Subarctic Pacific Ocean Research Articles

Climate Variability and Radiocarbon in the CM2Mc Earth System Model

Abstract The distribution of radiocarbon (14C) in the ocean and atmosphere has fluctuated on time scales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute 14C within the earth system on interannual to centennial time scales. The model, the Geophysical Fluid Dynamics Laboratory Climate Model version 2 (GFDL CM2) with Modular Ocean Model version 4p1(MOM4p1) at coarse-resolution (CM2Mc), is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory’s CM2M model, uses no flux adjustments, and is run here with a simple prognostic ocean biogeochemistry model including 14C. The atmospheric 14C and radiative boundary conditions are held constant so that the oceanic distribution of 14C is only a function of internal climate variability. The simulation displays previously described relationships between tropical sea surface 14C and the model equivalents of the El Niño–Southern Oscillation and Indonesian Throughflow. Sea surface 14C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974–76. Interannual variability in the air–sea balance of 14C is dominated by exchange within the belt of intense “Southern Westerly” winds, rather than at the convective locations where the surface 14C is most variable. Despite significant interannual variability, the simulated impact on air–sea exchange is an order of magnitude smaller than the recorded atmospheric 14C variability of the past millennium. This result partly reflects the importance of variability in the production rate of 14C in determining atmospheric 14C but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.

Quantifying the flux of CaCO3and organic carbon from the surface ocean using in situ measurements of O2, N2, pCO2, and pH

[1] Ocean acidification from anthropogenic CO2 has focused our attention on the importance of understanding the rates and mechanisms of CaCO3 formation so that changes can be monitored and feedbacks predicted. We present a method for determining the rate of CaCO3 production using in situ measureme nts of fCO2 and pH in surface waters of the eastern subarctic Pacific Ocean. These quantities were determined on a surface mooring every 3 h for a period of about 9 months in 2007 at Ocean Station Papa (50°N, 145°W). We use the data in a simple surface ocean, mass balance model of dissolved inorganic carbon (DIC) and alkalinity (Alk) to constrain the CaCO3: organic carbon (OC) production ratio to be approximately 0.5. A CaCO3 production rate of 8 mmol CaCO3 m−2 d−1 in the summer of 2007 (1.2 mol m−2 yr−1) is derived by combining the CaCO3: OC ratio with the a net organic carbon production rate (2.5 mol C m−2 yr−1) determined from in situ measurements of oxygen and nitrogen gas concentrations measured on the same mooring (Emerson and Stump, 2010). Carbonate chemistry data from a meridional hydrographic section in this area in 2008 indicate that isopycnal surfaces that outcrop in the winter in the subarctic Pacific and deepen southward into the subtropics are a much stronger source for alkalinity than vertical mixing. This pathway has a high enough Alk:DIC ratio to support the CaCO3:OC production rate implied by the fCO2 and pH data.

Fine-scale spatial and temporal variability of surface water dimethylsufide (DMS) concentrations and sea–air fluxes in the NE Subarctic Pacific

We report the results of a 2 year (6 cruises) survey of surface water dimethylsulfide (DMS) concentrations in the Subarctic Pacific Ocean using membrane inlet mass spectrometry for high spatial resolution measurements. Our results document strong variability in surface water DMS concentrations along a repeated coastal–oceanic transect. DMS concentrations were consistently low (~ 1–2 nM) in February, and increased to summertime mean values of ~ 8 nM. For both survey years, we observed higher DMS concentrations in near shore waters during early summer (June), but a reversal of this spatial gradient in August when the highest DMS concentrations were observed in open ocean regions. Sea–air DMS fluxes ranged from ~ 5 to 25 μmol m − 2 day − 1 and did not show the consistent spatial and temporal patterns observed for surface water concentrations. Fine-scale spatial variability was present for both surface water concentrations and sea–air fluxes. Some of this variability could be attributed to underlying physical and biologically heterogeneity, including the presence of localized phytoplankton blooms at the coastal–oceanic transition boundary. Using a simple statistical approach, we estimate that the mean length scale of DMS variability along our sampling transects was < 10 km, which was significantly shorter than that observed for physical variables (temperature and salinity), but somewhat larger than the variability length scale of chlorophyll a (chl a). Using multiple linear regression analysis, we were unable to produce a single consistent empirical relationship predicting surface DMS concentrations from a suite of biological and hydrographic variables. However, DMS did show significant correlations with some variables including calcite, chlorophyll a and mixed layer depth (MLD). Mean DMS concentrations across cruises were related to chl a/MLD ratios and the solar radiation dose, in a manner that was consistent with previously published algorithms. However, these algorithms failed to predict the mesoscale and sub-mesoscale variability along the cruise track. High frequency measurements of DMS during an extended occupation of Ocean Station Papa demonstrated large variability over periods ranging from ~ 12 to > 24 h. This variability could not be explained by simple empirical analysis, but showed some correspondence with daily cycles of incoming solar irradiance. Simple calculations suggest that biological turnover was likely a significant source of temporal variability in DMS concentrations at Ocean Station P. Our observations demonstrate the importance of high-resolution measurement techniques in furthering our understanding of ocean DMS dynamics.

