Distribution of pelagic blue-green algae in the North Pacific Ocean

The distribution of pelagic blue-green algae, especially ofTrichodesmium thiebautii, was investigated on the basis of the collection of theHakuhō Maru Cruise KH-69-4 along 155°W (50°N-15°S) in the North Pacific Ocean from September to November 1969.

1. Five species were identified:Trichodesmium thiebautii (most predominant),T. erythraeum, Oscillatoria sp.,Katagnymene spiralis andRichelia intracellularis.
2. T. thiebautii was most abundant in the western North Pacific central water and abundant next to it in the equatorial water, but it did not occur in the subarctic water.
3. T. thiebautii was ubiquitously distributed in the lower layer of 100–200 m in the equatorial water, though not in a large quantity.
4. T. thiebautii inhabited only the water warmer than 20°C. In its main habitat, nitrate and nitrite were almost zero, but ammonia and phosphate were present. There was not found any correlation between its occurrence and the salinity.
5. Blue-green algae were generally thinly populated in the water rich in diatoms.

     

Oil pollution in the Izu Islands waters

The oil pollution in the Izu Islands waters are observed as follows:

1. Globules and lumps of oil were collected 2.56 g on the average in wet weight by each taw(174 m³) of a larval net at 389 stations, up to the maximum collection of more than 500 g.
2. On the investigation of the Oshima—Torishima line (J1–J10), globules and lumps of oil were collected more at the south of the Kuroshio Current than at the north.
3. In the coastal region (T1–T19) of Japan, many globules and lumps of oil were collected when a cold water mass was located at the west side of the Izu Islands, from south to east side of where the Kuroshio Current took a roundabout and closed near to the Bōsō peninsula (Aug. 1971, Jun. 1972, Aug. 1972), or when the Kuroshio Current took a roundabout the cold water mass which extended as far as Oshima from the coast of the Bōsō peninsula (Sept. 1971).

     

Characteristics of deep currents off Cape Shiono-misaki before and after formation of the large meander of the Kuroshio in 1981

Deep currents measured by moored current meters over the shelf-slope off Cape Shiono-misaki, Kii Peninsula during the period from 28 April, 1981 to 4 May, 1982 are analyzed to determine characteristics of the deep current before and after the large meander of the Kuroshio formed. The observed deep currents show some different characteristics between the periods before and after the formation of the large meander of the Kuroshio,i.e.:

1. The mean current direction over the shelf slope changed to westward after the meander was formed, though it was eastward at two offshore stations before the meander was formed.
2. The eddy kinetic energy, \(ke((\overline {u'^2 } + \overline {\upsilon '^2 } )/2)\) became large at all stations after the meander formed.
3. It appears that there were current variations in the period band shorter than 10 days which propagated offshore before the meander formed but inshore after the meander formed.
4. After the meander formed, the current variations with a period of O(25 days) were amplified at two of the three stations. The current variations in this period band showed high coherence among the three stations.

Data from tidal stations showed that sea level variations with a period of O(30 days) were also amplified along the south coast of Japan after the meander was formed. But sea level variations were not coherent with current variations in this period band.     

Marine environmental studies on outbreaks of red tides in neritic waters

The cause of red tides appearing in neritic waters in Japan have been discussed by several workers in relation to the development of eutrophication. The author has been engaged in marine environmental studies on outbreaks of red tides, mainly, in the Seto Inland Sea since 1964 using a combination of the following approaches:

1. surveys of the red tides that have occurred,
2. elucidation of the oceanographical and biological characters of the water areas where red tides occur,
3. isolation of the red tide organisms for physiological studies of their growth,
4. chemical and biological assays of the growth promoting factors, and
5. estimation of the effects of pollutants on the growth of the red tide organisms.

