Further studies on the feeding habits ofSagitta nagae alvariño in Suruga Bay,central Japan

The feeding ofSagitta nagae was analyzed in connection with food chains in the pelagic and near-bottom communities of Suruga Bay. The following results are obtained: (1)S. nagae feeds on dominant copepod species both in the free water (upper water layers) and the near-bottom habitats; (2)S. nagae did not show any food-size selection in the range of 0.2 to 1.2 mm in cephalothorax width and height of copepods; (3) Feeding activity ofS. nagae in the free water is highest at night, while in the near-bottom habitat the species feeds more actively during the day than at night; and (4)S. nagae migrates between the bottom and free water and plays an important role in shallow water by connecting the near-bottom community with the pelagic community.     

Temporal variability of near-bottom particle resuspension and dynamics at the Porcupine Abyssal Plain, Northeast Atlantic

In order to study particle behaviour and its time-variability in the near-bottom layer on the Porcupine Abyssal Plain (48°50′N, 16°30′W, 4850 m), long-term measurements were made of currents, and nephelometry and particle samples were collected using an autonomous lander between mid-1996 and mid-1998. Water samples, collected in the Bottom Nepheloid Layer within 1000 m of the bottom, were filtered for suspended particles whose contents of organic carbon, nitrogen and pigments were determined. This study was co-ordinated with a water column flux study and a detailed programme of benthic studies to understand how the abyssal boundary layer responds to and modifies inputs of organic matter from the water column (MAST3/BENGAL programme).There were strong seasonal fluctuations in the near-bottom (2 m above the bottom, mab) particle flux, whose variation were correlated in time with the water column fluxes. During the periods of peak flux, the near-bottom flux was sometimes higher than that recorded higher up in the water column, but not always at other times. These excesses were attributed to the resuspension events, since we observed a correlation between current speed and nephelometry. However, in summer the peak in the particle resuspension flux could not be explained by the variations in the tidal amplitude. Instead we attribute it to the large quantities of fresh large particles (aggregations) that had just arrived on the bottom; it was probably linked to the feeding activity and sediment reworking by the rich and varied benthic and benthopelagic megafauna.In both 1997 and 1998, the nephelometry signal (directly related to fine particle concentration) and its variability increased after the peaks in large particle flux with a time-lag of 2–3 months. We assume that this time lag corresponds to the time it takes for the large fresh particles, once they have settled on the bottom, to be disaggregated into smaller particles, and hence become subject to resuspension in the quiet current conditions then prevailing in the BENGAL area. The suspended particle analyses confirm the vertical structure of the Bottom Nepheloid Layer, the lower part of which corresponds to the Bottom Mixed Layer (BML) where resuspension and mixing are higher.     

The diffusive fluxes of inorganic nitrogen across the intertidal sediment-water interface of the Changjiang Estuary in China

1IntroductionEstuaries are the main channels linking bothland and coastal seas,through which pass matterleached from land into rivers,together with anthro-pogenic matter discharged directly into the estuaries.During the past few decades,increasing concern…     

波浪作用下细颗粒泥沙悬移特性的试验研究

通过9组室内水槽试验，系统分析了不同周期和波高波浪作用下粉砂质细砂的悬移特性。试验结果表明，在波浪作用下，细颗粒泥沙在水体中悬浮产生明显的三层混浊结构：近底高浓度混浊层、中部混浊层和上部低浓度混浊层。其混浊度的大小与波浪动力条件相关，波浪周期的延长和波高的增加都会使近底层悬沙浓度显著增大，悬沙浓度的波动性在近底层表现得最为突出和复杂；波浪动力条件的改变对上层水体的悬沙浓度基本不产生影响。当波浪作用停止，悬沙开始静水沉降时，近底层水体含沙浓度的变化与波浪要素的比值相关，当波浪要素的比值大于临界值时，底层含沙浓度在一定时间段内反而增加，出现浓度返起现象。     

Foraminifer communities of the atlantic continental margin of Europe

Based on an analysis of the structures of the populations of benthic foraminifers at 639 stations located on the Atlantic continental margin of Europe, 33 communities were distinguished and mapped with respect to the abundance of the dominating species. The habitats of the communities are formed under the influence of environmental factors, which depend on the latitudinal and bathymetric zonations of the basin, the currents, the near-bottom upwellings, coastal water runoff, and topography of the boundaries between water masses. The communities recognized are confined to certain water masses, which helps one to trace the boundaries between the water masses and the regions of near-bottom upwellings.     

