Kosei Sasaoka

Bering Sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001

Abstract Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3+NO2) rich water (>25 μM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m−3) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy.

A description of temporal and spatial variability in the Bering Sea spring phytoplankton blooms (1997-1999) using satelite multi-sensor remote sensing

Abstract The Bering Sea is well known as a highly productive marginal sea. The objectives of this study were to clarify the interannual variability of spring bloom dynamics of the Bering Sea and to describe the spatial variability of this highly productive area using satellite multi-sensor remote sensing. We used multi-sensor remote sensing data sets of ocean color (OCTS and SeaWiFS), sea surface temperature (AVHRR), sea ice (SSM/I) and sea wind (SSM/I) to understand the complexity of the Bering Sea ecosystem. Phytoplankton biomass depends on the timing of sea ice melting and tends to increase when the melting is delayed. Wind stress is one of the important factors controlling the timing of the spring bloom. In 1997 and 1998, the east–west distribution of phytoplankton biomass exhibited a seesaw pattern, either high in west and low in east or low in west and high in east. We hypothesize that this seesaw pattern results from changes in the position and intensity of the Aleutian Low during spring and its relation to the El Nino–La Nina phenomena. During the El Nino period of 1998, the Aleutian Low shifted to the east of its normal position, and weak wind stresses facilitated the development of stratification and enhancement of spring bloom in the west. Conversely, when the Aleutian Low moved over into the western region in spring 1997, the same situation occurred in the east. Thus, the movements of the Aleutian Low promote a west-east seesaw pattern of sea surface wind stress, and consequently a corresponding seesaw pattern in phytoplankton biomass resulting from the subsequent variations in the depth of the mixed layer.

Basin-scale pCO2 distribution using satellite sea surface temperature, Chl a, and climatological salinity in the North Pacific in spring and summer

[1] An empirical method is presented for the estimation of basin-scale distribution of partial pressure of carbon dioxide (pCO 2 ) in the North Pacific using satellite-derived sea surface temperature (SST), chlorophyll-a concentrations (chl a), and climatological sea surface salinity (SSS). In this approach, multiple regression equations were developed to compute mixed layer dissolved inorganic carbon (DIC) based on SST, SSS and Chl a, whereas mixed layer total alkalinity (TA) was linearly regressed with SSS. The DIC-SST relation exhibited three different slopes at SST 27.5°C. Therefore data have been grouped with reference to SST. Regression equations were developed for two seasons (spring and summer). The regression errors for DIC and TA were 10.5 and 5 μmol kg -1 , respectively. The pCO 2 was computed from the estimated DIC and TA using dissociation constants given by Mehrbach et al. (1973), refit by Dickson and Millero (1987). The derived pCO 2 agreed with the shipboard pCO 2 observations within an error of 17-23 μatm. The sensitivity test on the regression equations for DIC estimation indicated that SSS is the most influencing parameter, followed by SST and Chl a. Using the monthly average SST and Chl a fields derived from the Advanced Very High Resolution Radiometer (AVHRR) and SeaWiFS (Sea-viewing Wide Field of view Sensor), respectively, and climatological SSS, monthly basin-scale pCO 2 fields were computed. The statistical model derived pCO 2 results are in agreement with underway pCO 2 in the North Pacific. This study strongly suggests that satellite-based techniques are promising tools for estimation of pCO 2 fields on a basin scale but the associated error bars are larger than required to study anthropogenic carbon uptake by the oceans. Incorporation of more in situ shipboard data may help in refining the estimating equations and reducing the errors further.

A Comparison of the Seasonality and Interannual Variability of Phytoplankton Biomass and Production in the Western and Eastern Gyres of the Subarctic Pacific Using Multi-Sensor Satellite Data

This study documents the results of a multi-sensor satellite investigation aimed at comparing the seasonality and interannual variability of phytoplankton biomass and primary productivity (PP) in the western and eastern gyres of the subarctic Pacific. Satellite data helped discern several features, most importantly the existence of significant east-west gradients in the supply of nitrate in winter, in the consumption of nitrate by phytoplankton and in phytoplankton production and biomass accumulation over the growth season. In the western subarctic gyre many of these features appear to be regulated by the strength of sea surface winds through increased iron and nitrate inputs. Multiple regression analysis of data extracted from 12 boxes spanning different hydrographic regimes in the subarctic Pacific, showed that over 65% of the variations in PP in the subarctic Pacific could be explained solely on the basis of changes in the strength of sea surface winds and the intensity of incident irradiance (PAR). The dependence of PP on sea surface wind stress was far greater in the western subarctic Pacific Gyre (WSG), than in the Alaskan Gyre (ALG) due to diminishing impact of surface winds towards the east. Spring accumulation of phytoplankton biomass was greater in the WSG than in the ALG despite the higher rates of PP in the latter. This study assumes particular significance because it helps ascertain the existence of several sub-regions within the two broader domains of the WSG and the ALG. In addition, large interannual variations in phytoplankton biomass and PP were observed in the subarctic Pacific following the onset of the El-Niño event of 1997 and the transition to La-Niña conditions in 1999. These variations were largely the result of differences in meteorological and oceanographic conditions across the subarctic Pacific following the development of the El-Niño.

Mesoscale eddy effects on temporal variability of surface chlorophyll a in the Kuroshio Extension

We investigated the relationship between chlorophyll a (Chl-a) concentrations estimated from satellite observations and the activity of eddies in the Kuroshio Extension region. High (low) area-averaged Chl-a concentrations were frequently observed in the core of cyclonic (anticyclonic) eddies. Such relationships between Chl-a concentrations and eddy cores were not frequently observed in the southern part of the recirculation gyre, and advection of background meridional gradient of Chl-a by eddy-edge currents accounted for Chl-a spatial variability. Decadal-scale changes of Chl-a concentrations around the Kuroshio Extension were strongly affected by eddy activity and transport but not by large-scale near-surface isopycnal heaving. We also found that decadal changes of nutrient concentrations near the main stream could affect Chl-a concentrations in the southern part of the recirculation gyre via southward transport of eddies and mean flow.