The red tide organisms surveyed wereEutreptiella sp. (Euglenophyceae),Dictyocha fibura (Chrysophyceae),Chattonella marina, C. antiqua, Fibrocapsa japonica, Heterosigma akashiwo (Chloromonadophyceae),Prorocentrum minimum, P. triestinum, Gymnodinium mikimotoi, G. nelsoni, Gymnodinium type '65,Noctiluca miliaris, Protogonyaulax catenella, Gonyaulax polygramma, Peridinium quinquecorne (Dinophyceae) andMesodinium rubrum (Ciliata). Most of these species were isolated and cultured for physiological studies. Algal growth potential in sea water was assayed in Hiuchi Nada and Beppu Bay. Growth promoting activity was found in sea water about 2km off shore from the industrial zone which locates along the coast of Beppu Bay. The effects of industrial wastes were examined forEutreptiella sp.,Heterosigma akashiwo andProrocentrum triestinum. Sulfite pulp waste caused marked promotion of growth in these organisms. The growth promoting activity of the waste was elucidated by chelation of the waste with iron. After the oil spill which occurred at Mizushima Bay in 1974, a heavy red tide ofNoctiluca miliaris appeared, followed byProrocentrum minimum. Polycyclic and polar fractions in heavy oil separated by chromatography were suspected to be the growth promoting substances. Natural growth promoting chemicals, uracil and thymine, were found in interstitial water of bottom mud of Beppu Bay in concentrations of 58–120, and 28–83?g per 100 g dry mud, respectively. The mechanism of fish kills caused by red tides was another subject of study.     

Vitamin B12 transudation from marine bottom muds

Transudation of vitamin B₁₂ from the surface layer of bottom muds in Maizuru Bay and in the Sea of Hiuchi was investigated in this report. The total counts of heterotrophs and bacteria producing vitamin B₁₂ in the bottom muds in these coastal regions were also examined. Results obtained were as follows:

1. In the marine bottom in these coastal regions, vitamin B₁₂ is continuously transuded from the surface layer of bottom muds to sea water. Accordingly, it is reasonably supposed that the surface layer of bottom muds, in which vitamin B₁₂ is produced and accumulated, is a source of provision of the vitamin in the coastal regions.
2. On an average, the quantity of transudation of vitamin B₁₂ from the surface layer of bottom muds to sea water is 2.6 μμg/cm²/hr and 5.2μμg/cm²/hr in Maizuru Bay and in the Sea of Hiuchi respectively.
3. In the surface layer of bottom muds in these coastal regions, the quantity of transudation of vitamin B₁₂ is correlated with the content of the vitamin and the counts of bacteria producing the vitamin.

     

Abyssal current along the northern periphery of Shikoku Basin

Abyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:

1. Mean currents with a magnitude of 5–10 cm sec^?1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.
2. At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.
3. At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed.

These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.     

New record of occurrence ofSergia lncens (Hansen) (crustacea,sergestidae) off Tung-kang,Taiwan, with special reference to phylogeny and distribution of the species

The commercially important sergestid shrimp,Sergia lucens (Hansen), has long been considered an endemic species of Japan because it had been found only in Suruga Bay and neighbouring waters. Recently, however, a considerable amount of a similar shrimp was caught by trawl nets off Tung-kang, southwestern Taiwan. This shrimp is distributed at depths of 100–300 m on the continental slope, around a deep submarine canyon adjacent to the mouth of the Kao-ping River. A morphological comparison with specimens from Suruga Bay reveals that the shrimp is identical toS. lucens. However, a slight difference in the patterns of water-soluble proteins was observed in a thin layer isoelectrophoretic examination. A difference was also observed in the spawning season of the two populations, indicating sexual and geographic isolation. Considering the distribution and phylogeny of the family Sergestidae, based on a possible evolutionary development from a benthic neritic organism to a pelagic oceanic one along the generaSicyonella-Sergestes-Sergia, it is assumed thatS. lucens entered a lower epipelagic habitat in the coastal waters from the warm oceanic mesopelagic habitat of the original stock. A hypothesis is proposed that speciation ofS. lucens from the original stock occurred when it was trapped in a semi-enclosed inlet (the paleo-East China Sea Gulf) that existed at the present Okinawa Trough during the late Pliocene to early Pleistocene. The inlet was deep, but had a neritic environment due to drainage from ancient large river systems, including the paleo-Yangtze River. The species expanded its distribution to the neighbouring waters during the warm interglacial period. However, a rapid rise in sea level after 14,000 years significantly changed the environmental conditions in the distributional area and the species could not expand into a neritic environment, which was too shallow for survival. Accordingly,S. lucens populations remained only in Suruga Bay and Tung-kang waters, where the environment has remained stable for the last 17,000 years or more. The two areas have the following common characteristics:

1. A large amount of fresh water is discharged into the deep submarine canyon adjacent to the river mouth.
2. The northeastern part is surrounded by land so that a direct inflow of boreal water into the environment is blocked.
3. Warm Kuroshio extension water intrudes and mixes with water from the rivers, and for Suruga Bay there is a current system that keeps distribution of the eggs and larvae restricted to the local habitats.

