Eastern Subarctic North Pacific Research Articles

Spatial Distribution and Seasonality of Halocline Structures in the Subarctic North Pacific

AbstractThe spatial distribution and seasonality of halocline structures in the subarctic North Pacific (SNP) were investigated using Argo profiling float data and various surface flux data collected in 2003–17. The permanent halocline (PH) showed zonal patterns in the spatial distributions of its depth and intensity and tended to be shallow and strong in the eastern SNP but deep and weak in the west. Mean distributions of PH depth and intensity corresponded to the winter mixed layer depth and sea surface salinity, respectively, indicating that it forms in association with the development of the winter mixed layer. In the Western Subarctic Gyre and Alaskan Gyre, where a relatively strong PH formed, PH intensity and depth showed clear seasonal variations, and deepening of the mixed layer compressed the underlying PH during the cooling period, resulting in intensification and development of the PH in late winter. In both regions, upwelling of high-salinity water also contributed to PH intensification. The summer seasonal halocline (SH) showed distinct zonal differences in frequency and intensity, which were opposite to the PH distribution. While an SH formed in the western and central SNP and coastal regions, it was seldom present in the eastern area. This zonal contrast of SH corresponded to freshening of the mixed layer during the warming period, primarily reflecting freshwater flux. Geostrophic and Ekman advection play important roles in spatial differences in SH intensity and depth. SH development contributed to PH intensification in the following winter, by decreasing salinity above the PH through entrainment.

Spatial patterns in abundance, taxonomic composition and carbon biomass of nano- and microphytoplankton in Subarctic and Arctic Seas

In the summers of 2007 and 2008, we studied assemblages of nano- and microphytoplankton from the subsurface chlorophyll maximum (SCM) across five broad oceanographic domains in the seas surrounding northern North America. These domains are the eastern Subarctic North Pacific (ESNP), Bering and Chukchi Seas (BE-CH), Beaufort Sea and Canada Basin (BS-CB), Canadian Arctic Archipelago (CAA), and Baffin Bay and Labrador Sea (BB-LS). Average abundance and total carbon biomass (C) of phytoplankton (>2 µm) varied ∼10-fold and ∼20-fold, respectively, across the five domains. In the BE-CH, CAA and BB-LS, diatoms averaged 35–70% and dinoflagellates 11–45% of total phytoplankton C (>2 µm), whereas in the ESNP and BS-CB, unidentified flagellates/coccoids (2–8 µm) represented a greater proportion of total C (27% and 39% respectively) than in the other domains.In the BE-CH and BB-LS, phytoplankton C (>2 µm) was dominated by dinoflagellates of the genus Gymnodinium, centric diatoms including Thalassiosira spp. and Chaetoceros spp., unidentified flagellates/coccoids (2–8 µm), and cryptomonads. In contrast, diatoms such as Thalassiosira spp. and its resting spores dominated C in the CAA, with dinoflagellates being less significant than in the BE-CH and BB-LS. Unidentified flagellates/coccoids (2–8 µm), Gymnodinium spp., and cryptomonads dominated in the ESNP, and particularly in the BS-CB, where diatoms contributed only 18% of the very low levels of total phytoplankton C (>2 µm).Phytoplankton C (>2 µm) to chlorophyll a ratios (phyto C:chl a) averaged only 31 g C g chl a−1 in the oligotrophic BS-CB domain, and 51–150 g C g chl a−1 in the other domains, whereas ratios of biogenic silica to phytoplankton C (>2 µm) (bSi:phyto C) were lowest in the eastern domains. Estimates of phytoplankton C were highly sensitive to the choice of C to cell volume equations (C:vol) adopted in the calculations, particularly in diatom-rich areas.This study highlights how diatoms and dinoflagellates are the main drivers of large-scale variations in C biomass for phytoplankton (> 2 µm), whereas unidentified flagellates/coccoids (2–8 µm) make a significant contribution to C biomass in oligotrophic domains, such as BS-CB, where diatoms and dinoflagellates are less abundant. Reduced surface water density (σT) was associated with deeper SCM layers, and with decreased C biomass of unidentified flagellates/coccoids (2–8 µm). These observations confirm recent studies highlighting the role of surface water stratification caused by melting sea ice in shaping nano- and microphytoplankton assemblages.

