响浮游植物生物量和群落结构。高营养盐Anadyr水团以及白令陆架水控制海域,表现出高Chl a且浮游植物以硅藻为主,相反,低营养盐如阿拉斯加沿岸流控制海域,Chl a生物量低且以微型,微微型浮游植物为主。在外陆架海区,海冰覆盖情况影响着水团的物理特征及营养盐浓度水平,相应地显著影响浮游植物群落结构。在海冰覆盖区域,硅藻生物量站到总Chl a生物量的75%以上;在靠近门捷列夫深海平原海区,受相对高盐的冰融水影响（MW-HS）,营养盐浓度和Chl a浓度相对海冰覆盖区略高,浮游植物结构中微型、微微型藻类比重增加,硅藻比例则降至33%;南加拿大海盆无冰海区（IfB）,表层水盐度最淡,营养盐浓度最低,相应地显示出低Chl a生物量,表明海冰消退,开阔大洋持续时间延长,将导致低生物量及激发更小型浮游植物的生长,并不有利于有机碳向深海的有效输出。     

Interannual Changes of the Winter to Early Spring Biomass and Composition of Mesozooplankton in the Kuroshio Region in Relation to Climatic Factors

Interannual variations of biomass of major mesozooplankton groups (Cnidaria, Chaetognatha, Copepoda, Crustacea other than copepods, Tunicata) in the January to March period were examined in the slope, Kuroshio and offshore waters off the Pacific coast of western Japan (western region) from 1971 to 1988 and off central Japan (central region) from 1971 to 1989. The mean biomass for each year of most of the mesozooplankton groups was high in the early 1970s and tended to decrease (in the western region) or to have dropped to a lower level (in the central region) after the mid-1970s. Stepwise multiple linear regression analyses of the mean biomass for each year of each mesozooplankton group in the Kuroshio in both regions against climatic factors revealed that the biomass was related positively to wind speed. It is therefore considered that the nutrient supply to the upper layers limits the production of many of the mesozooplankton groups examined in the Kuroshio, even in winter. Similar relationships were also found for the biomass of Copepoda, non-copepod Crustacea and Tunicata in the offshore water in the western region. The percentage of copepods in the biomass in the central region seemed to decrease under high water temperature conditions, while that of Chaetgnatha tended to increase. Climatic factors thus largely influenced the interannual variations of biomass and composition of mesozooplankton in and near the Kuroshio during the winter to early spring period. This revised version was published online in July 2006 with corrections to the Cover Date.     

Export Production in the Subarctic North Pacific over the Last 800 kyrs: No Evidence for Iron Fertilization?

The subarctic North Pacific is a high nitrate-low chlorophyll (HNLC) region, where phytoplankton growth rates, especially those of diatoms, are enhanced when micronutrient Fe is added. Accordingly, it has been suggested that glacial Fe-laden dust might have increased primary production in this region. This paper reviews published palaeoceanographic records of export production over the last 800 kyrs from the open North Pacific (north of ∼35°N). We find different patterns of export production change over time in the various domains of the North Pacific (NW and NE subarctic gyres, the marginal seas and the transition zone). However, there is no compelling evidence for an overall increase in productivity during glacials in the subarctic region, challenging the paradigm that dust-born Fe fertilization of this region has contributed to the glacial draw down of atmospheric CO₂. Potential reasons for the lack of increased glacial export production include the possibility that Fe-fertilization rapidly drives the ecosystem towards limitation by another nutrient. This effect would have been exacerbated by an even more stable mixed layer compared to today. This revised version was published online in July 2006 with corrections to the Cover Date.     

Community Structure and Dynamics of Phytoplankton in the Western Subarctic Pacific Ocean: A Synthesis

