Short-term variation in copepod community and physical environment in the waters adjacent to the Kuroshio Current

A continuous survey examined short-term variations in the zooplankton community and physical ocean environment from the northeastern Izu Islands to Boso Peninsula in Japan. High copepod abundance and small upwellings in the surface layer and salinity minimum layer in the subsurface were observed on the north side of coastal fronts in the westernmost transect, moving southward as the Kuroshio Current left the Boso Peninsula. Thus, the salinity minimum layer might be a key factor forming upwelling and the fronts, leading to large abundance of coastal copepods off the northeastern Izu Islands. A community structure analysis of calanoid copepods revealed an intermediate belt assemblage between coastal and offshore (Kuroshio) assemblages. Copepod abundance was remarkably low and Ctenocalanus vanus dominated (nearly 37%) in the intermediate belt zone, indicating that C. vanus has a relatively high tolerance to adverse environments for calanoid copepods. As the Kuroshio Current left the Boso Peninsula, the coastal assemblage expanded in the same direction, and the intermediate belt assemblage off the northeastern Izu Islands disappeared. The largest population of Calanus sinicus was found along the two western transects off the northeastern Izu Islands (>1000 m depth), which was assumed to be transported from Sagami Bay and advanced southwestward while growing from copepodite stages CIII to CV. Larvae of C. sinicus would be an important food for fish larvae in addition to Paracalanus parvus s.l., the numerically dominant species in the coastal assemblage, and C. vanus under the adverse conditions for coastal copepods.     

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.

     

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.     

Trichodesmium in the East China Sea

A study is made on the surface distribution ofTrichodesmium thiebautii Gomont in the East China Sea during the years 1961–1967.

1. This alga is very scarce or absent in the period from January to March, and begins to increase in May and June, reaching the maximum population density from July to September, and again decreases in October and November.
2. In summerT. thiebautii is predominantly distributed in the area along the Kuroshio current with high densities of 10²–10³ filaments/l, occasionally exceeding 10³ filaments/l. This alga is estimated to occupy the major part or almost all the parts of chlorophylla in situ contained in the surface water of this season.
3. The occurrence ofT. thiebautii off the southern coasts of Central Japan seems to be caused rather by the multiplication in its own habitat than by the transportation by the Kuroshio current from the East China Sea.

     

Observations of upwelling around the Izu Peninsula,Japan: May 1982

Hydrographic observations between the Izu Peninsula and Oshima Island, Japan, in May 1982, showed upwelling around the tip of the Peninsula and possibly also in the lee of Oshima Island. The upwelling introduced water as cold as 18 C with nitrate concentrations of 3M to the surface. Temperature/salinity analyses indicated that the upwelled water was of Kuroshio characteristics. Slightly fresher water was advected out of Sagami Bay in a coastal counter current.Contribution number 470 from the Shimoda Marine Research Center, University of Tsukuba.     

Sessile animals on tar globules in the waters around the Ryukyu Islands

A large number of tar globules with sessile organisms were collected from the surface tows taken with larval nets in the waters around the Ryukyu Islands during November and December, 1973. Bryozoans (one species), tubeworms (Serpulidae,Janua (Dexiospira) foraminosa (Moore &Bush)) and goose barnacles (Lepas pectinata Spengler,L. anatifera Linné) were the most important species of sessile animals found on these tar globules. Sinking of tar globules byLepas of middle or large size was suggested from the differences in their specific gravity.     

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.     

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.     

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.

     

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.

     

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.     

