Takashi Midorikawa

Temporal increases of phosphate and apparent oxygen utilization in the subsurface waters of western subarctic Pacific from 1968 to 1998

30-years time series of AOU and phosphate in the Oyashio sub-surface domain showed an increasing trend superimposed on bidecadal oscillation. AOU and phosphate trend on isopycnals between 26.7 and 27.2 σθ increased by an average of 0.9±0.5 and 0.005±0.003 µmol/kg/y, respectively. Salinity on these isopycnals also showed an average linear increase of 0.0008 psu/y. Salinity and density of winter mixed layer, on the other hand, was found to have decreased during the observation period. Observed bidecadal oscillation (average period 20±1 y) in subsurface AOU negatively correlated with that of North Pacific Index (r=−0.88±0.06). As the cause of the linear increase of subsurface phosphate and AOU, we speculated that vertical water exchange in the upper layers of the subarctic North Pacific might have been diminished during this period. A decreasing trend of salinity and density of winter mixed layer in Oyashio that was observed during the same period supported this speculation.

Iron(III) hydroxide solubility and humic-type fluorescent organic matter in the deep water column of the Okhotsk Sea and the northwestern North Pacific Ocean

Abstract Vertical distributions of Fe(III) hydroxide solubility were studied in the Okhotsk Sea and the northwestern North Pacific Ocean during May and June 2000. Fe(III) solubility minima (0.35– 0.45 nM ) were present in a narrow depth range (80– 100 m ) below the surface mixed layer at all stations. In general, the Fe(III) solubility levels in intermediate and deep waters are characterized by mid-depth maxima (0.76– 0.86 nM ) at 800– 1250 m depth and, below that, a slight decrease to 0.4– 0.6 nM with depth in association with increase in nutrient, apparent oxygen utilization (AOU) and humic-type fluorescence intensity. The most significant correlation between the Fe(III) solubility and humic-type fluorescence in intermediate and deep waters suggests that the distribution of humic-type fluorescent organic matter may control the distribution of Fe(III) solubility in deep ocean waters. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters may be controlled primarily by Fe(III) complexation with natural organic ligands, such as marine dissolved humic substances released through the oxidative decomposition and transformation of biogenic organic matter in intermediate and deep waters. In addition, high Fe(III) hydroxide solubility values (1.0– 1.6 nM ) were observed in the surface mixed layer at a station in the northwestern North Pacific Ocean where a phytoplankton bloom was observed. The higher Fe(III) solubility in the surface waters was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators, which may be released by dominant phytoplankton and/or bacteria during the spring bloom and probably have a different chemical composition from those found in intermediate and deep waters.

Decreasing pH trend estimated from 25-yr time series of carbonate parameters in the western North Pacific

We estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3°N to 33°N along the repeat hydrographic line at 137°E in the western North Pacific Ocean. Estimates were based on the observational records of oceanic CO2 partial pressure and related surface properties over the last two decades. The computed pH time series both for 25 yr in winter (late January.early February) and for 21 yr in summer (June.July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer. The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr-1 (average, 0.0018 ± 0.0002 yr-1) in winter and from 0.0008 to 0.0019 yr-1 (average, 0.0013 ) 0.0005 yr-1) in summer. The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25°N) in summer. We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.

Molecular masses and chromophoric properties of dissolved organic ligands for copper(II) in oceanic water

Abstract The distribution of molecular masses of organic ligands for copper(II) in oceanic water was investigated. The bulk dissolved organic matter (DOM) was fractionated by ultrafiltration and organic ligands were extracted from the resultant fractions by using immobilized metal ion affinity chromatography (IMAC). Contributions of total organic ligands were 2.0–4.4% of the bulk DOM in surface waters, as determined by the UV absorbance. In the distribution of molecular masses of organic ligands, relative contribution of the fraction with low molecular masses ( 10,000 Da fraction. The distribution of molecular masses of organic ligands shifted to higher molecular masses, as compared with that of the bulk DOM. The fluorescence intensities of organic ligands were shown to be associated with carboxyl contents, based on peak excitation/emission wavelengths and the pH-dependence of fluorescence. Two ligand classes with different conditional stability constants (log K CuL ′≈7 and 9) were determined from fluorescence quenching of ligand fractions during copper(II) titration. Organic ligands in low molecular mass fractions were relatively weak and strong ligands occurred in higher molecular mass fractions. It is suggested that the weaker ligand sites would consist of two or more carboxyl groups (log K HL ′=4), whereas carboxyl groups (log K H 2 L ′=2), which are protonated at lower pH, and primary amine may additionally contribute to the formation of more stable copper(II) complexes of the stronger ligand.