Oceanographic conditions influencing silicoflagellate flux assemblages in the Bering Sea and subarctic Pacific Ocean during 1990–1994

To determine the influence of oceanographic conditions on silicoflagellate assemblages, sinking fluxes and composition of silicoflagellates were studied from August 1990 to July 1994 at two long-term monitoring sites: Station AB in the Bering Sea (53.5°N, 177°W) and Station SA in the northern subarctic Pacific Ocean (49°N, 174°W). Total silicoflagellate flux (TSF) increased annually in spring and/or fall at both stations. Interannual variation in the timing of the spring TSF increase was probably related to the depth of the mixed layer during winter and to the starting time of stratification. Composition of the silicoflagellate assemblage differed between the two stations. The assemblage at Station AB was composed almost entirely of Distephanus speculum, similar to the assemblage in the Western Subarctic Gyre. The temperate-subtropical genus, Dictyocha, occurred only sporadically. The assemblage at Station SA was more variable, reflecting changes in the climate–ocean relationships indicated by the Pacific Decadal Oscillation Index (PDOI). Dictyocha increased during the positive phase of the PDOI, due to intensification of the Alaskan Stream. In colder water, Distephanus with heptagonal skeletons were more abundant than hexagonal Distephanus. These results will be useful for paleoceanography studies of the area.

Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Autotrophic picoplankton in mesozooplankton guts: evidence of aggregate feeding in the mesopelagic zone and export of small phytoplankton

Zooplankton play a key role in affecting the efficiency by which organic matter is exported to depth. Mesozooplankton consumption of detrital aggregates has been hypothesized as a mechanism for enhancing the export of picoplankton from surface layers. We analyzed the gut con- tents of mesopelagic copepods and ostracods using light and epifluorescence microscopy to deter- mine if cyanobacteria and eukaryotic phytoplankton too small to be ingested individually were pre- sent. Hind-guts were dissected from multiple species collected in discrete depth intervals between 0 and 1000 m during the day and night, at contrasting sites in the subtropical (Hawaii Ocean Time- series site ALOHA) and subarctic (Japanese time-series site K2) Pacific Ocean. Autofluorescing cyanobacteria and small eukaryotic phytoplankton were found in the guts of nearly all species sam- pled from all depths, indicating consumption of aggregates. Some of the cyanobacteria and other small cells ingested may have originated from inside the guts, or as symbionts, of microzooplankton, which were also common in the guts of many of these species. At both sites, most species' guts con- tained higher concentrations of cyanobacteria and small phytoplankton at night than during the day. Ostracod guts at ALOHA contained higher densities of picoplankton than those at K2, reflecting the predominance of smaller cells at ALOHA. Guts of diel vertical migrators still contained picoplankton at their deep, daytime residence depths, indicating active export of these cells. Our results indicate mesozooplankton grazing on aggregates is a pathway by which flux of picoplankton can be enhanced.