Topographic Effect of Izu Ridge on the Distribution of the North Pacific Intermediate Water South of Japan

The topographic effect of the Izu Ridge on the horizontal distribution of the North Pacific Intermediate Water (NPIW) south of Japan has been studied using observational data obtained by the Seisui-Maru of Mie University (Mie Univ. data) and those compiled by Japan Oceanographic Data Center (JODC data). Both data sets show that water of salinity less than 34.1 psu on potential density () surface of 26.8 is confined to the eastern side of the Izu Ridge, while water of salinity less than 34.2 psu is confined to the southern area over the Izu Ridge at a depth greater than 2000 m and to the southeastern area in the Shikoku Basin. It is also shown by T-S analysis of Mie Univ. data over the Izu Ridge that water of salinity less than 34.2 psu dominates south of 30°N, where the depth of the Izu Ridge is deeper than 2000 m and NPIW can intrude westward over the Izu Ridge. JODC data reveal that relatively large standard deviations of the salinity on surface of 26.7, 26.8 and 26.9 are detected along the mean current path of the Kuroshio and the Kuroshio Extension. Almost all of the standard deviations are less than 0.05 psu in other area with the NPIW, which shows that the time variation in the salinity can be neglected. This observational evidence shows that the topographic effect of the Izu Ridge on the horizontal distribution of the NPIW, which is formed east of 145°E by the mixing of the Kuroshio water and the Oyashio water, is prominent north of 30°N with a depth shallower than 2000 m.     

Sea level variations at the Izu Islands and typical stable paths of the Kuroshio

Differences in daily mean sea level between Kushimoto and Uragami and daily mean sea levels at Miyake-jima and HachijÔ-jima in the Izu Islands are examined during the 1964–1980 period, and characteristics of the typical paths of the Kuroshio corresponding to the dominant sea level states are described.Sea levels at the islands show three dominant states: high and low sea level states (45 % and 31 %) in the non-large-meander period (October 1963 –7 August 1975) and high sea level state (64 %) in the large-meander period (8 August 1975–15 March 1980). This indicates the existence of three typical paths of the Kuroshio, and the states correspond to the nearshore and offshore non-large-meander paths and the typical large-meander path, respectively. The first path is located near the coast throughout the whole southern area off Japan, the second path leaves the coast around the Izu Ridge and passes south of HachijÔ-jima, and the third path is located near the coast over the ridge after meandering far to the south of Enshû-nada.The positions of the three typical paths are almost the same in the farthest upstream and downstream regions south of Japan between 131E and 142E. The nearshore and offshore non-large-meander paths overlap between Kyûshû and the Kii Peninsula, being invariably close to the coast, while the typical large-meander path south of Shikoku is located offshore and changes its position meridionally.At the mid-depth of 400 m the nearshore non-large-meander and typical large-meander paths pass the Izu Ridge through the deep channel between Miyake-jima and HachijÔ-jima, while the offshore non-large-meander path passes through the deep region south of HachijÔjima. The path of the Kuroshio at mid-depth is well constrained by the bottom topography of the Izu Ridge.     

The FAO guidelines for the development and use of indicators for sustainable development of marine capture fisheries and an Australian example of their application

Guidelines for developing and implementing sustainability indicators for marine capture fisheries were drafted during a recent expert consultation jointly organised by Australia and FAO in Sydney (Australia). This paper provides an overview of the guidelines and illustrates their application with an Australian example. As a background to the guidelines, the concept of sustainable development (SD) for marine capture fisheries was discussed and an agreed set of definitions and usage of common terms developed. The guidelines then outline the five sequential steps that need to be addressed in developing a meaningful set of indicators in the context of a Sustainable Development Reference System (SDRS). The five steps are:

• 1.specifying the scope of the SDRS;
• 2.developing a framework to agree on components within the system;
• 3.specifying criteria, objectives, potential indicators and reference values;
• 4.choosing the set of indicators and reference values;
• 5.specifying the method of aggregation and visualisation.

These steps are further elaborated in terms of how to scope the SDRS, define the dimensions and hierarchical levels to be included in the system, set multiple objectives and link these to indicators and reference values (e.g. targets, thresholds and/or standards). The guidelines also provide some examples of possible indicators, criteria for selecting some indicators over others and present ways of aggregating and visualising the indicators so that progress towards achieving sustainable development can be communicated easily to decision-makers.     

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.     