The carbonate system of Amur Bay (sea of Japan) under conditions of hypoxia

In the course of the expedition of the Pacific Oceanological Institute in August 2007, extensive hypoxia was found in the near-bottom layer of the Amur Bay water mass. The hypoxia’s formation was immediately reflected in the values and distribution of the carbonate parameters in the near-bottom waters of the bay. The maximum values of the carbon dioxide partial pressure, the dissolved inorganic carbon, and the total alkalinity were associated with the areas of the minimum oxygen content. The microbial destruction of the dead phytoplankton greatly increased the partial pressure of carbon dioxide, which was over 2000 μatm in the hypoxia centers at a depth of about 20 m. At the same time, the carbon dioxide partial pressure in the surface waters of Amur Bay was considerably lower than that in the atmosphere. Hence, the bay’s aquatic area was a sink for the atmospheric CO₂ despite its high content in the near-bottom waters. It was shown that the excess alkalinity associated with the hypoxia sites in the near-bottom layer of water was caused by the sulfate reduction proceeding in the upper layer of the sediment.     

Diel Vertical Migration of the Marine Cladoceran Podon Leuckarti: Variations with Reproductive Stage

Diel vertical migration (DVM) of the marine cladoceran Podon leuckarti was investigated with reference to their reproductive stages and size. Males and females, except for gamogenic females with an advanced resting egg (GA), aggregated at the near-bottom layer or at the surface during the day and dispersed into the water column at night. Both the near-bottom aggregation and the surface aggregation during daytime are suggested to be behavior that reduces the predation risk from visual predators. However, GA aggregated in the near-bottom layer during daytime and avoided the surface layer. The near-bottom aggregation might be more effective behavior for GAs to reduce risk of visual predation than the surface aggregation because of the conspicuous resting egg they carried. These results show that carrying an advanced resting egg influenced the DVM of P. leuckarti.     

Thermohaline structure of abyssal and near-bottom waters of the Black Sea

The paper considers the thermohaline structure of Black Sea abyssal and near-bottom waters on the basis of exact measurements recorded in recent years under the HYDROBLACK and COMSBLACK international projects. The surface water mass (along with the cold mixed layer), the intermediate, abyssal, and near-bottom masses are identified. The upper boundary of deep waters is defined as a depth level where the density ratio decreases down to 0.1 This depth roughly corresponds to the 900–1100 m depth levels. Furthermore, the paper provides potential temperature and salinily values in the near-bottom layer, with an error of, respectively, 0.002°C and 0.002‰, at most. Translated by Vladimir A. Puchkin.     

The geochemistry of deepwater particulate matter over the hydrothermal field at 9°d50′ N (the East Pacific Rise)

The results of geochemical studies of particulate matter in the water mass over the hydrothermal field at 9°50′ N on the East Pacific Rise are presented. The particulate matter was tested in background waters, in the buoyant plume, and in the near-bottom waters. The contents of Si, Al, P, C_org, Fe, Mn, Cu, Zn, Ni, Co, As, Cr, Cd, Pb, Ag, and Hg were determined. No definite correlations were found between the ele-ments in the background waters. Many of the chemical elements correlated with Fe and associated with its oxyhydroxides in the buoyant plume. In the near-bottom waters, microelements are associated with Fe, Zn, and Cu (probably, to their sulfides formed under fluid mixing with seawater). The matter precipitated in a sed-imentation trap was similar to the near-bottom particulate matter in the elemental composition.     

Long-term changes of dissolved oxygen,hypoxia, and the responses of the ecosystems in the East China Sea from 1975 to 1995

Observation data obtained in the 32°N transect (transect E) in 1975–1995 were used to analyze the long-term changes in dissolved oxygen (DO) concentration and near-bottom hypoxic water in the East China Sea (ECS). A declining trend in annual average DO concentration and the degree of DO saturation was observed. Consequently, the apparent oxygen utilization in the western waters of transect E was on the rise. There was a seasonal hypoxic phenomenon in near-bottom water in the western water of transect E. The width of hypoxic water formed in summer gradually extended eastward along the continental shelf (transect E) at the rate of 3.12 km year⁻¹. Three potential reasons might have caused the formation and maintenance of near-bottom hypoxic water. First, the special hydrological topography and hypoxic deep water of the Taiwan Warm Current provided a backdrop for the hypoxic zone. Second, in summer, the strength of water column stratification restricts water exchange. Third is the occurrence and decay of the phytoplankton bloom. In surface water, nutrient concentrations increased gradually, and chlorophyll (Chl a), primary production, and phytoplankton biomass in summer increased. On the other hand, the community structure of phytoplankton, zooplankton, and zoobenthos became simple. Blooming phytoplankton consumed plenty of nutrients in the surface, but the upwelling of nutritious bottom water was suppressed by the strong thermocline. As a result, sinking of phytoplankton was enhanced because of nutrient deficiency. In recent years, a serious lack of zoobenthos in the study area corresponded to a higher degree of hypoxia. This phenomenon would have a major effect on the evolution of ecological dynamic systems in the ECS.     