Trophic niche breadth variability differs among three Neocalanus species in the subarctic Pacific Ocean

Trophic niche breadth variability differs among three Neocalanus species in the subarctic Pacific Ocean

Net biological oxygen production in the ocean—II: Remote in situ measurements of O 2 and N 2 in subarctic pacific surface waters

Net community biological production in the euphotic zone of the ocean fuels organic matter and oxygen export from the upper ocean, which has a large influence on the atmospheric pressure of carbon dioxide and is the driving force for metabolite distributions in the sea. We determine the net annual biological oxygen production in the mixed layer of the northeast subarctic Pacific Ocean from in situ O 2 and N 2 measurements. Temperature, salinity, total gas pressure and O 2 were measured every 3 h for 9 months in 2007 at about 3 m depth on a surface mooring at Station P (50°N, 145°W). The concentration of nitrogen gas, N 2, determined from separate total gas pressure and pO 2 measurements, was used as an inert tracer of the physical processes that induce gas departure from thermodynamic equilibrium with the atmosphere. We use a simple model of the ocean’s mixed layer along with the nitrogen concentration to constrain the importance of bubbles, gas exchange and horizontal advection, which are then used in the oxygen mass balance to derive net biological oxygen production. The mixed-layer oxygen mass balance is dominated by exchange with the atmosphere, and we determine a mean summertime oxygen production of 24 mmol O 2 m −2 d −1. The annual pattern in the difference between the supersaturation of oxygen and nitrogen in the surface waters reveals very little net oxygen production during the winter at this location. The calculated annual net community production (NCP) of carbon from this new method, 2.5 mol m −2 yr −1, agrees to within its error of about×40% with previous determinations at this location from oxygen mass balance, NO 3 − draw down and 234Th measurements. This value is either indistinguishable from or lower than annual NCP measurements in the subtropical North Pacific, indicating that there is no experimental evidence for differences in annual NCP between the subarctic and subtropical North Pacific Ocean.

Dynamics of dissolved and particulate organic matter during the spring bloom in the Oyashio region of the western subarctic Pacific Ocean

We evaluated the dynamics of dissolved organic matter (DOM) and particulate organic matter (POM) at various stages of the spring bloom in the Oyashio region of the western subarctic Pacific Ocean by means of samples obtained during 3 cruises in April, May, and June 2003. In April, the average concentration (±1 SD) of nitrate plus nitrite ions was 23.3 ± 0.39 µmol l -1 in surface waters, and this decreased to below the detection limit (<0.05 µmol l -1 ) during the bloom period. About 80% of newly accumulated organic carbon was partitioned into POM. Whereas the C:N ratios of bulk DOM ranged from 13 to 20, the average C:N ratio of newly accumulated DOM was 7.3 ± 1.5. The lability of DOM was assessed by seawater culture-type incubation experiments on surface sea- water samples from each observation site. Although the newly accumulated DOM was relatively rich in nitrogen, most of it escaped rapid (d to wk) bacterial consumption in the seawater culture experi- ments.

Metabolism and elemental composition of the euphausiids Euphausia pacifica and Thysanoessa inspinata during the phytoplankton bloom season in the Oyashio region, western subarctic Pacific Ocean

Rates of oxygen consumption ( R: μl O 2 [individual] −1 h −1), and ammonia excretion ( E: μg NH 4-N [individual] −1 h −1), O:N ratios (by atoms) and body water contents (% of wet mass [ WM], as an index of lipid accumulation) of Euphausia pacifica and Thysanoessa inspinata were monitored during 9–14 March and 6–30 April 2007, including the onset of the spring phytoplankton bloom in the Oyashio region. Regression analyses revealed that variations in R and E were generally correlated with dry body mass ( DM) for both euphausiids. When R and E were standardized to a body size of 10 mg DM, R std , E std , O:N ratios and water contents fluctuated from 6.3 to 10.1, 0.07 to 0.34, 31 to 232 and 73.8 to 78.1, respectively, for E. pacifica, and from 8.7 to 11.3, 0.06 to 0.71, 24 to 295 and 75.1 to 82.2, respectively, for T. inspinata. None of these variables were significantly correlated with temporal variations in SST (1.7 to 5.7 °C) or chlorophyll a standing stock (29.4 to 252.6 mg m −2, 0–100 m) at the study site. However, April E and O:N ratio data pooled for both euphausiids were significantly less and greater, respectively, than the pooled values in March, suggesting preferential utilization of dietary protein for body growth or reproduction under conditions of abundant food supply as found in April. No substantial differences were observed in water content, ash, C, N or C:N ratios (by mass) of the two euphausiids collected in either March or April. Water content, C, N, C:N ratio and ash data averaged for E. pacifica and T. inspinata over the March and April cruises were similar: 76.3 and 78.1% of WM, 36.1 and 37.5% of DW, 9.4 and 10.1% of DW, 3.82 and 3.71 by mass, and 10.6 and 10.8% of DM, respectively. Combining the R-DM and E-DM relationships established with the population structure data of Kim et al. (2010), we estimate that daily ingestion and ammonia-N regeneration by the combined populations of the two euphausiids were 1.5–24.1% (integrated mean: 4.9%) and 0.18–1.32% (0.41%), respectively, of primary production and associated N demand during the bloom.