Basin-scale distribution of NH4 + and NO2 − in the Pacific Ocean

We used more than 25,000 nutrient samples to elucidate for the first time basin-scale distributions and seasonal changes of surface ammonium (NH4 +) and nitrite (NO2 −) concentrations in the Pacific Ocean. The highest NH4 +, NO2 −, and nitrate (NO3 −) concentrations were observed north of 40°N, in the coastal upwelling region off the coast of Mexico, and in the Tasman Sea. NH4 + concentrations were elevated during May–October in the western subarctic North Pacific, May–December in the eastern subarctic North Pacific, and June–September in the subtropical South Pacific. NO2 − concentrations were highest in winter in both hemispheres. The seasonal cycle of NH4 + was synchronous with NO2 −, NO3 −, and satellite chlorophyll a concentrations in the western subtropical South Pacific, whereas it was synchronous with chlorophyll-a but out of phase with NO2 − and NO3 − in the subarctic regions.

Variability in the correlation between satellite-derived liquid cloud droplet effective radius and aerosol index over the northern Pacific Ocean

The relationship between aerosol index (AI) and cloud effective radius (CER) was examined over five sea regions in the northern Pacific using daily Moderate Resolution Imaging Spectroradiometer data from 2002–2014. The results show that there tends to be a negative relationship between AI and CER at lower AI, becoming positive at higher AI, suggesting the Twomey effect held in low aerosol environment. Over the entire AI range, the correlation between AI and CER was significantly positive over the two marginal seas (the Bohai–Yellow Sea and the Sea of Japan), and it was significantly negative over the three open oceans (the North Pacific subtropical gyre, the western and eastern subarctic North Pacific) for all seasons, except that the correlations in summer were significantly positive over the two subarctic North Pacific regions. A series of statistical analyses showed that the AI–CER relationship (the regression slope) is significantly correlated with relative humidity (RH) and precipitable water vapour (PWV); on average, PWV accounts for 60% of the variance of the slope for the two marginal seas and higher for the three open oceans (79%). The slope can change from negative to positive at high PWV levels, suggesting that water vapour plays an important role in the variability of the slope. Aerosol hygroscopic growth and the growth of CER in humid condition may counteract the Twomey effect at high AI, particularly over the two marginal seas. The error high contribution of the low liquid cloud fraction (LCF) to AI–CER relationship may partially account for the positive slope over the two marginal seas, while the low LCF has a negligible impact on the three open oceans. Additionally, over the three open oceans, stable thermodynamic state may prevent the effect of aerosol on cloud at high AI.

Using the natural spatial pattern of marine productivity in the Subarctic North Pacific to evaluate paleoproductivity proxies

AbstractSedimentary proxies used to reconstruct marine productivity suffer from variable preservation and are sensitive to factors other than productivity. Therefore, proxy calibration is warranted. Here we map the spatial patterns of two paleoproductivity proxies, biogenic opal and barium fluxes, from a set of core‐top sediments recovered in the Subarctic North Pacific. Comparisons of the proxy data with independent estimates of primary and export production, surface water macronutrient concentrations, and biological pCO2 drawdown indicate that neither proxy shows a significant correlation with primary or export productivity for the entire region. Biogenic opal fluxes, when corrected for preservation using 230Th‐normalized accumulation rates, show a good correlation with primary productivity along the volcanic arcs (τ = 0.71, p = 0.0024) and with export productivity throughout the western Subarctic North Pacific (τ = 0.71, p = 0.0107). Moderate and good correlations of biogenic barium flux with export production (τ = 0.57, p = 0.0022) and with surface water silicate concentrations (τ = 0.70, p = 0.0002) are observed for the central and eastern Subarctic North Pacific. For reasons unknown, however, no correlation is found in the western Subarctic North Pacific between biogenic barium flux and the reference data. Nonetheless, we show that barite saturation, uncertainty in the lithogenic barium corrections, and problems with the reference data sets are not responsible for the lack of a significant correlation between biogenic barium flux and the reference data. Further studies evaluating the factors controlling the variability of the biogenic constituents in the sediments are desirable in this region.