A submersible study in the western Blanco fracture Zone,N.E. Pacific: Structure and evolution during the last 1.6 Ma

During July and August 1991, the French-American Blanconaute dive program used the French submersibleNautile to investigate the West Blanco Depression (WBD), a deep, elongate trough located at the intersection of the Blanco Transform Fault Zone with the southern Juan de Fuca Ridge (JdFR). Twenty dives were carried out along the north wall of the WBD, which exposes the upper oceanic crust over a 65 km distance, from the JdFR axis (to the west) to the oblique trace of an ancient propagator (to the east, crustal age around 2 Ma). Thirteen of these dives were precisely located within a 13 × 7 km zone of the north wall, covered by a high-resolution sonar mapping operation during the Blancotrough cruise in 1987. This series of geological traverses, plus 4 dives across the south wall of the WBD (one dive) and the adjacent Parks Plateau (3 dives), collected 242 rock samples. We report here the main results of the dive program and preliminary laboratory studies:

1. Transform-related tectonic activity has recently abandoned the southern margin of Parks Plateau, and is presently located inside the WBD area, mainly along its northern wall. The tectonic features observed are compatible with a right-lateral strike-slip system, with a NE-SW extensional component.
2. Three main lithological units are exposed along the north wall of the WBD. From top to bottom, they are: (1) a Volcanic Unit, forming a steep upper cliff, made of massive and pillow flows and basaltic dikes, with an estimated average thickness of 800 m; (2) a less steep Transition Zone, about 150 to 400 m thick, largely masked by rubble but exposing both diabase outcrops and pillow flows; and (3) a massive Diabase Unit, exposed over 700–800 m, with a dike complex structure visible from place to place, and cut by a net of hydrothermal veins. Deep crustal rocks such as gabbros were not observed.
3. Spectacular mass-wasting features are visible all along the north wall of the WBD. About 60% of the face of the wall is masked by talus cones, rubble, rock avalanche deposits and slide blocks. Three main landslides, of approximately one km³ in volume each, were tentatively identified. One of them was mapped in detail and consists of an approximately 300 m thick (0.85 km³), coherent slide block detached from a zone where intense hydrothermal alteration and faulting have obviously weakened the bedrock, that is in places entirely altered to blue clays.
4. The basaltic lavas of the WBD north wall show a remarkable evolution with time, from east to west. Around the tip of the ancient propagator, they are restricted to primitive, olivine-rich picritic basalts. Proceeding westward, they exhibit a wide range of differentiation, including highly fractionated, FeTi-rich ferrobasalts at about 35–45 km from the JdFR axis. When approaching the JdFR axis, the FeTi enrichment decreases gradually, and the ferrobasalts evolve towards slightly differentiated MORB-type basalts, typical of the southern JdFR. This magmatic evolution marks the transition from the end of a propagating rift regime to a steady-state accretion regime.
5. The WBD north wall also permits the study of weathering and hydrothermal alteration processes and their evolution in space and time. Vertically, the alteration products evolve from oceanic weathering and zeolite facies (Volcanic Unit) to the greenschist facies (Transition Zone and Diabase Unit). Horizontally, the evolution with time is mainly a general hydration of the crust that is, however, very irregularly distributed.
6. Several vertical magnetic traverses along the north wall of the WBD, using a bottom magnetometer attached to the basket of the submersible, have shown a sharp 5000 to 7000 nT positive anomaly at about 3500 m depth. This anomaly corresponds exactly to the first appearance of extrusive pillow-lava outcrops, and confirms the dramatic decrease in magnetic anomaly amplitude below that depth, detected during the Blancotrough cruise in 1987. The vertical magnetic profiles thus appear to have imaged the base of the magnetic source layer.

     

Distribution des sédiments sur la marge du golfe de Tehuantepec (Pacifique Oriental). Exemple d'interaction tectonique-eustatisme