Taiwan annually exported some tens of metric tons of the driedS. lucens to Japan for the last few years, but the standing stock of Tung-kang waters is probably not as large as that in Suruga Bay.     

Blooms of <Emphasis Type="Italic">Trichodesmium erythraeum</Emphasis> in the South Eastern Arabian Sea during the onset of 2009 summer monsoon

This study presents in situ evidence for the blooms of Trichodesmium erythraeum observed in the shelf waters of the South Eastern Arabian Sea (SEAS) during the onset of the southwest monsoon in June 2009. Evidence showed that water surface discoloration was caused by the accumulation of T. erythraeum, and that the water column contained a colony of T. thiebautii. The surface water color in the bloom region varied from pale brown to pinkish red. Pale brown indicated healthy algae at the peak of its photosynthetic activity, while pinkish red indicated the presence of photosynthetically less active filaments. Zooplankton abundance, especially copepodites, in the bloom area substantiated the theory that Trichodesmium filaments are excellent epiphytes to which the copepodites cling. The bloom area was very fertile with copious quantities of dissolved oxygen (6.85 ml L^?1), PO₄-P (0.108 μmol L^?1) and SiO₄ (1.29 μmol L^?1). Lower NO₃-N (0.028 μmol L^?1) values in the bloom area did not appear to affect Trichodesmium growth from molecular nitrogen fixation. However, lower NO₃-N values altered the normal phytoplankton composition of this area.     

Distribution,behavior and fate of PCBs in the marine environment

I am extremely grateful and honored for being awarded the Okada Prize (1985) for my study. The present article reviews my research on the distribution, behavior and fate of PCBs in the marine environment. The outline of this study is summarized as follows:

1. Polychlorinated biphenyls (PCBs) were detectable in the wide range of environmental media and biota of the Seto-Inland Sea, Japan, in which much high concentrations were found in sediment and biological samples due to their hydrophobic, lipophilic and less biodegradable properties as well as their extensive production and use in estuarine and coastal regions.
2. PCBs extend the boundaries of their distribution all over the global environment, being evidenced by their occurrence in open ocean atmosphere, hydrosphere and biosphere, even in Antarctica. The global contamination of PCBs is much more prominent in northern hemisphere than in southern hemisphere.
3. The sinking rate of PCBs from surface to deeper layers in open ocean water column is relatively slower in tropical waters than in high latitude ones. This implies the possible prolonged contamination of persistent synthetic organic chemicals in the tropical marine environment.
4. The bioaccumulation processes in marine ecosystems can be explained by the physicochemical and biochemical properties of PCBs and the metabolic capacity of organisms. In higher animals, additional factors such as parturition and lactation are also related to this process.
5. Total PCB load in global environment was estimated to be about 370 thousand tons. Of this, most amounts were in coastal sediment and open ocean water. Presently, about 780 thousand tons of PCBs are still in use mainly in electrical equipments. In order to reduce the PCB levels in marine environment, pertinent measures to prevent the further discharge and safe disposal of PCBs are required.

     

On the mean sea levels at various locations along the coasts of Japan

The yearly variations of mean sea levels at various locations along the coasts of Japan have been investigated, based upon the data of observation taken at 55 tide-gauge stations from 1953 to 1970, and some discussions have been made on the relationships between those variations and meteorological and oceanographical conditions, etc. The results obtained are as follows:

