Physical behavior of the SEEDS iron-fertilized patch by sulphur hexafluoride tracer release

The first iron (Fe) – fertilization experiment in the western North Pacific was carried out using SF₆ to trace the Fe-fertilized water mass. A solution in 10,800 liters of seawater of 350 kg of Fe and 0.48 M of SF₆ tracer was released into the mixed layer over a 8 × 10 km area. On the first underway transects through the patch after the Fe release, we observed a significant increase of dissolved Fe (ave. 2.89 nM). The fertilized patch was traced for 14 days by on-board SF₆ analysis. A Lagrangian frame of reference was maintained by the use of a drogued GPS buoy released at the center of the patch. The patch moved westward at a rate of 6.8 km d⁻¹. Mixed layer depth increased from 8.5 to 15 m during the experiment. Horizontal diffusivity was determined by the change of SF₆ concentration in the patch. The horizontal diffusivity increased during the experiment. We evaluate here the fate of Fe in a Fe-fertilized patch using the dilution rate determined from sulphur hexafluoride (SF₆) concentration. Dissolved Fe concentrations subsequently decreased rapidly to 0.15 nM on Day 13. However, the dissolved Fe half-life of 43 h was relatively longer than in previous Fe-enrichment studies, and we observed a larger increase of the centric diatom standing stock and corresponding drawdown of macro-nutrients and carbon dioxide than in the previous studies. The most important reason for the larger response was the phytoplankton species in the western North Pacific. In addition, the smaller diffusivity and shallower mixed layer were effective to sustain the higher dissolved Fe concentration compared to previous experiments. This might be one reason for the larger response of diatoms in SEEDS.     

Mesozooplankton responses to iron-fertilization in the western subarctic Pacific (SEEDS2001)

A mesoscale iron-fertilization experiment was carried out in the western subarctic Pacific during summer 2001. The iron-patch was traced for 14 days after the fertilization, and the abundance and behavior of mesozooplankton were compared with those outside of the patch. The phytoplankton biomass in the patch rapidly increased to over 15 times the initial level by the later half of the observation period, and was composed of large-sized (>10 mm), centric diatoms. Dominant zooplankton species in the upper 200-m depth were large copepods: Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica. Mesozoplankton biomass as well as species composition did not change significantly in the patch over the observation period. Furthermore, no changes of vertical distribution or diel vertical migration were observed for any species or stages of mesozooplankton throughout the observation period. However, the abundance of the first copepodite stages of N. plumchrus and E. bungii increased several fold in the patch after the diatom bloom formation compared to the densities outside the patch. The increases of both species are considered to be due to lowered mortality during the egg and nauplius stages. Spawning of N. plumchrus takes place at depth using lipid storage, while spawning of E. bungii takes place in the surface layer supported by grazing. These facts suggest that the relative importance of nauplii in the diets of the large copepods was decreased in the patch by the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 4–18 times, and the maximum values were observed during the bloom peak. However, the grazing impact on phytoplankton was low throughout the experiment, especially during the bloom period (<6% of the primary production).     

Functional roles of interzonal migrating mesozooplankton in the western subarctic Pacific

Grazing experiments and production estimation based on life-history analysis of Neocalanus copepods (N. cristatus, N. plumchrus and N. flemingeri) were carried out in the Oyashio region to understand the carbon flows associated with the interzonal migrating copepods. These copepods, and also Eucalanus bungii, fed on nano- and micro-sized organisms non-selectively throughout the season. However, diatoms were the dominant food resource until May and organisms, such as ciliates were the major resource after May. Daily growth rate was estimated from the Ikeda–Motoda, Huntley–Lopez and Hirst–Sheader models. Since the growth rates were considered to be overestimates for the Huntley–Lopez model and underestimates for the other two models, we applied the weight-specific growth rates previously reported for these species in the Bering Shelf. Surface biomass of Neocalanus increased rapidly in June during the appearance of C5, and a successive increase of overwintering stock was evident in the deeper layer. The deep biomass decreased gradually from September to May during the dormant and reproduction period. N. cristatus has the largest annual mean biomass (2.3 gC m⁻²), followed by N. plumchrus (1.1) and N. flemingeri (0.4). Daily production rate of Neocalanus varied from 0.4 to 363.4 mgC m⁻² day⁻¹, to which N. cristatus was the largest contributor. Annual production was estimated as 11.5 gC m⁻² year⁻¹ for N. cristatus, 5.7 for N. plumchrus and 2.1 for N. flemingeri, yielding annual P/B ratio of 5 for each species. The annual production of Neocalanus accounted for 13.2% of the primary production in the Oyashio region. Their fecal pellets were estimated to account for 14.9% (0.7 gC m⁻² year⁻¹) of sinking flux of organic carbon at 1000-m depth. Moreover, their export flux by ontogenetic vertical migration, which is not measured by sediment trap observations, is estimated to be 91.5% (4.3 gC m⁻² year⁻¹) of carbon flux of sinking particles at 1000-m depth. These results suggest the important role of interzonal migrating copepods in the export flux of carbon.     

