We investigated phenological changes in phytoplankton in the subarctic North Pacific and the relationship to climatic forcing variability from 1998 to 2006, using ocean color satellite data combined ...with climatological data. The interannual to 9‐year mean variability in the timing, magnitude, and duration of phytoplankton blooms were examined using SeaWiFS data. Based on 10‐day composites of SeaWiFS data, the date of bloom initiation, peak chlorophyll a concentration, and bloom duration were estimated each year for every 2 × 2 degree grid within the range of 40–60°N and 130°E–120°W. The peak chlorophyll a concentration, date of bloom initiation, and bloom duration were derived by a Gaussian curve fitting technique. In addition, to investigate the geographic pattern of phytoplankton phenology, we classified the oceanic regions of the subarctic North Pacific into three groups based on K‐means clustering: group A was distributed in the coastal regions of marginal seas, waters around the Kamchatka Peninsula, and along the Aleutian Islands; group B was mainly distributed offshore in western and central regions; and group C was in southeastern and central regions. The timing of spring blooms of group A was earlier in El Niño phase and later in La Niña phase, whereas the opposite pattern was seen in group B. Our study clearly revealed regional differences in phytoplankton seasonality, phenology patterns, and relationships between interannual variability of phytoplankton phenology and El Niño Southern Oscillation‐scale variation.
Key Points
Phytoplankton phenology in the North Pacific was examined by using SeaWiFS
K‐means clustering was classified oceanic regions into three major groups
Interannual variability of bloom timing is related to ENSO‐scale variation
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We investigated the seasonal variability of the phytoplankton community in the western subarctic gyre (WSG) of the northwestern North Pacific with respect to structure (abundance, size, and taxonomic ...composition) and photophysiological state from 2006 to 2012 by using the chemotaxonomy program CHEMTAX, microscopy, and fast-repetition-rate fluorometry. Chlorophyll a standing stock (∫Chl a) varied seasonally from 20 to 52 mg m−2 and increased frequently to > 40 mg m−2 in June and July. Diatoms (20–35%) and prymnesiophytes (13–23%) comprised major portions of the ∫Chl a during the bloom period. Diatoms decreased to < 23% during the postbloom period, and prymnesiophytes became the most abundant group (24–35%). Mean Fv :Fm ratios (potential photochemical efficiency of photosystem II) in the mixed layer were relatively high (0.41–0.47) in winter and early spring, decreased rapidly to 0.32–0.39 concomitant with bloom development, and remained at low levels in the summer and autumn, although macronutrients NO⁻₃, PO3−₄, and Si(OH)₄ in the mixed layer were not depleted at any time. In June 2012, onboard-ship iron (Fe)-enrichment experiments stimulated increases of Chl a concentrations (from 0.64 to 7.15 mg m−3) and Fv :Fm (from 0.33 to 0.44). Seasonal variability of the phytoplankton community in the WSG is controlled mainly by Fe, with light and temperature limitation occurring in winter and early spring. Our study also suggests that the magnitude and duration of blooms in the WSG are strongly affected by Fe availability.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
In March 2011, an accident at the Fukushima Daiichi nuclear power plant (FNPP-AC) was caused by the Tohoku earthquake and tsunami. Here we show the distribution of artificial caesium-134 and -137 ...(134Cs and 137Cs) in the western North Pacific one month after the FNPP-AC. In surface seawater, 137Cs concentrations were from several times to two orders of magnitude higher than before the FNPP-AC. 134Cs was also detected, and in many seawater samples the 134Cs/137Cs ratio was about 1. These findings indicate that radionuclides from the FNPP dispersed quickly in the western North Pacific. 134Cs and 137Cs concentrations in suspended solids and zooplankton at stations K2 and S1 were also one to two orders higher than before the accident. Numerical simulation results show that the higher caesium observed in the western North Pacific one month after the FNPP-AC was transported not only by diffusion and advection of seawater but also via the atmosphere as an aerosol.