1. In cases covering 90% of all the tide-gauge stations, the magnitude of the annual mean rate of variation of ground level is less than 10 mm/year, and the mean value of the magnitude for all the stations is ?3.17 mm/year, while that exclusive of Osaka is ?1.7 mm/year.
2. The effect of atmospheric pressure variation on the height of annual mean sea level is of the order of several centimeters in view from both time and place.
3. In view of the characteristic types of variations, the coasts of Japan may be divided into five regions of similar mean sea level deviations. And in the yearly variations of mean sea levels, there can be seen a kind of variation which corresponds to the variation of oceanographical conditions such as abnormal fall of seawater temperature.
4. The mean sea level deviations at various locations along the coasts of Japan, referred to the standard sea level “T.P.” are different for different places. Namely, (1) on the coast of the Japan Sea, the west coast of Kyushu, the coast of Sanriku and the coast of Hokkaido, the mean sea level is higher than on the Pacific coasts from Southern Kyushu to Southern Honshu. (2) Along the coast of the Seto-Inland Sea, the mean sea level is generally higher.

As for the leading causes of the above deviations, we may safely enumerate the following ones, viz. (a) the effect of the deflecting force of the earth's rotation on currents, and (b) the effect of variation of seawater density. However, as regards the relative importance of these two effects, no decisive conclusion can yet be given for the present.     

Chemical environment for red tides due toChattonella antiqua in the seto inland sea,Japan

Severe red tides due toChattonella antiqua occur sporadically during summer in the Seto Inland Sea, Japan, and cause significant damage to the fishing industry. In order to assess the chemical environment with respect to the outbreak ofC. antiqua, environmental factors that affect the growth ofC. antiqua were monitored around the Ie-shima Islands, the Seto Inland Sea, in the summer of 1986. In addition, a growth bioassay of the seawater usingC. antiqua was conducted under a semicontinuous culture system. Although temperature, salinity and light intensity were optimum for the growth ofC. antiqua, red tides by this species did not occur. Concentrations of NH ₄ ⁺ , NO ₃ ^? and PO ₄ ^3? were low (<0.4, <0.2, <0.06 µM, respectively) above the thermocline (8–12 m) and high below it (0.6–2, 4–8, 0.4–0.8 µM, respectively). Vitamin B₁₂ concentrations did not change significantly between the surface (0 m) and below the thermocline (25 m) in the level of 2–4 ng·l^?1. The growth bioassay revealed that in the surface waters, concentrations of N- as well as P- nutrients were too low to support a rapid growth ofC. antiqua. At the depth of 25 m, neither N, P nor B₁₂ limited the growth rate. In order to obtain more quantitative information on the growth rate as a function of the concentrations of N- and P- nutrients,C. antiqua was grown in a semicontinuous culture system by changing nutrient concentrations systematically. The observed growth rate (μ) can be approximated as follows: $$\mu = \mu _{\max } .\frac{{S_N }}{{K_g ^N + S_N }}.\frac{{S_{PO4} }}{{K_g ^P + S_{PO4} }},$$ whereS _N is the concentration of NO ₃ ^? plus NH ₄ ⁺ (0–6 µM),S _PO, the concentration of PO ₄ ^3? (0–0.6 µM), μ_max (0.97 d^?1) the maximal growth rate,K ₀ ^N (1.0 µM) andK ₀ ^P (0.11 µM) the half saturation constants for NO ₃ ^? and PO ₄ ^3? , respectively. Using the above equation with nutrient concentrations measured, the rate at which seawater supports the growth ofC. antiqua can be estimated and this can be used for the assessment of chemical environments with respect to the outbreak ofC. antiqua.     

Microbiological and Chemical Studies in the Seas of Hiuchi and Bingo—II

The distribution of inorganic nitrogen compounds and the metabolic rates of these compounds by microorganisms as a whole were investigated in the Seas of Hiuchi and Bingo. The results obtained are as follows:

1. Of inorganic nitrogen compounds, the contents in sea water, those in bottom muds, the uptake or liberation rates of microorganisms as a whole in sea water, and the liberation rates from bottom muds to sea water are 0.2～4.0 μg at. N/l, 3～60 μg at.N/100 g, 0.01～0.5 μg at.N//lhr, and 0.3～1.9 μg at.N/100 cm²/hr, respectively, and these contents or rates of ammonia usually are the largest of these inorganic nitrogen compounds.
2. From the above-mentioned results and the others, it is suggested that the nitrogen in the seas circulates mainly in sea water itself and the course of nitrogen cycle, which passes through bottom muds, is not so important, and further that, of the cycle of inorganic nitrogen compounds, the main course is the course which ammonia is liberated from organic nitrogen compounds and it is immediately uptaked by microorganisms, and the course which it is oxidized to nitrate and the others are not so important.