Wind fluctuations near a cold vortex-tropopause funnel system observed by the MU radar

Vertical and temporal variations of three-dimensional wind velocity associated with an upper-tropospheric cold vortex-tropopause funnel system were observed by an MST radar in Japan (the MU radar). Marked changes of vertical velocity and horizontal wind direction were found between the inside and outside of the cold vortex. The vertical velocity activity outside the vortex was asymmetric; it was most active in a sector before the vortex. Unsaturated internal gravity waves in their generation stage contribute predominantly to the vertical velocity activity, suggesting that tropospheric occluded cyclones may be a possible source of middle-atmospheric gravity waves through the geostrophic adjustment process.     

Internal gravity wave selection in the upper troposphere and lower stratosphere observed by the MU radar: Preliminary results

Marked wavelike variations of the lower stratospheric wind observed on 7–10 May, 1985 by an MST radar in Japan (by the MU radar) are analyzed assuming that they are induced by monochromatic internal inertio-gravity waves. These variations are mainly composed of two modes (periods: 22 and 24 hours), both of which have zonal phase velocities (C _X ) slower than the mean westerly wind (). A statistical analysis of the zonal phase velocity shows thatC _X above andC _X below the tropopause jet stream, which is considered to be a vivid proof of wave selection due to the tropospheric mean flow and upward wave emission from the tropopause jet. A comparison between the MU radar results and routine meteorological observations leads to the conclusion that the marked waves appear when the jet stream takes a maximum wind speed.     

Single particle analysis of oceanic suspended matter during the SEEDS II iron fertilization experiment

Suspended particles collected from surface seawater during the SEEDS II (Subarctic Iron Experiment for Ecosystem and Dynamics Study II) experiment were analyzed individually using an electron probe X-ray micro analyzer and characterized by size and elemental composition. Their numbers, relative abundances, and relative particle volume all showed clear differences between samples collected inside vs. outside the phytoplankton bloom that developed following the addition of iron. Throughout the study, Si-rich, Ca-rich and Organic particles were dominant and their number increased inside the fertilized patch; these particles accounted for 21%, 13% and 58% of the particles examined, respectively. Si-rich or Ca-rich particles commonly consisted of fragments of diatom frustules and coccolithophorids. There was consistently greater percentage of Ca-rich particles and lower percentage of Si-rich particles inside the patch than outside of it in number, but both types of these particles apparently occupied a larger volume inside the patch than outside of it. Organic particles, that showed having peaks in smaller diameter particles, increased apparently inside the patch with time after iron fertilization. The Organic particles had a more diverse mixture of both bio-related and crustal trace elements than the other types of particles. These results suggest that the increase in suspended particles following the iron enrichment was due to a combination of detrital material and live phytoplankton.     