Eolian dust input to the Subarctic North Pacific

Eolian dust is a significant source of iron and other nutrients that are essential for the health of marine ecosystems and potentially a controlling factor of the high nutrient-low chlorophyll status of the Subarctic North Pacific. We map the spatial distribution of dust input using three different geochemical tracers of eolian dust, 4He, 232Th and rare earth elements, in combination with grain size distribution data, from a set of core-top sediments covering the entire Subarctic North Pacific. Using the suite of geochemical proxies to fingerprint different lithogenic components, we deconvolve eolian dust input from other lithogenic inputs such as volcanic ash, ice-rafted debris, riverine and hemipelagic input. While the open ocean sites far away from the volcanic arcs are dominantly composed of pure eolian dust, lithogenic components other than eolian dust play a more crucial role along the arcs. In sites dominated by dust, eolian dust input appears to be characterized by a nearly uniform grain size mode at ∼4 μm.Applying the 230Th-normalization technique, our proxies yield a consistent pattern of uniform dust fluxes of 1–2 g/m2/yr across the Subarctic North Pacific. Elevated eolian dust fluxes of 2–4 g/m2/yr characterize the westernmost region off Japan and the southern Kurile Islands south of 45° N and west of 165° E along the main pathway of the westerly winds. The core-top based dust flux reconstruction is consistent with recent estimates based on dissolved thorium isotope concentrations in seawater from the Subarctic North Pacific. The dust flux pattern compares well with state-of-the-art dust model predictions in the western and central Subarctic North Pacific, but we find that dust fluxes are higher than modeled fluxes by 0.5–1 g/m2/yr in the northwest, northeast and eastern Subarctic North Pacific. Our results provide an important benchmark for biogeochemical models and a robust approach for downcore studies testing dust-induced iron fertilization of past changes in biological productivity in the Subarctic North Pacific.

Distribution and composition of suspended biogenic particles in surface waters across Subarctic and Arctic Seas

Suspended biogenic particles were examined across Subarctic and Arctic Seas during the Canada‐Three‐Oceans (C3O) program in the summers of 2007 and 2008. Particulate organic carbon (POC) and nitrogen (PON), biogenic silica (bSiO2), and size‐fractionated chlorophyll a (chl a) concentrations were measured within the euphotic zone throughout five domains: Eastern Subarctic North Pacific (ESNP), Bering and Chukchi Seas (BE‐CH), Southern Beaufort Sea and Canada Basin (BS‐CB), Canadian Arctic Archipelago (CAA), and Baffin Bay and Labrador Sea (BB‐LS). Despite large variability along the sampling transect and vertically throughout the euphotic zone, domain averages of depth‐integrated POC and PON were relatively invariant across the ESNP, BE‐CH, CAA, and BB‐LS, ranging from 546 to 670 mmol m−2 (POC) and 101 to 154 mmol m−2 (PON), but were much lower in BS‐CB (199 and 78 mmol m−2, respectively). In contrast, depth‐integrated bSiO2 averages were highest in BE‐CH and CAA (>100 mmol m−2), intermediate in ESNP and BB‐LS (34–48 mmol m−2), and low in BS‐CB (6 mmol m−2). Similarly to the bSiO2 distribution, the biomass of the >5 μm chl a size‐fraction averaged 68–69% of total chl a in BE‐CH and CAA, 56% in BB‐LS, and 22–23% in ESNP and BS‐CB. Ratios of bSiO2:PON averaged 1.10–1.30 in BE‐CH and CAA, 0.30–0.34 in ESNP and BB‐LS, and 0.08 in BS‐CB. This study highlights the relative importance of siliceous phytoplankton such as diatoms in BE‐CH and CAA, smaller nonsiliceous cells in BS‐CB and ESNP, and a mixed assemblage of variably sized phytoplankton with a less siliceous component in BB‐LS.