Nutrient and diatom dynamics during late winter and spring in the Oyashio region of the western subarctic Pacific Ocean

We investigated the nutrient and diatom dynamics during late winter and spring (9-March to 1-May 2007) in the Oyashio region as part of the OECOS-WEST research cruises. Macronutrients, iron, chlorophyll a (Chl- a) and biogenic silica (BSi) concentrations in the upper mixed layer varied remarkably ranges were 1.88–18.8 μmol L −1 for NO 3+NO 2, 0.64–1.85 μmol L −1 for PO 4, 3.14–35.7 μmol L −1 for Si(OH) 4, 0.14–0.54 nmol L −1 for D-Fe, 0.64–24.6 nmol L −1 for T-Fe, 0.30–17.4 μg L −1 for Chl- a, and 0.34–14.1 μmol L −1 for BSi. Mixed-layer depth (MLD) varied from 8–190 m during the cruises. The growth rate of in situ phytoplankton communities, dominated by centric diatoms, varied in shipboard culture experiments from 0.55 d −1 for iron-replete to 0.14 d −1 for iron-limited conditions. A relationship between BSi and Chl- a concentrations indicates that the in situ diatom community in the warmer water system (>4 °C) was heavily silicified, probably due to iron-limitation. The in situ macronutrient and dissolved iron concentrations below the MLD and estimated macronutrient concentrations during winter were negatively correlated to temperature (1–6 °C), that is to the relative proportion of warm modified Kuroshio Water mixed into the colder Oyashio water system. The rate of decrease in Si(OH) 4 per °C increase was greater than the rates for NO 3+NO 2 and PO 4 for both in situ and estimated winter values. These results suggest that the spring bloom in the cold-water system with high macronutrients and iron concentrations would progress rapidly and intensely, and then be terminated by nitrogenous nutrient depletion. However, the diatom bloom in warmer waters with lower macronutrients and iron concentrations would be terminated by Si- and/or iron-limitation of heavily-silicified diatoms. In the OECOS study, variation of macronutrients and iron due to the surface intrusions of several water masses and modification from different chemical conditions during winter were the most important factors regulating the progression, magnitude and probably fate of the spring phytoplankton bloom in the Oyashio region.

Reproduction, grazing, and development of the large subarctic calanoid Eucalanus bungii: is the spring diatom bloom the key to controlling their recruitment?

The response of large calanoid, Eucalanus bungii, to environmental fluctuation, particularly in relation to the spring diatom bloom in the Oyashio region, western subarctic Pacific Ocean, was examined by investigating egg production, grazing, development and starvation tolerance. Mean in situ egg production rate increased with ambient chlorophyll-a concentration, ranging from 0 to 47 eggs female−1 d−1, while no diurnal synchronous spawning behavior was observed. Under the spring bloom condition, E. bungii showed prey preference for less mobile and larger-sized prey (≥30 μm ESD) and bloom-forming diatom Thalassiosira spp. accounted for >80% of ingested carbon. In the laboratory, E. bungii was successfully reared from newly hatched nauplii to adult with the diatom, Thalassiosira nordenskioldi, as a food resource. Nauplii newly hatched from eggs reached the adult stage in ca. 150 days (5°C) with a sigmoidal developmental pattern and no sexual difference in development pattern. Starvation experiments indicated that the starved copepodids (C1–C4) became more vulnerable to high temperature with the progression of developmental stage, suggesting that the post-bloom condition with low food availability and increased temperature is harsh for their copepodids. The results of this study in conjunction with previous findings suggest that E. bungii is well adapted to utilize large-sized phytoplankton, such as a bloom-forming diatoms and, therefore, their recruitment processes, including egg production, development and mortality would be strongly affected by the duration and intensity of the spring bloom.

Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Dissolved inorganic carbon (DIC) was measured from 1992 to 2008 at two time-series sites in the subarctic western North Pacific; this region is a source of atmospheric CO2 in winter due to vertical mixing of deep waters rich in DIC. To estimate the decadal DIC increase resulting from CO2 uptake from the atmosphere, we corrected DIC for the contribution of biological activity below the temperature minimum (Tmin) layer (¡«100 m), which is the remnant of the mixed layer from the preceding winter. Decadal DIC increases in the Tmin layer and upper intermediate water (1.3¨C1.5 ¦Ìmol kg-1 yr-1; 100¨C200 m) were higher than those expected from oceanic equilibration with increasing atmospheric CO2 and those previously reported in the open North Pacific. The increase in water column CO2 was estimated to be 0.40 ¡À 0.08 mol m-2 yr-1. The decadal DIC change in the Tmin layer affects winter CO2 emission. The increase of atmospheric xCO2 in winter (2.1 ± 0.0 ppm yr-1) is higher than that of oceanic CO2 (0.7 ¡À 0.5 ppm yr-1) that calculated from DIC and total alkalinity in the Tmin layer. This difference suggests reduction of CO2 emission in winter is possibly controlled by the increase of total alkalinity.

Late Pleistocene stratigraphy and palaeoceanographic implications in northern Bering Sea slope sediments: evidence from the radiolarian species Cycladophora davisiana

AbstractA high‐resolution abundance curve of the radiolarian species, Cycladophora davisiana, was obtained from a Late Pleistocene sediment core, PC‐23A, which was obtained from the northern slope of the Bering Sea. The abundance pattern of this species was high during the glacial period and low during the Holocene. This general stratigraphic pattern was correlated with a previously reported pattern for the subarctic Pacific Ocean. However, planktonic foraminifera‐based accelerator mass spectrometry 14C dating revealed that a significant peak in the C. davisiana curve during the glacial period correlates to 15.5 ka, which is younger than that previously estimated as being the Last Glacial Maximum. The higher abundance of this species implies that there was well‐ventilated water with a large amount of organic matter in the intermediate‐water layer. An unusually high propagation of C. davisiana during the earliest Holocene period, Bølling–Allerød (B‐A) and Dansgaad–Oescheger interstadial event # 12, was most likely related to a significant increase in the amount of organic matter released into the intermediate‐water depths by a brine pump system via ventilation. In addition, centennial‐scale variations in the abundance of C. davisiana during the B‐A period suggest that the intermediate‐water ventilation was sensitively affected by brine rejection during sea ice formation as well as large‐scale atmospheric circulations such as the Arctic Oscillation. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The response of the virus community to the SEEDS II mesoscale iron fertilization

Although the important role of viruses in marine biogeochemical cycles has been established in recent years, virus activity (including changes in this activity) has been largely ignored during mesoscale iron (Fe)-fertilization experiments relative to other processes. This is of particular interest as viruses have been shown to be critical to the transformation of Fe from the particulate (i.e., biological) to the dissolved pools. The goal of the present study was to evaluate changes in the virus-mediated lysis of heterotrophic bacterial cells following a shift in ecosystem trophic status brought about by a mesoscale Fe addition in the subarctic Pacific Ocean. Virus production rates, estimated by a reduction and reoccurrence assay, were coupled with transmission electron microscopy estimates of burst size and direct counts of virus and bacterial abundance. Fe fertilization of the upper mixed layer resulted in significant yet weak increases in virus production rates during the 12 days of observation immediately after fertilization, although the burst size (viruses produced per lytic event) and the percentage of visibly infected cells remained constant. The results imply that increases in virus production rates were most likely tied to a decreased lytic cycle length or the stimulation of lysogenized cells following the stimulation of primary and secondary productivity by the addition of Fe. The results also indicate that virus-induced cell-lysis regenerated an estimated nearly 200 pmol L−1 Fe daily, providing a significant return of Fe back to the water column, which may be critical in the maintenance of this added Fe as resident.