Sinking particles collected by drifting sediment traps at 100 and 200 m depths at two observation sites, K2 and S1, located in the subarctic and subtropical gyres in the western North Pacific, ...respectively, were fractionated in 5 ranges of sinking velocities between 5 and 1000 m d−1 using an elutriation system. The velocity distributions were divided into two distinct types over both sites: type-S containing more particles at the relatively slow end of the velocity range and type-F including a peak in the middle range (50–150 m d−1). These distributions showed little change between 100 and 200 m, although fluxes of particulate organic carbon (POC) decreased vertically. The averaged sinking velocities (wpoc) calculated from the velocity distributions of POC were 31 ± 16 and 63 ± 26 m d−1 at K2 and S1, respectively. For S1 particles, a positive correlation was found between wpoc and content of CaCO3. This result indicates that particles containing large amounts of denser CaCO3 sink faster than those containing large amounts of organic matter with low densities. Particles at K2, which were mainly composed of opal and organic matter, did not exhibit a clear relationship between wpoc and the content of denser opal. Instead of opal, wpoc had a positive correlation with δ15N of the sinking particles and was small (large) when the surface layer was stratified (well-mixed). The attenuation of POC flux with depth resulted from physical fragmentation of particles by turbulence at K2 and from biological decomposition and fragmentation at S1.
•Sinking velocity of particulate matter measurement by an elutriation system.•Particles containing large amount of CaCO3 sink faster.•Particles in the subarctic region sink faster in the well-mixed season.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Decadal- to multi-decadal variations have been reported in many regional ecosystems in the North Pacific, resulting in an increasing demand to elucidate the link between long-term climatic forcing ...and marine ecosystems. We detected phenological and quantitative changes in the copepod community in response to the decadal climatic variation in the western subarctic North Pacific by analyzing the extensive zooplankton collection taken since the 1950s, the Odate Collection. Copepod species were classified into five seasonal groups depending on the timing of the annual peak in abundance. The abundance of the spring community gradually increased for the period 1960-2002. The spring-summer community also showed an increasing trend in May, but a decadal oscillation pattern of quasi-30-year cycles in July. Phenological changes coincided with the climate regime shift in the mid-1970s, indicated by the Pacific decadal oscillation index (PDO). After the regime shift, the timing of the peak abundance was delayed one month, from March-April to April-May, in the spring community, whereas it peaked earlier, from June-July to May-June, in the spring-summer community, resulting in an overlap of the high productivity period for the two communities in May. Wintertime cooling, followed by rapid summertime warming, was considered to be responsible for delayed initiation and early termination of the productive season after the mid-1970s. Another phenological shift, quite different from the previous decade, was observed in the mid-1990s, when warm winters followed by cool summers lengthened the productive season. The results suggest that climatic forcing with different decadal cycles may operate independently during winter-spring and spring-summer to create seasonal and interannual variations in hydrographic conditions; thus, combinations of these seasonal processes may determine the annual biological productivity.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We investigated temporal and spatial variations in the zooplankton community structure in the Oyashio and Transition region of the subarctic western North Pacific from 1960 to 1999 using principal ...component analysis (PCA) and zooplankton samples from the historical Odate Collection. In particular, we examined the influence of Kuroshio and Oyashio decadal dynamics on geographical variations in the zooplankton community. The first principal component (PC1) closely represented the interannual variation in cold-water, large copepod species, while the second PC (PC2) represented the variation in warm-water, small copepod species. Using Rodionov's regime-shift method, we detected a significant increase in the PC score after 1976 and 1981 for PC1 and PC2, respectively. After the shift years, (1) warm-water species increased in the Transition zone, (2) the distribution center of the cold-water species shifted southward, and (3) copepod abundance and species diversity increased in the Transition zone as a result of (1) and (2). The timing of these shifts in the zooplankton community roughly coincided with the North Pacific climatic regime shift in 1976/1977. From the mid-1970s to the early 1980s, the southern boundary of the Oyashio shifted southward and increased geostrophic transport was observed in the Kuroshio, indicating spin up of the Kuroshio-Oyashio system. Change in atmospheric circulation during the 1976/1977 regime shift is thought to have caused the spin up of these currents, which subsequently affected the regional zooplankton community through advective processes.