Seasonal variability of primary production and phytoplankton biomass in the western Pacific subarctic gyre: Control by light availability within the mixed layer

A distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time series station K2 in the western Pacific subarctic gyre (WSG). The mean depth-integrated primary production was highest (569 ± 162 mg C m−2 d−1) in summer and lowest (101 ± 16 mg C m−2 d−1) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high-nutrient, low-chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth-integrated chlorophyll a was observed in summer (54.6 ± 13.4 mg m−2), but chlorophyll a remained high in winter (45.3 ± 7.7 mg m−2). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.

Temperature and zooplankton size structure: climate control and basin-scale comparison in the North Pacific.

The global distribution of zooplankton community structure is known to follow latitudinal temperature gradients: larger species in cooler, higher latitudinal regions. However, interspecific relationships between temperature and size in zooplankton communities have not been fully examined in terms of temporal variation. To re-examine the relationship on a temporal scale and the effects of climate control thereon, we investigated the variation in copepod size structure in the eastern and western subarctic North Pacific in 2000–2011. This report presents the first basin-scale comparison of zooplankton community changes in the North Pacific based on a fully standardized data set obtained from the Continuous Plankton Recorder (CPR) survey. We found an increase in copepod community size (CCS) after 2006–2007 in the both regions because of the increased dominance of large cold-water species. Sea surface temperature varied in an east–west dipole manner, showing the typical Pacific Decadal Oscillation pattern: cooling in the east and warming in the west after 2006–2007. The observed positive correlation between CCS and sea surface temperature in the western North Pacific was inconsistent with the conventional interspecific temperature–size relationship. We explained this discrepancy by the geographical shift of the upper boundary of the thermal niche, the 9°C isotherm, of large cold-water species. In the eastern North Pacific, the boundary stretched northeast, to cover a large part of the sampling area after 2006–2007. In contrast, in the western North Pacific, the isotherm location hardly changed and the sampling area remained within its thermal niche throughout the study period, despite the warming that occurred. Our study suggests that while a climate-induced basin-scale cool–warm cycle can alter copepod community size and might subsequently impact the functions of the marine ecosystem in the North Pacific, the interspecific temperature–size relationship is not invariant and that understanding region-specific processes linking climate and ecosystem is indispensable.

Interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s

Abstract Surface chlorophyll a concentrations were measured at seven stations located at 1° latitude intervals between 52°30′ N and 58°30′ N along longitude 179°30′ W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993–95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179°30′ W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991–2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 μg l−1 in the Bering Sea basin and the values were usually

Seasonal changes in the mesozooplankton biomass and community structure in subarctic and subtropical time-series stations in the western North Pacific

Seasonal changes in mesozooplankton biomass and their community structures were observed at time-series stations K2 (subarctic) and S1 (subtropical) in the western North Pacific Ocean. At K2, the maximum biomass was observed during the spring when primary productivity was still low. The annual mean biomasses in the euphotic and 200- to 1000-m layers were 1.39 (day) and 2.49 (night) g C m−2 and 4.00 (day) and 3.63 (night) g C m−2, respectively. Mesozooplankton vertical distribution was bimodal and mesopelagic peak was observed in a 200- to 300-m layer; it mainly comprised dormant copepods. Copepods predominated in most sampling layers, but euphausiids were dominant at the surface during the night. At S1, the maximum biomass was observed during the spring and the peak timing of biomass followed those of chlorophyll a and primary productivity. The annual mean biomasses in the euphotic and 200- to 1000-m layers were 0.10 (day) and 0.21 (night) g C m−2 and 0.47 (day) and 0.26 (night) g C m−2, respectively. Copepods were dominant in most sampling layers, but their mean proportion was lower than that in K2. Mesozooplankton community characteristics at both sites were compared with those at other time-series stations in the North Pacific and with each other. The annual mean primary productivities and sinking POC fluxes were equivalent at both sites; however, mesozooplankton biomasses were higher at K2 than at S1. The difference of biomasses was probably caused by differences of individual carbon losses, population turnover rates, and trophic structures of communities between the two sites.

Sixteen-year phytoplankton biomass trends in the northwestern Pacific Ocean observed by the SeaWiFS and MODIS ocean color sensors

Using multisensor/platform biophysical data collected from 1997 to 2013, we investigated trends of the concentrations of phytoplankton biomass (Chl) in the northwestern Pacific Ocean (NWPO) and the probable responsible factors. The trend of rising sea surface temperature (SST) was the main factor maintaining phytoplankton positive net growth and resulted in a trend of increasing Chl at high latitudes in all seasons. At latitudes of 36–46°N, east of 160°E, the trend of rising SST was accompanied by a trend of declining Chl, markedly in spring and fall, which could be ascribed to strengthened stratification. The trends of environmental variables in the Oyashio area have modified conditions in a way detrimental to phytoplankton growth, the result being a trend of declining Chl from spring to fall. Chl south of roughly 36°N exhibited different trends in different seasons because of the different trends of vertical stratification. Whereas the observed 16-year Chl trends were not primarily influenced by interannual climate variability, to some degree they were likely modified by decadal variability associated with a weakened Aleutian Low pressure. This work prompts further comprehensive studies to investigate the probable ecological consequences of the observed Chl trend for high-trophic-level marine organisms in the NWPO.