Pelagic primary productivity and upper ocean nutrient dynamics across Subarctic and Arctic Seas

Phytoplankton and nutrient dynamics were investigated during the 2007 and 2008 summers in the euphotic zone of five broad domains across Subarctic and Arctic Seas: the Eastern Subarctic North Pacific Ocean, ESNP; Bering and Chukchi Seas, BE‐CH; Beaufort Sea and Canada Basin, BS‐CB; Canadian Arctic Archipelago, CAA; and Baffin Bay and Labrador Sea, BB‐LS. Average concentrations of nutrients (NO3−, NH4+, Si(OH)4, and PO43−) decreased markedly from west to east, with minima in NO3− and NH4+ in surface BS‐CB waters, but relatively invariant urea‐N concentrations across the entire region. In the BS‐CB domain, low uptake rates of nitrate (ρNO3−) and ammonium (ρNH4+) were exceeded by uptake of urea (ρUrea‐N). Whereas average ρNO3− was highest in the BE‐CH domain, ρUrea‐N was maximal in BB‐LS. Average depth‐integrated f‐ratios ranged from 0.27 in the BS‐CB domain to 0.57 in BE‐CH, while chlorophyll a (chl a) and primary productivity (ρC) were highest in BE‐CH and BB‐LS, and consistently low in the BS‐CB domain. The >5 µm phytoplankton fraction dominated ρC and ρNO3− in the BE‐CH and CAA domains, whereas ESNP and BS‐CB were dominated by the <5 µm fraction. In the BB‐LS domain, the larger cells were responsible for ∼50% of ρC, ρNO3−, and ρUrea‐N. This study highlights the contrast in ice‐corrected average new production between the BE‐CH (396 mg C m−2 d−1) and BS‐CB (5.50 mg C m−2 d−1) domains in summer, and the larger contribution of urea‐N uptake to total N uptake in central and eastern regions where NO3− concentrations were lower.

Decadal variation of temperature inversions along Line P

Hydrographic data measured for 50 years along Line P between the North American west coast and mid Gulf of Alaska as well as data from profiling float observations were analyzed to study the formation and variation of temperature inversions in the eastern subarctic North Pacific. Remarkable decadal to inter-decadal variation was observed in the magnitude of temperature inversions. This variation was mostly attributed to the variation of southward Ekman transport, eastward geostrophic transport, and surface cooling.

Dissolved zinc in the subarctic North Pacific and Bering Sea: Its distribution, speciation, and importance to primary producers

The eastern subarctic North Pacific, an area of high nutrients and low chlorophyll, has been studied with respect to the potential for iron to control primary production. The geochemistry of zinc, a critical micronutrient for diatoms, is less well characterized. Total zinc concentrations and zinc speciation were measured in near‐surface waters on transects across the subarctic North Pacific and across the Bering Sea. Total dissolved zinc concentrations in the near‐surface ranged from 0.10 nmol L−1 to 1.15 nmol L−1 with lowest concentrations in the eastern portions of both the North Pacific and Bering Sea. Dissolved zinc speciation was dominated by complexation to strong organic ligands whose concentration ranged from 1.1 to 3.6 nmol L−1 with conditional stability constants (K′ZnL/Zn′) ranging from 109.3 to 1011.0. The importance of zinc to primary producers was evaluated by comparison to phytoplankton pigment concentrations and by performing a shipboard incubation. Zinc concentrations were positively correlated with two pigments that are characteristic of diatoms. At one station in the North Pacific, the addition of 0.75 nmol L−1 zinc resulted in a doubling of chlorophyll after 4 days.