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Multiplatform observations of ocean biogeochemical data were used to elucidate meridional differences in the factors that limit phytoplankton biomass (Chl-a) and the mechanisms that trigger the ...seasonal winter or spring phytoplankton bloom in the northwestern Pacific Ocean (NWPO). During the winter, Chl-a north (south) of 30°N is limited by light (nutrients). During the spring and fall, Chl-a in much of the area east of the Japan/Kuril Islands and/or north of 40°N (south of 35°N) is limited by light (nutrients). During the summer, nutrients limit Chl-a over much of the NWPO, except in the areas east of the Japan/Kuril Islands and north of 45°N. In the area south of around 31°N, phytoplankton biomass is nutrient limited throughout the year, and the seasonal bloom emerges in the winter, begins in the fall which is associated with mixed layer deepening. Between 31°N and 40°N, the spring bloom onset is mainly associated with a cessation of mixed layer deepening. In much of the area north of 40°N, including the Oyashio area, the onset of the spring bloom is consistent with Sverdrup’s critical depth hypothesis. The spatial extents of the light- and nutrient-limited areas and the areas associated with a single bloom onset mechanism are by no means constant. They are expected to undergo meridional shifts as a result of large-scale climatic changes and global warming.
•We elucidated Chl-a limiting factors in northwestern Pacific•Light- and nutrient-limited areas shifted meridionally•There existed meridional differences in the mechanisms initiating seasonal blooms
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock ...and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Pelagic ecosystems of the western North Pacific have experienced dramatic changes in recent decades. Prior retrospective analysis of several regions has shown that ecosystem or population changes ...have occurred coincident with or shortly following significant changes in ocean climate and atmospheric forcing. Here, we summarize these changes and attempt to identify specific mechanisms responsible for the changes in three regions in the western North Pacific: the Oyashio (OY), the subtropical water (ST), and the southern Japan /East Sea (JES). A cooling condition after the climate regime shift of 1976-1977 deepened the winter mixed layer depth (MLD) in both the OY and ST, but influenced lower-trophic level productivity differently between these regions. The deep MLD reduced winter phytoplankton and zooplankton biomass in the subarctic OY, presumably due to a decrease in wintertime light availability. Concurrently, it increased spring plankton biomass in the ST, presumably due to the replenishment of springtime nutrients. When a warming condition became prevalent after the 1988-1989 regime shift, the shallow winter MLD increased the winter plankton biomass in the OY, but decreased the spring plankton biomass in the ST through the same mechanism controlling light and nutrient availability. In the JES, with a complex water column structure consisting of the surface warm current and dense subsurface cold water, the hydrographic conditions and ecosystem responses could not be explained by the mechanisms for the OY and ST. We also detected cooling and warming phase shifts induced a phenological change in the OY. A model hindcast for the OY estimated an average 5-day (max. 20-day) delay in the timing of the primary production peak during the cool phase after the mid-1970s. This timing became earlier after the mid-1990s, several years after the 1988-1989 regime shift, reaching the level before the mid-1970s. The observed increase and decrease in winter and spring phytoplankton biomass, respectively, during the warm phase in the 1990s, indicate a moderate but prolonged productive season, which differs from the conventional seasonal variation pattern of the OY with an extensive but short spring bloom. This alternate phytoplankton seasonality coupled with the direct influence of a warm temperature anomaly might dramatically alter the zooplankton community structure in the OY after the 1990s, as suggested by the marked increase in the occurrence of warm-water copepods in spring. Based on these findings, we warn that retrospective analyses based on annual averages and lacking careful consideration of seasonal variation may result in erroneous findings of causes and consequences.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted ...between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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