During the PROGEMA I cruise which took place on the south-western Pacific coast of Mexico (between 14° and 16° N, 92° and 96° W), 3.5 kHz echosounding profiles, surface samples and cores as well as hydrological and oxygen minimum measurements down to 2250 m water depth were made in the Gulf of Tehuantepec. Chemical, sedimentological, mineralogical, X-ray and BEM analyses of waters, core and surface samples provided new data which permit a better knowledge of the Gulf of Tehuantepec margin, where tectonic activity is due to an active subduction.On the shelf of the gulf, the distribution pattern of quaternary deposits is affected by a double tectonic forcing and by eustatic oscillations of the sea level. The first tectonic control is linked to the existence of north-south tectonic troughs in the Tehuantepec Isthmus, in which the atmospheric circulation between the Gulfs of Mexico and Tehuantepec is accelerated by the Venturi effect. The resulting winds, called “Tehuanos”, play an important role in the forcing of surface circulation in the Gulf of Tehuantepec, inducing a deviation of the general circulation and thus forcing the distribution pattern of terrigenous sediment supply according to two physiographically and mineralogically distinct provinces, inherited from the past tectonic evolution, which are separated by the Tehuantepec axis.

• •-|A southeastern province related to the Caribbean Plate and fed by: (1) the rivers draining the Chiapas Massif; and (2) the Costa-Rica Current driving tropical and subtropical waters from Central America.
• •-|A northwestern province part of the North American Plate and fed by: (1) the Rio Tehuantepec which drains the Sierra de Oaxaca; and (2) the Californian Current.

The second tectonic control is linked to the tectonic structure of the margin influenced by:

• •-|shallow banks where the products of a present-day phosphatogenesis are concentrated because they are swept by upwellings. The latter are reinforced because the banks are situated south of the wind corridor;
• •-|tectonic depressions acting as sediment traps;
• •-|numerous canyons cutting the slope along brittle accidents, driving weak turbidity currents but which play an important role in the forcing of upwelling currents. Antarctic diatoms are found which are conveyed by the deep circulation from the South-Eastern Pacific and provide evidence that Antarctic Cold Water masses (AABW) rise towards the shelf.

The influence of eustatism is attested by the presence on the shelf of the Gulf of Tehuantepec of three paleoshorelines at 40 , 75 and 130 m water depth, linked to the Würmian sea level and to the postglacial sea level rise as on many passive margins.     

Interactions between oceanic and gravity tides,as analysed from world-wide earth tide observations and ocean models

The problem of interactions between earth tides and oceanic tides is rather complex as it involves effects of newtonian attraction, loading and associated change of earth potential, tangential pressure and friction on the moving ocean floor which are not always easy to evaluate, principally for coastal or island stations. This paper takes advantage of two facts:

1. By the end of 1983 the International Center of Earth Tides has collected and evaluated a considerable amount of data from 223 stations including those of the Trans World Profiles developed by the same group of authors (102 stations). This ensures, for the first time, a World wide distribution including the tropical areas and the southern hemisphere.
2. In 1978–80, new oceanic cotidal maps of high quality, established by E.W. Schwiderski, became available.

We have calculated, for the eight principal tidal waves, the correlations between the observed gravity variations and those resulting from a calculation based upon the Schwiderski maps. This correlation is highly significant. At the level of accuracy of the best transportable gravimeters the agreement is perfect except at a few places where effects of lateral heterogeneities in the mantle can perhaps be suspected. These cotidal maps can therefore be safely used as working standards for other geodetic and geophysical applications.     

Hydrographic structure in association with deep boundary current in the north of the Shikoku Basin

High quality CTD data were collected in the north of the Shikoku Basin where an abyssal boundary current has been observed through direct current measurements. Analyses of hydrographic data showed:

1. Colder and saltier water (heavier water) compared to surrounding waters is found above the continental shelf-toe and the eastern flank of the Kyushu-Palau Ridge where the existence of the abyssal boundary current has been expected. The heavier water has a horizontal extent of about 50 km.
2. The heavier water has the vertical scale of 2000 m from the sea bottom, and is associated with a thermal wind shear which enhances a component of the flow toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left in the abyss. The assumed “level of no motion” at about 2500 m depth gives the geostrophically estimated current in a good agreement with the directly measured current.

A volume transport associated with the colder and higher salinity water is estimated to be about 2 Sv off Cape Shiono-misaki which may include a recirculation above the Nankai Trough. This is about twice of the transport estimated in the interior of the Shikoku Basin through a vorticity balance between the stretching term and latitudinal variation of the planetary vorticity.     

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.