Seasonal variation in total inorganic carbon and its controlling processes in surface waters of the western North Pacific subtropical gyre

Abstract Seasonal variation in total inorganic carbon (TCO 2 ) in surface waters of the western North Pacific (137°–152°E) subtropical gyre was analyzed on the basis of measurements of TCO 2 and partial pressure of CO 2 ( p CO 2 sw). The controlling processes including vertical mixing, horizontal advection, and net air–sea CO 2 transport, as well as biological activity, were quantified. The seasonal increase in normalized TCO 2 (NTCO 2 ) from autumn to winter, ranging from 19 to 37 μmol kg −1 in the northern part of the subtropical gyre between 24°N and 30°N, was predominantly accounted for by the upward supply of TCO 2 due to enhanced vertical mixing. The contribution of horizontal advection, estimated from monthly meridional NTCO 2 distributions and the monthly advection field of the Meteorological Research Institute (MRI)s 3D-ocean general circulation model, was insignificant. Analyses of the mixed-layer NTCO 2 budget revealed that biological activity was playing an important role in the decrease in surface NTCO 2 from winter to summer. Annual net community production reached 48±19 gC m −2 between 24°N and 30°N, and 19±16 gC m −2 between 15°N and 23°N.

Temporal variations in lower trophic level biological environments in the northwestern North Pacific Subtropical Gyre from 1950 to 1997

Abstract An examination of large archives (1950–1997) of the oceanographic and atmospheric data from the northwestern North Pacific Subtropical Gyre has revealed clear linkages between atmospheric forcing factors, physical processes and biological events. Large changes in the winter and spring biomass of phytoplankton and macroplankton observed over annual, decadal and inter-decadal time scales could clearly be attributed to climate-related changes in oceanographic processes. Interannual changes in the intensity of the winter-time East Asian Monsoon had a significant impact on the extent of convective overturning, on nitrate inputs into the euphotic zone and the concentrations of chlorophyll a in winter and during the following spring. A prolonged period of deeper winter mixed layers observed from the mid-1970s to the mid-1980s led to a sizeable increase in winter mixed-layer nitrate concentrations. This change resulted in a decrease in winter-time phytoplankton biomass. Spring-time chlorophyll a , in contrast, showed a steady increase during this period. The decline in winter phytoplankton biomass could be attributed to the depths of mixed layer. A deeper mixed layer prevents phytoplankton from remaining in the euphotic zone for long enough to photosynthesize and grow, leaving substantial amounts of nutrients unutilised. However, as a result of stratification of the water column in spring following each of these winters, phytoplankton could take advantage of the enhanced ambient concentrations of nutrients and increase its biomass. Another noteworthy observation for the period from the mid-1970s to the early 1980s is that the western subtropical gyre progressively became phosphate limited. The period of diminishing mixed-layer phosphate concentrations was observed in our study area from the early 1990s onwards was consistent with recent observations at Station ALOHA in the eastern subtropical gyre.

Extraction and characterization of organic ligands from oceanic water columns by immobilized metal ion affinity chromatography

Abstract Organic ligands for Cu(II) were extracted from oceanic water columns by immobilized metal ion affinity chromatography (IMAC). Separation of organic ligands from bulk dissolved organic matter (DOM) allowed the chemical characterization of organic ligands. Measurements of complexing abilities, as well as fluorescence and chemical analyses, indicated that natural ligands were a mixture of, at least, two different types of organic ligand. One type was prominent in the surface water, it was weakly fluorescent but rich in both primary amines and carbohydrates. The other type was predominant in deep water and had the converse characteristics, namely, low levels of both primary amines and carbohydrates, but relatively strong fluorescence. The measurements of the complexing ability of organic ligands from the surface waters suggested the existence of a natural ligand (log K′CuL ≈ 9) that has one or two primary amines as copper-binding sites.