Semidiurnal tides from the extended Canadian Middle Atmosphere Model (CMAM) and comparisons with TIMED Doppler interferometer (TIDI) and meteor radar observations

The extended Canadian Middle Atmosphere Model (extended CMAM) is a general circulation model, which extends from the surface to about 210 km. Spatial complex spectral analysis is applied to horizontal winds simulated by the extended CMAM to obtain semidiurnal tidal amplitudes and phases (from e5 to w5) in the mesosphere and lower thermosphere (MLT) region. The dominant w2 migrating component and the presence of eight nonmigrating tides (w3, w4, w5, e1, e2, e3, e4 and e5) in the mid-latitudes are identified. Components w1 and s0, which tend to maximize at high latitudes, will be discussed separately in a later paper. The migrating semidiurnal tide (w2) has amplitudes reaching over 20 m s⁻¹ for both zonal and meridional winds in the mid-latitude region. Its form compares well to the published results. The amplitudes of nonmigrating semidiurnal tides are non-negligible compared with the migrating semidiurnal tides. The amplitudes for w3 and e2 exceed 12 and 8 m s⁻¹, respectively.Comparisons are made with four nonmigrating semidiurnal components (w3, w4, e1 and e2) derived from the TIMED Doppler interferometer (TIDI) wind measurements between 85 and 105 km altitude and between 45°S and 45°N latitude. Overall, the basic CMAM and TIDI latitudinal structures of the amplitudes agree well and the agreement between the annual mean amplitudes varies with component. Relative to the TIDI results, the CMAM seasonal variations of w4 are in good agreement, of e2 are in reasonable agreement, of w3 are in partial agreement and of e1 are in poor agreement.The 11 semidiurnal components from the model are superimposed to generate the total semidiurnal winds at Jakarta (6°S, 106°E) and Kototabang (0°, 100°E) and are compared with measurements from two equatorial meteor radar stations at these sites. The relative contributions of components to the reconstructed amplitude vary from month to month. The CMAM reconstructions are generally larger than the radar results by a factor varying between one and two. The phases in the radar data are typically stationary with respect to height, whereas they generally decrease with height in the CMAM reconstruction.     

Vertical distributions of large ontogenetically migrating copepods in the Oyashio region during their growing season

Neocalanus flemingeri, Neocalanus plumchrus, Neocalanus cristatus and Eucalanus bungii are large and dominant mesozooplankton occurring throughout the subarctic Pacific. They are an important trophic link and transporter of organic matter to the mesopelagic zone. Vertical distributions of these copepods were investigated from March to October 2000 in the Oyashio region of the western subarctic Pacific. Neocalanus plumchrus and N. flemingeri were distributed in the surface layer (0–50 m) and N. cristatus and E. bungii in the subsurface layer (50–100 m). However, when examined in detail, clear seasonal and vertical differences were observed. Neocalanus plumchrus was concentrated in the top 20 m from late April to the end of July, and N. flemingeri showed a little deeper distribution from May to July. Neocalanus cristatus showed a deeper distribution than that of grazing individuals of E. bungii from April to early July, but grazing individuals of E. bungii (C3–C6) showed a deeper distribution than that of N. cristatus from the end of July to October. Early copepodites of E. bungii were distributed much shallower than late copepodite stages and overlapped with copepodites of N. plumchrus and N. flemingeri. These results suggest that the four species of large copepods have established habitat segregation by season, vertical distribution and food resource partitioning in the Oyashio region as well as other regions of the North Pacific.     

When human capital does not matter: local contexts of reception and immigrant wages in Japan

Although research on immigrants in the US provides strong evidence that human capital is more important than social capital in determining their wages, data from Hamamatsu, Japan indicates that social capital variables are the primary determinant of immigrant earnings and human capital does not have a significant effect. The divergent impact of these two variables on the earnings of immigrants are a result of the different economic and social conditions that immigrants encounter in Japan compared to the United States. In a recent country of immigration like Japan where immigrant labor markets are relatively undeveloped and foreign workers are confined to unskilled, marginal jobs, the human capital that they acquire over time is not reflected in better jobs with higher earnings. In contrast, immigrants with access to social capital in the form of immigrant networks, gender, and ethnicity are able to obtain jobs with higher wages in Japan. Because foreign workers are still temporary target earners, they therefore rely heavily on their immigrant social networks to find better-paying jobs. In addition, Japan is a country with significant gender and ethnic discrimination where employers strongly prefer male foreign workers and ethnically similar nikkeijin (Japanese descendants born and raised abroad) and are willing to pay them significantly higher wages. Therefore, depending on the local context of immigrant reception, the relative importance of human versus social capital in explaining economic outcomes among immigrants can vary considerably.