Pan-North Pacific comparison of long-term variation in Neocalanus copepods based on stable isotope analysis

Regional differences in the mechanisms of temporal variation in the lower trophic levels in the western, central, and eastern subarctic North Pacific were studied using the nitrogen stable isotope ratio (δ15N) of the major copepod species, Neocalanus cristatus, Neocalanus flemingeri, and Neocalanus plumchrus. We used formalin-preserved specimens collected in the Oyashio region (OY), three sections from north to south along the 180° longitudinal line (180LineSA, TN, and TS), off Vancouver Island (Off-Van), and at Sta. P, during the periods of 1960–2000, 1979–1997, 1981–2007, and 1996–2007, respectively. The regional mean δ15N of the three species roughly corresponded to the surface nitrate distribution and the extent of its drawdown from winter to spring; it was higher in regions of larger seasonal drawdown as observed in the coastal regions OY and Off-Van (7–10‰), but lower in regions with less seasonal drawdown, such as in the offshore regions at St. P and stations along the 180Line (3–6‰). Time series analysis revealed possible region-specific mechanisms for temporal variation in Neocalanus δ15N. First, δ15N indicated shifts in feeding strategies between herbivorous to omnivorous/carnivorous at OY and 180LineSA, where δ15N tended to be lower in the years with warmer winters, suggesting that Neocalanus took advantage of enhanced phytoplankton production under favorable light availability due to increased stratification. Conversely, wind-induced latitudinal advection of surface water was considered to be the initial cause of interannual variation in Neocalanus δ15N at 180LineTN, 180LineTS, and Off-Van, where δ15N was higher in the years with strong southerly or westerly winds at 180LineTN and TS, and the Off-Van site. This suggests that pole-ward transport of relatively oligotrophic, southern water might enhance the uptake of the heavier isotope by phytoplankton, which Neocalanus feed upon. Another possibility at the Off-Van site, where high δ15N was observed (c.a., 8–10‰), is a switch in the Neocalanus feeding strategy induced by decreased phytoplankton availability. This study demonstrated the usefulness of zooplankton δ15N as an indicator of interannual variation in lower trophic level environments and food web structures, which are caused by region-specific mechanisms.

On Turbulence Production by Swimming Marine Organisms in the Open Ocean and Coastal Waters

Abstract Microstructure and acoustic profile time series were collected near Ocean Station P in the eastern subarctic North Pacific and in Saanich Inlet at the south end of Vancouver Island, British Columbia, Canada, to examine production of turbulent dissipation by swimming marine organisms. At Ocean Station P, although a number of zooplankton species are large enough to generate turbulence with Reynolds numbers Re > 1000, biomass densities are typically less than 103 individuals per cubic meter (<0.01% by volume), and turbulent kinetic energy dissipation rates ɛ were better correlated with 16-m vertical shear than acoustic backscatter layers. In Saanich Inlet, where krill densities are up to 104 individuals per cubic meter (0.1% by volume), no dramatic elevation of dissipation rates ɛ was associated with dusk and dawn vertical migrations of the acoustic backscatter layer. Dissipation rates are a factor of 2 higher [〈ɛ〉 = 1.4 × 10−8 W kg−1, corresponding to buoyancy Re = 〈ɛ〉/(νN 2) ∼ 140] in acoustic backscatter layers than in acoustically quiet waters, regardless of whether they are vertically migrating. The O(1 m) thick turbulence patches have vertical wavenumber spectra for microscale shear commensurate with the Nasmyth model turbulence spectrum. However, the turbulence bursts of O(10−5 W kg−1) proposed to occur in such dense swarms appear to be rare. Thus far, intense turbulent bursts have been found infrequently, even in very dense aggregations O(104 individuals per cubic meter) characteristic of coastal and high-latitude environs. Based on sampling to date, this corresponds to a frequency of occurrence of less than 4%, suggesting that turbulence production by the marine biosphere is not efficient.