Seasonal variability in the lower trophic level environments of the western subtropical Pacific and Oyashio Waters—a retrospective study

The seasonal cycles of phytoplankton and mesozooplankton and their environments in the western subtropical Pacific and Oyashio Waters have been compared and described, using all available historical oceanographic and atmospheric data from the regions. Remarkable differences exist in the seasonal cycles of water properties in the upper layers of these two regimes, primarily due to fundamental dissimilarities in the hydrographic and nutrient structures, and meridional differences in the amplitudes of the annual cycles of wind stress and solar irradiance in the two regions. In the Subtropical Water, seasonal variations in phytoplankton and mesozooplankton biomass are constrained within narrow bounds, because of semi-permanent stratification and downwelling typically associated with the subtropical anticyclonic gyres. A prominent feature here is that near-surface chlorophyll a concentrations increase during the deepening of the winter mixed layer when nutrient supply is greatest. This indicates that phytoplankton populations rapidly respond to higher nutrient flux during this time, since light levels are high enough for their growth. In contrast, in the Oyashio Water, large winter nutrient replenishment is followed by marked spring increases in phytoplankton and mesozooplankton biomass, initiated primarily by water-column stabilization following an increase in solar irradiance and shallowing of the mixed layer. An outstanding characteristic of the Oyashio Water is that the remarkable coincidence of the spring peak of mesozooplankton biomass with that of phytoplankton. From this it follows that newly recruited young copepodites arriving at the surface layer are capable of taking advantage of an abundant supply of food during the spring phytoplankton bloom to growand build up a large standing stock. Another conspicuous difference observed in the two water masses is that the N:P ratios in the Subtropical Water exceed the Redfield value in all seasons, with highest N:P centered around 24.5–25.5st in the core of the North Pacific Subtropical Mode Water. We speculate that N2 fixation may be an important source of newnitrogen in the Subtropical Water. r 2002 Elsevier Science Ltd. All rights reserved.

Distributions and variations in the partial pressure of CO2 in surface waters (pCO2w) of the central and western equatorial Pacific during the 1997/1998 El Niño event

Measurements of partial pressure of CO2 in surface waters (pCO2w) and overlying air (pCO2a) were made in the central and western equatorial Pacific from October 1997 to February 1998 within the period of the 1997/1998 El Nino, which was reported to be the strongest El Nino event on record. The distribution of the pCO2w showed a pattern driven by the eastward movement of western Pacific warm pool and thermodynamic effects (temperature and salinity), which was different from those of the moderate 1986/1987 El Nino and non-El Nino periods. Due to the eastward movement of the warm pool with sea surface temperature (SST) higher than 28.5 °C and sea surface salinity (SSS) lower than 34.5, the pCO2w between 180° and 163°W (347–364 μatm) was almost equal to that of the air (351 μatm). Between 143°E and 180°, the pCO2w tended to increase toward the west (387 μatm at 0°, 144°E in December 1997) along with the SST and SSS. West of 143°E in January 1998, a steep change in pCO2w ranging from 320 to 365 μatm occurred while retaining high SST (>28.5 °C) and SSS (>34.5). This was caused by the advection of surface water from the southern low latitudes that had been affected by biological activity (New Guinea Coastal Current). From December 1997 to January/February 1998, the SSS was usually higher than 34.5 west of 180°, which was significantly high compared to the western equatorial Pacific warm pool. This was probably due to the decrease of the net fresh water input for the western equatorial Pacific and/or the northward migration of surface water from the Southern Hemisphere. The CO2 outflux from the central and western equatorial Pacific (5.5°S–5.5°N, 139.5°E–159.5°W) was estimated to be 0.027 Pg-C/year in December 1997 and 0.038 Pg-C/year in January/February 1998. This presents a significant decrease from the CO2 outflux during the non-El Nino periods (0.34 Pg-C/year in January/February 1989, 0.11 Pg-C/year in September/November 1990) and a slight one from the moderate 1986/1987 El Nino period (0.055 Pg-C/year in January/February 1987). Following the El Nino–Southern Oscillation phenomena, CO2 outflux varied largely in the central equatorial Pacific and little in the western equatorial Pacific.

Detection of a strong ligand for copper in sea water and determination of its stability constant

A method for the detection and characterization of an organic ligand with a high conditional stability constant but a low concentration in natural waters is proposed. The method is based on a combination of a procedure that involves the ligand-exchange reaction between natural ligands and chelating ragents and the analysis of the experimental results on metal speciation by a simple equilibrium model. Application of this method to samples of sea water from different marine environments suggested the presence of a strong ligand for copper in most samples of sea water examined. The lower limits of the concentration of the strong ligand in oceanic water were evaluated to be < 0.01–0.20 nM. The conditional stability constant for the copper complex with this ligand was evaluated to be higher than 1013.8–1014.3 1 mol−1 at an ionic strength lower than 10−5 M at pH 5.71 and 4°C. In coastal water, the concentration of the strong ligand was higher than in oceanic water, but the conditional stability constant was the same. The conditional stability constant for the strong ligand, evaluated by the present method, is equal to or one order of magnitude greater than that of ethylenediaminetetraacetic acid under the same conditions.