The phytoplankton community in the western subarctic Pacific (WSP) is composed mostly of pico- and nanophytoplankton. Chlorophyll a (Chl a) in the <2 μm size fraction accounted for more than half of the total Chl a in all seasons, with higher contributions of up to 75% of the total Chl a in summer and fall. The exception is the western boundary along the Kamchatka Peninsula and Kuril Islands and the Oyashio region where diatoms make up the majority of total Chl a during the spring bloom. Among the picophytoplankton, picoeukaryotes and Synechococcus are approximately equally abundant, but the former is more important in term of carbon biomass. Despite the lack of a clear seasonal variation in Chl a concentration, primary productivity showed a large seasonal variation, and was lowest in winter and highest in spring. Seasonal succession in the phytoplankton community is also evident with the abundance of diatoms peaking in May, followed by picoeukaryotes and Synechococcus in summer. The growth of phytoplankton (especially >10 μm cell size) in the western subarctic Pacific is often limited by iron bioavailability, and microzooplankton grazing keeps the standing stock of pico- and nano-phytoplankton low. Compared to the other HNLC regions (the eastern equatorial Pacific, the Southern Ocean, and the eastern subarctic Pacific), iron limitation in the Western Subarctic Gyre (WSG) may be less severe probably due to higher iron concentrations. The Oyashio region has similar physical condition, macronutrient supply and phytoplankton species compositions to the WSG, but much higher phytoplankton biomass and primary productivity. The difference between the Oyashio region and the WSG is also believed to be the results of difference in iron bioavailability in both regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long-term diatom fluxes in response to oceanographic conditions at Stations AB and SA in the central subarctic Pacific and the Bering Sea, 1990–1998

Long-term monitoring of diatom fluxes during 1990–1998 was conducted at Station AB in the Bering Sea and Station SA in the central subarctic Pacific in order to decipher the relationships between sinking diatom and the upper water mass environments. The total diatom flux at Station AB was generally twice as high as that at Station SA. The dominant species in the sinking flora was primarily Neodenticula seminae at both stations, which was a significant contributor to organic carbon flux. The flora at Station AB was represented by relatively abundant coastal taxa including Chaetoceros resting spores. These results suggest more favorable conditions for diatom production at Station AB compared with those at Station SA. The possible influences of oligotrophic and temperate water masses were discerned from the positive SST anomaly and the occurrence of a few specimens of temperate species. The cumulative annual fluxes of total diatoms at Stations AB and SA are apparently related to the variation in the mean annual depth of mixed layer. At Station SA, annual mean of total diatom flux showed a negative correlation with the Pacific Decadal Oscillation (PDO) Index, which suggests a significant relationship between surface water turbidity and diatom production. At Station AB, the annual change of coastal diatom flux was correlated with the PDO and the winter value of the Arctic Oscillation, which may suggest a significant influence of Alaskan Stream waters via the Aleutian Islands and intensity of surface water mixing.     

Processes Causing the Temporal Changes in Si/N Ratios of Nutrient Consumptions and Export Flux during the Spring Diatom Bloom

Two processes are generally explained as causes of temporal changes in the stoichiometric silicon/nitrogen (Si/N) ratios of sinking particles and of nutrient consumption in the surface water during the spring diatom bloom: (1) physiological changes of diatom under the stress of photosynthesis of diatom and (2) differences of regeneration between silicon and nitrogen. We investigated which process plays an important role in these changes using a one-dimensional ecosystem model that explicitly represents diatom and the other non-silicious phytoplankton. The model was applied to station A7 (41°30′ N, 145°30′ E) in the western North Pacific, where diatom regularly blooms in spring. Model simulations show that the Si/N ratios of the flux exported by the sinking particles at 100 m depth and of nutrient consumptions in the upper 100 m surface water have their maxima at the end of the spring diatom bloom, the values and timings of which are significantly different from each other. Analyses of the model results show that the differences of regeneration between silicon and nitrogen mainly cause the temporal changes of the Si/N ratios. On the other hand, the physiological changes of diatoms under stress can hardly cause these temporal changes, because the effect of the change in the diatom's uptake ratio of silicon to nitrogen is cancelled by that in its sinking rate.     

One Dimensional Ecosystem Model Simulation of the Effects of Vertical Dilution by the Winter Mixing on the Spring Diatom Bloom

A one-dimensional ecosystem model has been used to investigate the processes relevant to the spring diatom bloom which play important roles in the biogeochemical cycle in the western subarctic Pacific. The model represents the plankton dynamics and the nutrient cycles in the spring diatom bloom; its results show the importance of dilution by deep mixing in winter. It is supposed that the vertically integrated biomass of phytoplankton decreases in the winter due to the decrease of photosynthesis, because the deep mixing transports phytoplankton to a layer with a low light level. However, the observed integrated diatom biomass increases as the mixed layer deepens. This is because the decrease of concentration due to dilution by mixing causes the diatom grazed pressure to be less significant than diatom photosynthesis. In other words, the effect of dilution on the grazed rate is more significant than the effect on the photosynthesis rate because the grazed rate depends on the concentrations of both diatom and grazer, whereas the photosynthesis rate depends only diatom concentration. The average specific diatom grazed rate, defined as grazed rate divided by diatom biomass, decreases by 35% associated with the deepening, while the average specific photosynthesis rate of diatom decreases by 11%. As a result, the average specific net diatom growth rate during the deep mixing is about 70% of its maximum during the spring diatom bloom. The deep mixing significantly affects the amplitude of the spring diatom bloom not only by the supply of nutrients but also by the dilution which drastically decreases the grazed pressure. This revised version was published online in July 2006 with corrections to the Cover Date.     