     

Influence of bottom topography on an upwelling current: Generation of long trapped filaments

We investigate the influence of bottom topography on the formation and trapping of long upwelling filaments using a 2-layer shallow water model on the f-plane. A wind forced along-shore current, associated with coastal upwelling along a vertical wall, encounters a promontory of finite width and length, perpendicular to the coast.In the lower layer, topographic eddies form, which are shown to drive the formation of a filament on the front. Indeed, as the upwelling current and front develop along the coast, the along shore flow crosses the promontory, re-arranging the potential vorticity structure and generating intense vortical structures: water columns with high potential vorticity initially localized upon the promontory are advected into the deep ocean, forming cyclonic eddies, while water columns from the deep ocean with low potential vorticity climb on the topography forming a trapped anticyclonic circulation. These topographic eddies interact with the upper layer upwelling front and form an elongated, trapped and narrow filament.Sensitivity tests are then carried out and it is shown that:

• •baroclinic instability of the front does not play a major role on the formation of long trapped filaments;
• •increasing the duration of the wind forcing increases the upwelling current and limits the offshore growth of the filament;
• •modifying the promontory characteristics (width, length, height and slopes) has strong impact on the filament evolution, sometimes leading to a multipolarisation of the potential vorticity anomaly structure which results in much more complicated patterns in the upper layer (numerous shorter and less coherent filaments). This shows that only specific promontory shapes can lead to the formation of well defined filaments;
• •adding bottom friction introduces a slight generation of potential vorticity in the bottom layer over the promontory, but does not significantly alter significantly the formation of the filament along the outcropped front in the present configuration;
• •modifying the stratification characteristics, in particular the density jump between the layers, has only a weak influence on the dynamics of topographic eddies and on filament formation;
• •the influence of capes is also modest in our simulations, showing that topography plays the major role in the formation of long and trapped upwelling filaments.

     

Structure of plankton phytocoenoses in the shelf waters of the northeastern Black Sea during the <Emphasis Type="Italic">Emiliania huxleyi</Emphasis> bloom in 2002–2005

It is shown that, in 2002–2005, the mass development of the coccolithofore Emiliania huxleyi on the Gelendzhik shelf occurred annually and in May–June its abundance reached 1.5 × 10⁶ cells/l. In 2004–2005, the bloom of E. huxleyi was accompanied by a mass development of the diatom alga Chaetoceros subtilis var. abnormis f. simplex (0.6–0.9 × 10⁶ cells/l); for the first time, it was registered as a dominating form of the Black Sea phytoplankton. Small flagellates and picoplankton algae played a noticeable role in the phytoplankton throughout the entire period of the studies. Meanwhile, in the early summer period, the bulk of the biomass consisted of coccolithophores (50–60%), while, in the late summer period, diatomaceous algae dominated (50–70%). Among the ecological factors that favor the coccolithophore development one may note the microstratification of the upper mixed layer at a high illumination level and high temperature in the surface waters (18–21°C). The terrigenous runoff during the rainy period had a negative effect on the E. huxleyi development, while storms dispersed the population over the upper mixed layer. The wind-induced near-shore upwelling stimulated the development of diatoms.     

Dimethylsulfide in coastal zone of the East China Sea

Dimethylsulfide (DMS) in seawater were observed four times from February 1993 to August 1994 along a fixed section (PN line) in the East China Sea. The DMS concentrations showed remarkable temporal and spatial variations. The DMS concentrations were generally higher in the upper euphotic layer of the continental shelf zone in summer. The spatial variation, however, was more pronounced even in well mixed winter water, where the concentration of DMS varied widely from 3 to 106 ng-S/l in the continental shelf zone while the salinity was vertically almost uniform. This means that DMS in seawater is rapidly produced and decomposed with a time scale less than one month in the water column. The largest value of 376 ng-S/l was obtained at 5 m depth near the mouth of Changjiang River in August 1994. The mean concentrations in the surface 30 m layer in the continental shelf zone were 21, 54, 126 and 57 ng-S/l in February, October, June and August, respectively, which were about twice as large as those in the Kuroshio region. The mean fluxes of DMS from the East China Sea to the atmosphere are estimated to be 49 g-S/m²/day in winter and 194 g-S/m²/day in summer in the continental shelf zone, and to be 32 and 107 g-S/m²/day in the Kuroshio region.     