Primary productivity, bacterial productivity and nitrogen uptake in response to iron enrichment during the SEEDS II

Abstract Primary productivity (PP), bacterial productivity (BP) and the uptake rates of nitrate and ammonium were measured using isotopic methods ( 13 C, 3 H, 15 N) during a mesoscale iron (Fe)-enrichment experiment conducted in the western subarctic Pacific Ocean in 2004 (SEEDS II). PP increased following Fe enrichment, reached maximal rates 12 days after the enrichment, and then declined to the initial level on day 17. During the 23-day observation period, we observed the development and decline of the Fe-induced bloom. The surface mixed layer (SML) integrated PP increased by 3-fold, but was smaller than the 5-fold increase observed in the previous Fe-enrichment experiment conducted at almost the same location and season during 2001 (SEEDS). Nitrate uptake rates were enhanced by Fe enrichment but decreased after day 5, and became lower than ammonium uptake rates after day 17. The total nitrogenous nutrient uptake rate declined after the peak of the bloom, and accumulation of ammonium was obvious in the euphotic layer. Nitrate utilization accounted for all the requirements of N for the massive bloom development during SEEDS, whereas during SEEDS II, nitrate accounted for >90% of total N utilization on day 5, declining to 40% by the end of the observation period. The SML-integrated BP increased after day 2 and peaked twice on days 8 and 21. Ammonium accumulation and the delayed heterotrophic activity suggested active regeneration occurred after the peak of the bloom. The SML-integrated PP between days 0 and 23 was 19.0 g C m −2 . The SML-integrated BP during the same period was 2.6 g C m −2 , which was 14% of the SML-integrated PP. Carbon budget calculation for the whole experimental period indicated that 33% of the whole (particulate plus dissolved) PP (21.5 g C m −2 ) was exported below the SML and 18% was transferred to the meso-zooplankton (growth). The bacterial carbon consumption (43% of the whole PP) was supported by DOC or POC release from phytoplankton, zooplankton, protozoa and viruses. More than a half (56%) of the whole PP in the Fe patch was consumed within the SML by respiration of heterotrophic organisms and returned to CO 2 .

Iron-induced alterations of bacterial DMSP metabolism in the western subarctic Pacific during SEEDS-II

Abstract The effect of added iron on bacterial cycling of the climate-active gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was tested during the second Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS II) from 19 July to 21 August 2004 aboard the R/V Hakuho-Maru. The study area in the northwest Pacific Ocean (48°N 165°E) was enriched with Fe and the conservative tracer, SF6, allowing the fertilized patch to be tracked. Microbial DMSP cycling rates were determined in the surface mixed layer (5 m) during incubations using the 35S-DMSP technique. The addition of iron resulted in a 4-fold increase in concentrations of chlorophyll a (chl a) within the surface mixed layer (5 m depth), and the length of the sampling period allowed the observation of both bloom and post-bloom conditions. Inside the fertilized patch, the alleviation of resource limitation gave rise to the concurrent increase in bacterial abundance and production. Changes in the phytoplankton community within the Fe-enriched patch translated into a sustained decrease in chl a-normalized particulate DMSP (DMSPp) concentrations, suggesting a preferential stimulation of the growth of DMSPp-poor phytoplankton species. Despite short-lived peaks of DMSPp within the Fe-enriched area, concentrations of DMSPp generally remained stable during the entire sampling period inside and outside the fertilized patch. During the Fe-induced bloom, microbial DMSP-sulfur (DMSP-S) assimilation efficiency increased 2.6-fold inside the Fe-enriched area, which indicated that as bacterial production increased, a greater proportion of DMSP-S was assimilated and possibly diverted away from the bacterial cleavage pathway (i.e. production of DMS). Our results suggest that iron-induced stimulation of weak DMSPp-producers and DMSP-assimilating bacteria may diminish the potential production of DMS and thus limit its flux towards the atmosphere over the subarctic Pacific Ocean.