North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects

We survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE (SubArctic Gyre Experiment), including important results obtained from related projects. Intensive observations have provided the transport distributions relating to NPIW and revealed the existence of the cross-wind-driven gyre Oyashio water transport that flows directly from the subarctic to subtropical gyres through the western boundary current as well as the diffusive contribution across the subarctic front. The anthropogenic CO₂ transport into NPIW has been estimated. The northern part of NPIW in the Transition Domain east of Japan is transported to the Gulf of Alaska, feeding the mesothermal (intermediate temperature maximum) structure in the North Pacific subarctic region where deep convection is restricted by the strong halocline maintained by the warm and salty water transport originating from NPIW. This heat and salt transport is mostly balanced by the cooling and freshening in the formation of dense shelf water accompanied by sea-ice formation and convection in the Okhotsk Sea. Intensive observational and modeling studies have substantially altered our view of the intermediate-depth circulation in the North Pacific. NPIW circulations are related to diapycnal-meridional overturning, generated around the Okhotsk Sea due to tide-induced diapycnal mixing and dense shelf water formation accompanied by sea-ice formation in the Okhotsk Sea. This overturning circulation may possibly explain the direct cross-gyre transport through the Oyashio along the western boundary from the subarctic to subtropical gyres. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abrupt changes of intermediate-water oxygen in the northwestern Pacific during the last 27 kyr

An oxygen minimum zone (OMZ) currently exists at intermediate water depths on the northern Japanese margin in the northwestern Pacific. The OMZ results largely from a combination of high surface–water productivity and poor ventilation of intermediate waters. We investigated the late Quaternary history (last 27 kyr) of the intensity of this OMZ using changes in benthic foraminiferal carbon isotopes and assemblages in a sediment core taken on the continental slope off Shimokita Peninsula, northern Japan, at a water depth of 975 m. The core was located well within the region of the present-day OMZ and high surface–water productivity. The benthic foraminiferal δ¹³C values, which indicate millennial-scale fluctuations of nutrient contents at the sediment–water interface, were 0.48‰ lower during the last glacial maximum (LGM) than during the late Holocene. These results do not indicate the formation of glacial intermediate waters of subarctic Pacific origin, but rather the large contribution of high-nutrient water masses such as the Antarctic Intermediate Water, implying that the regional circulation pattern during the LGM was similar to that of modern times. Benthic foraminiferal assemblages underwent major changes in response to changes in dissolved oxygen concentrations in ocean floor sediments. The lowest oxygen and highest nutrient conditions, marked by dysoxic taxa and negative values of benthic foraminiferal δ¹³C, occurred during the Bølling/Allerød (B/A) and Pre-Boreal warming events. Dysoxic conditions in this region during these intervals were possibly caused by high surface–water productivity at times of reduced intermediate–water ventilation in the northwestern Pacific. The benthic assemblages show dysoxic events on approx. 100- to 200-year cycles during the B/A, reflecting centennial-scale productivity changes related to freshwater cycles and surface–water circulation in the North Pacific.     

Iron deficiency in micro-sized diatoms in the Oyashio region of the Western subarctic Pacific during spring