Temporal variability of winter mixed layer in the mid-to high-latitude North Pacific

Temperature and salinity data from 2001 through 2005 from Argo profiling floats have been analyzed to examine the time evolution of the mixed layer depth (MLD) and density in the late fall to early spring in mid to high latitudes of the North Pacific. To examine MLD variations on various time scales from several days to seasonal, relatively small criteria (0.03 kg m−3 in density and 0.2°C in temperature) are used to determine MLD. Our analysis emphasizes that maximum MLD in some regions occurs much earlier than expected. We also observe systematic differences in timing between maximum mixed layer depth and density. Specifically, in the formation regions of the Subtropical and Central Mode Waters and in the Bering Sea, where the winter mixed layer is deep, MLD reaches its maximum in late winter (February and March), as expected. In the eastern subarctic North Pacific, however, the shallow, strong, permanent halocline prevents the mixed layer from deepening after early January, resulting in a range of timings of maximum MLD between January and April. In the southern subtropics from 20° to 30°N, where the winter mixed layer is relatively shallow, MLD reaches a maximum even earlier in December–January. In each region, MLD fluctuates on short time scales as it increases from late fall through early winter. Corresponding to this short-term variation, maximum MLD almost always occurs 0 to 100 days earlier than maximum mixed layer density in all regions.

Formation and variation of temperature inversions in the eastern subarctic North Pacific

Hydrographic data from profiling float observations for 2001–2005 and from expendable bathythermograph observations for 1993–2005 were analyzed to study the formation and variation of temperature inversions (T‐inversions) in the eastern subarctic North Pacific (SNP). The formation and variation of T‐inversions differed significantly between the northern and southern regions of the eastern SNP. In the northern region, the temperature minimum (T‐min) at the top of T‐inversions outcropped to the sea surface and was cooled in the mixed layer nearly every winter. This process caused a seasonal cycle in the magnitude of T‐inversions (ΔT), with a maximum in winter. In the southern region, the winter T‐min outcropped relatively infrequently and the ΔT did not exhibit a significant maximum in winter during most years. The T‐min in the southern region was likely to outcrop upstream near the date line roughly one year earlier and was then advected to the southern region.

Temperature Inversions in the Subarctic North Pacific

Abstract Hydrographic data from the World Ocean Database 2001 and Argo profiling floats were analyzed to study temperature inversions in the subarctic North Pacific Ocean. The frequency distribution of temperature inversions [F(t-inv)] at a resolution of 1° (latitude) × 3° (longitude) was calculated. Temperature inversions seldom occurred around 50°N in the eastern subarctic North Pacific but were more common in the northern Gulf of Alaska and the southeastern subarctic North Pacific (42°–48°N, 140°–170°W). Large temperature inversions occurred throughout the year in the western and central subarctic North Pacific (north of 42°N and west of 180°) except near the Aleutian and Kuril Islands. Near those islands, F(t-inv) was characterized by pronounced seasonal variations forced by surface heating/cooling and strong tidal mixing.

Seasonal and interannual variability of temperature inversions in the subarctic North Pacific

Hydrographic data from profiling floats obtained during 2001–2004 were analyzed to study seasonal and interannual variability of temperature inversions (T‐invs) in the subarctic North Pacific (SNP). In the western SNP and Bering Sea, the temperature minimum at the top of T‐invs outcropped and was renewed every winter, causing a seasonal cycle in the magnitude of T‐invs, with the maximum at the end of winter. In the Gulf of Alaska in the eastern SNP, the temperature minimum outcropped in winters 2002 and 2004, but scarcely outcropped in winter 2003. Consequently, the magnitude of the T‐invs showed remarkable interannual variation; its monotonic decrease through winter 2003 overwhelmed the seasonal cycle. The year‐to‐year variation of the magnitude of the T‐invs in each region of the SNP was consistent with and thereby attributable to that of the winter sea surface temperature anomaly there.