北太平洋西部亜寒帯親潮域の基礎生産を支えるアムール川流域 －「巨大」魚付林仮説－

北太平洋西部亜寒帯親潮域の基礎生産を支えるアムール川流域 －「巨大」魚付林仮説－

Downward carbon transport by diel vertical migration of the copepods Metridia pacifica and Metridia okhotensis in the Oyashio region of the western subarctic Pacific Ocean

Seasonal change in the downward carbon transport due to respiration and mortality through diel vertical migration (DVM) of the calanoid copepods Metridia pacifica and Metridia okhotensis was estimated in the Oyashio region, western subarctic Pacific during six cruises from June 2001 to June 2002. M. pacifica (C4, C5 and adult females) was an active migratory species throughout the year though its DVM amplitude varied among seasons and stages. The mean distribution depths of adult females during the daytime were positively related with the illumination level in the water column, being shallowest in April and deepest in January. M. okhotensis generally showed less-extensive migrations than M. pacifica. Therefore, together with their lower abundance, this species is considered to be a less-important mechanism of downward transport of carbon except for April when their DVM was more active and descended deeper than M. pacifica, which remained in the upper 150 m even during the daytime. The mean migrating biomass of the two Metridia species was 558 mg C m −2 d −1 and was high during summer to winter (263–1676 mg C m −2 d −1) and low during spring (59–63 mg C m −2 d −1). Total downward flux through DVM fluctuated between 1.0 and 20.0 mg C m −2 d −1 with an annual mean of 8.0 mg C m −2 d −1. Contribution of the respiratory flux was greater than the mortality flux and accounted for 64–98% of total migratory flux throughout the year except for January when contribution of both fluxes was equal. Overall the annual carbon transport by DVM of Metridia spp. was estimated as 3.0 g C m −2 year −1, corresponding to 15% of the annual total POC flux at 150 m at the study site, suggesting that DVM is a significant process for carbon export in the subarctic region as well as that in tropical and subtropical oceanic regions. Since DVM in M. pacifica is more active during the non-bloom season when the gravitational flux of particulate matter is low, this species plays an important role in driving the biological pump in the subarctic Pacific during summer to winter.

High feeding rates on large particles by Neocalanus flemingeri and N. plumchrus, and consequences for phytoplankton community structure in the subarctic Pacific Ocean

The open subarctic Pacific Ocean is a high nitrate low chlorophyll (HNLC) system characterized by low concentrations of phytoplankton, a community dominated by small cells, and iron-limited growth of, especially, the larger phytoplankton. In such systems the main energy and material flow is through the microbial web, with large copepods considered primarily to be grazers on the larger microzooplankton occupying the top of this web. Consistent with this is the recognition that much of the nutrition of the dominant copepods in this system, Neocalanus flemingeri, N. plumchrus and N. cristatus, is derived from microzooplankton. Also, these copepods consume only a small fraction of the total phytoplankton production. In this paper, we show that the contribution made by N. flemingeri and N. plumchrus to establishing and maintaining the community structure of this ecosystem should be re-evaluated. Our experiments indicate these grazers have high clearance rates on large particles, including both large phytoplankton and microzooplankton, and this selective removal contributes to establishment and maintenance of the observed foodweb structure in the Gulf of Alaska. These high feeding rates combined with large populations of these two Neocalanus species concentrated in the upper layer of the ocean, result in population-based feeding rates approximately equal to the growth rates of large phytoplankton under iron-limited conditions. We conclude that N. flemingeri and N. plumchrus populations (a) directly prevent the accumulation of large phytoplankton cells by selectively feeding on them at high rates, and (b) indirectly stimulate the accumulation of the smaller phytoplankton by consumption of their major grazers, the microzooplankton.