In order to detect iron (Fe) stress in micro-sized (20–200 μm) diatoms in the Oyashio region, western subarctic Pacific during spring, immunological ferredoxin/flavodoxin assays were applied to samples collected from the surface layer in May 2005. Concomitantly, the community composition of the micro-sized phytoplankton and hydrographic conditions, including dissolved Fe and macronutrient concentrations, were also examined. Chlorophyll (Chl) a concentrations were <2 mg m⁻³ at all sampling stations, except at a station where the Chl a level was 9.0 mg m⁻³ and a micro-sized diatom bloom occurred. A high abundance of ferredoxin in micro-sized diatoms was detected only at a rather near-shore station where dissolved Fe and macronutrient concentrations were higher, indicating that the micro-sized diatoms did not suffer from iron deficiency. On the other hand, flavodoxin in micro-sized diatoms was often observed at the other stations, including the bloom station, where macronutrients were replete but dissolved Fe concentration was low (0.31 nM). A significant amount of chlorophyllide a, a degradation product of Chl a, was also observed at the bloom station, suggesting a decline of the diatom bloom. The micro-sized phytoplankton species at all the stations were mainly composed of the diatoms Thalassiosira, Chaetoceros, and Fragilariopsis spp. Our study indicates that micro-sized diatoms were stressed by Fe bioavailability during the spring season in the Oyashio region     

Balance of Volume Transports between Horizontal Circulation and Meridional Overturn in the North Pacific Subarctic Region

Horizontal and meridional volume transports on timescales from intra-seasonal to interannual in the North Pacific subarctic region were investigated using a reanalysis dataset for 1993–2001 that was constructed from an assimilation of the TOPEX altimeter and in situ data into an eddy-permitting North Pacific ocean general circulation model. The barotropic flow is excited along east of the Emperor Seamounts by the western intensification dynamics. The volume transport of this flow compensates for that across the interior region east of the Seamounts below the summit depth of the Seamounts. The Oyashio, which is also considered as a compensation flow for the transport in the whole interior region, includes baroclinic as well as barotropic components. Baroclinic transports in the whole interior region exceed those in the western boundary region in the upper (200–1000 m) and lower (2000–5000 m) layers, and the total transport is northward (southward) in the upper (lower) layer. These excesses of the baroclinic transport are balanced by a vertical transport of the meridional overturn. The meridional overturn has a complementary relation to the basin-scale baroclinic circulation in the North Pacific subactic region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Iron regeneration and organic iron(III)-binding ligand production during in situ zooplankton grazing experiment

To elucidate iron regeneration and organic iron(III)-binding ligand formation during microzooplankton and copepod grazing on phytoplankton, incubation experiments were conducted in the western subarctic Pacific. During 8 days of dark incubation of ambient water and that amended with plankton concentrate, dissolved iron and organic iron(III)-binding ligands accumulated, approximately proportionally to the decrease in chlorophyll a. The observed increases in dissolved iron concentration were much greater than those expected from the consumption of phytoplankton biomass and previously reported Fe:C value of cultured algal cells, suggesting resolution from colloidal or particulate iron adsorbed onto the algal cell surface. When copepods were added to the ambient water, organic iron(III)-binding ligands accumulated more rapidly than in the control receiving no copepod addition, although consumed phytoplankton biomass was comparable between the two treatments. Bioassay experiment using filtrates collected from the incubation experiment showed that organic ligands formed during microzooplankton grazing reduced the iron bioavailability to phytoplankton and suppressed their growth. Moreover, picoplankton Synechococcus sp. and Micromonas pusilla were more suppressed by the organic ligands than the diatom Thalassiosira weissflogii. In conclusion, through microzooplankton and copepod grazing on phytoplankton, organic iron(III)-binding ligands as well as regenerated iron are released into the ambient seawater. Because the ligands lower iron bioavailability to phytoplankton through complexation and the degree of availability reduction varies among phytoplankton species, grazing by zooplankton can shift phytoplankton community structure in iron-limited waters.     

Impact of mesoscale eddies on particulate organic carbon flux in the western subarctic North Pacific

The mechanism that controls particulate organic carbon (POC) flux in the deep sea differs depending on the season and sea. The POC produced in the western subarctic North Pacific are known to be transported to the deep sea efficiently, but the direct relationship between the POC flux and physical processes is still unclear. In this study, we evaluated the effect of mesoscale eddies on POC flux in the western subarctic North Pacific. The seasonal and interannual variabilities of POC flux were investigated using data from a time-series sediment trap deployed at 4810 m at station K2 (47°N, 160°E) from 2005 to 2018. POC flux was high during May–November, appearing to reflect spring and fall blooms at the ocean surface. POC flux also showed interannual variability, with twelve peaks that were mostly affected by enhanced bloom just before the peak. Nine peaks of the twelve peaks were affected by mesoscale eddies, which enhanced bloom around K2 by extending the area with a high chlorophyll-a concentration along the coastal region into the offshore region, suggesting that mesoscale eddies strongly impact the interannual variability of POC flux at K2.