Relationship between bacterial community structure, light, and carbon cycling in the eastern subarctic North Pacific

Biogeochemical controls on the community structure of heterotrophic marine bacteria are not well understood, and these organisms play a critical role in the global carbon cycle. Through terminal restriction fragment length polymorphism (T‐RFLP) analyses of bacterial 16S rRNA genes along a zonal transect in the eastern subarctic North Pacific, variations in community structure were compared to commonly obtained chemical and biological oceanographic measurements. It was found that heterotrophic bacterial community structure was strongly, but independently, related to both phytoplanktonic production and light. Based on computer‐simulated T‐RFLP analyses of database 16S rRNA gene sequences, it appeared that the relationship between bacterial community structure and light observed along the transect was driven by the distribution of α‐proteobacteria with phototrophic capabilities. In addition, experiments were conducted at sea to measure the growth of heterotrophic bacteria in response to amendments of amino acids, protein, N‐acetyl glucosamine, and chitin. Variation in the response of heterotrophic bacteria in the experiments did not correlate significantly with variation in community structure along the transect, which suggests that the ability of the heterotrophic bacterial community to use specific components of the dissolved organic carbon reservoir was not related to its phylogenetic structure as detected by T‐RFLP. However, a possible relationship between community structure and the cycling of chitin is discussed. Overall, the results stress the importance of both heterotrophic and phototrophic metabolisms when considering environmental controls on the structure of bacterial communities in the sea.

Size‐fractionated iron distributions and iron‐limitation processes in the subarctic NW Pacific

Comparison of vertical profiles of size‐fractionated iron between the western and eastern subarctic North Pacific clearly showed higher labile particulate iron concentrations towards the west and this result strongly supports the higher iron supply in the western region. Additionally, the results of the SEEDS experiment, the first meso‐scale iron enrichment experiment in the subarctic North Pacific, clearly showed that artificially enriched iron in the dissolved fraction (mainly in colloidal fraction) was rapidly transformed to suspended labile particulate iron during phytoplankton growth and was retained in the surface mixed layer. Probably, this same rapid transformation process occurs naturally after sporadic atmospheric iron supply and the labile particulate iron is retained in the western region. Furthermore, this transformation process reduces dissolved concentration of iron and its bioavailability. Therefore, the transformation process is important for understanding how phytoplankton became iron limited and the biogeochemical iron cycle in the western subarctic North Pacific.

Chlorophyll-a and primary production during spring in the oceanic region of the Oyashio Water, the north-western Pacific

The concentration of phytoplankton chlorophyll-a (chl-a) and primary production within the euphotic zone (at depths corresponding to 100, 30, 10 and 2% light) were measured in the oceanic region of the Oyashio Water, the north-western Pacific, in spring of 1993, 1994 and 1995. The chl-a concentrations ranged from 0.15 to 6.98 mg m−3 and the range of chl-a standing stock integrated from the surface to the 2% light depth was between 4 and 43 mg m−2. The daily primary production integrated from the surface to the 2 and 0.2% light depths was estimated to be between 94 and 1695 mg C m−2 d−1 and between 114 and 2046 mg C m−2 d−1, respectively. Phytoplankton bloom was noticed when the depths of the euphotic zone and the upper mixed layer were similar. The maximum chl-a concentration and daily primary production in the upper 0.2% light depth were comparable to those reported previously in the spring bloom in the coastal, offshore and oceanic regions of the Oyashio Water, and in the summer bloom of the Western Subarctic Gyre in the North Pacific. The magnitude of daily primary production in the phytoplankton bloom in the western subarctic North Pacific (the Oyashio Water and the Western Subarctic Gyre) was comparable with those in the eastern subarctic North Pacific and the subarctic North Atlantic. The magnitude of chl-a in the phytoplankton bloom in the western subarctic North Pacific was higher than those in the other subarctic regions. Varied contribution of large diatoms to the phytoplankton biomass, caused by different iron and silicate conditions was hypothesized to be a leading factor for variation in magnitude of the phytoplankton biomass.

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.