Circulation of the East China Sea,a numerical study

A three-dimensional, primitive-equation model is developed to study how the Kuroshio, the monsoon, the Yangtze River outflow and the buoyancy forcing from the South China Sea affect the circulation of the East China Sea. It is found that the Kuroshio water usually intrudes into the East China Sea from both sides of Taiwan Island. Winter winds enhance the Kuroshio intrusion from northeast of Taiwan, but weaken it from the Taiwan Strait. Summer winds act in the opposite way. The increased presence of the Kuroshio water in the East China Sea in winter can be largely attributed to the shoreward surface Ekman drift associated with the northerly wind. In summer, the-shaped plume emanating from the Taiwan Strait is, to a large extent, produced by the buoyancy forcing from the South China Sea.In summer, the bimodal distribution of the Yangtze River outflow is initially produced by the upwelling-favorable wind. Away from the Yangtze River, the far-field dispersal of the fresher water depends on the strength of the Kuroshio. A stronger Kuroshio enhances the seaward dispersal of the northern branch of the Yangtze outflow north of Taiwan, but reduces the southward penetration of the southern branch. In winter, downwelling-favorable winds confine the Yangtze River outflow to a narrow band forming nearshore coastal jet penetrating southward. The northern tip of Taiwan acts as a conduit, channeling the seaward dispersal of the fresher water. The model results interpret the observed circulation patterns.     

Recent advances in ocean-circulation research on the Yellow Sea and East China Sea shelves

Recent advances in ocean-circulation research on the Yellow Sea and East China Sea shelves are summarized. Observations using acoustic Doppler current profilers (ADCPs) suggest that the connectivity of mean-volume-transports is incomplete between the Tsushima (2.6 Sverdrups; 1 Sv = 10⁶ m³/s) and Taiwan Straits (1.2 Sv). The remaining 1.4-Sv transport must be supplied by onshore Kuroshio intrusion across the East China Sea shelf break. The Yellow Sea Warm Current is not a persistent ocean current, but an episodic event forced by northerly winter monsoon winds. Nevertheless, the Cheju Warm Current is detected clearly regardless of season. In addition, the throughflow in the Taiwan Strait may be episodic in winter when northeasterly winds prevail. The throughflow strengthens (vanishes) under moderate (severe) northeasterly wind conditions. Using all published ADCP-derived estimates, the throughflow transport (V) in the Taiwan Strait is approximated as

Application of chemical tracers to an estimate of benthic denitrification in the Okhotsk Sea

To estimate benthic denitrification in a marginal sea, we assessed the usefulness of \({\text{N}}_{2}^{*}\) , a new tracer to measure the excess nitrogen gas (N₂) using dissolved N₂ and argon (Ar) with N* in the intermediate layer (26.6–27.4σ _θ ) of the Okhotsk Sea. The examined parameters capable of affecting \({\text{N}}_{2}^{*}\) are denitrification, air injection and rapid cooling. We investigated the relative proportions of these effects on \({\text{N}}_{2}^{*}\) using multiple linear regression analysis. The best model included two examined parameters of denitrification and air injection based on the Akaike information criterion as a measure of the model fit to data. More than 80 % of \({\text{N}}_{2}^{*}\) was derived from the denitrification, followed by air injection. Denitrification over the Okhotsk Sea shelf region was estimated to be 5.6 ± 2.4 μmol kg^?1. The distribution of \({\text{N}}_{2}^{*}\) was correlated with potential temperature (θ) between 26.6 and 27.4σ _θ (r = ?0.55). Therefore, we concluded that \({\text{N}}_{2}^{*}\) and N* can act complementarily as a quasi-conservative tracer of benthic denitrification in the Okhotsk Sea. Our findings suggest that \({\text{N}}_{2}^{*}\) in combination with N* is a useful chemical tracer to estimate benthic denitrification in a marginal sea.