Indication of bottom transport in shallow marine regions based on the data of satellite ocean color scanners

By example of a testing area in the northern Caspian Sea, a case study has been conducted to examine the feasibility of indication of near-bottom water transport from the data of ocean color scanners (OCS) for the basins whose bottom is shallower than the lower limit of the water-leaving radiance origination layer. The testing area has provided the desirable diversity of radiance origination conditions owing to an isolated underwater ridge. Based on the archived materials of the OCS SeaWiFS for 2000–2004, statistics of spectral normalized water-leaving radiances of the testing area have been computed. Hypothetically, the displacements of a radiance maximum in reference to the ridge were due to different mechanisms of interaction of water motions with bottom relief and bottom sediments. The correspondence between the patterns of radiance maximum displacements and the current notions of general water transport in the vicinity of the testing area has been established. Hopefully, the archived data of OCSs can be useful for revealing the patterns of seasonal and interannual variability of the near-bottom water transport at shallows with favorable bottom properties.     

Major-element composition of the particulate matter in the near-bottom nepheloid layer of the gulf of Mexico

During cruises 71-A-12 and 73-A-3 of the R/V “Alaminos” eighty-six samples of suspended matter at eleven near-bottom stations in the Gulf of Mexico and northwestern Caribbean Sea were collected, and simultaneously, values for light scattering were measured. Selected samples of the suspended matter were analyzed for particulate aluminum, silicon, iron, calcium, magnesium, organic carbon and inorganic carbon. The results indicate that a permanent but highly variable near-bottom nepheloid layer exists in the Gulf of Mexico but not in the northwestern Caribbean Sea. Average total-suspended loads in the Gulf of Mexico nepheloid layer are two times higher than in the clear water above the nepheloid layer.Since there is a significant increase in the alumino-silicate fraction and a corresponding decrease in the organic fraction of the suspended matter in the nepheloid layer relative to the overlying water, it appears that sediments are the most probable source of the increased concentrations of suspended matter in the nepheloid layer. This hypothesis is supported by X-ray diffraction analyses on the nepheloid material collected at one station which show the same mineral assemblages as the underlying sediments.Time studies over periods of one week and one and one-half years showed large total-suspended-matter variations which indicate that non steady-state processes, primarily vertical eddy diffusion and possibly advection, are controlling the distribution of suspended matter in the nepheloid layer.     

Phytopigments and fatty acids as molecular markers for the quality of near-bottom particulate organic matter in the North Sea

In February, May and August 1994, four stations in the North Sea (viz. at the Broad Fourteens, Frisian Front, German Bight and Skagerrak) were visited to sample near-bottom particulate organic matter. Samples, taken by means of a pump, a sediment trap and a sediment recorder, were analysed on organic carbon, total nitrogen, phytopigments and fatty acids. These molecular markers were used to describe the nature and quality of the organic particles in the near-bottom water. Principal component analysis showed chlorophyll a, phaeopigments and fatty acids to be useful markers for the quality of organic matter and yield complementary information.The quality of the near-bottom particles appeared to be related to the local hydrography and depositional circumstances. The Broad Fourteens station, a non-depositional sandy site along the Dutch coast, showed organic particles to be relatively fresh, little influenced by resuspended sedimentary material. Near-bottom organic particles on this site contained relatively high shares of chlorophyll a and polyunsaturated fatty acids, characteristic of algal matter. On the other hand the particulate organic material on the two depositional locations, the Frisian Front and the German Bight stations, was influenced by resuspension of sedimentary organic particles poor in pigments and fatty acids. Amounts of carbon trapped in the near-bottom environment at the Skagerrak station were lower than expected from the literature.     

Spatial and temporal variability of the North Korean Cold Water leading to the near-bottom cold water intrusion in Korea Strait

In the deepest region of Korea Strait, the surface temperature is highest in August (lowest in March), while the near-bottom temperature is lowest in September (highest in May). Cross-spectral analysis of the monthly temperature data between the two layers shows high coherence at the annual frequency with phase of 154°. Why and how does such a nearly opposite phasing occur between the surface and the near-bottom temperatures there? This study aims at answering these questions using historical and recently observed data.Cold and relatively fresh subsurface water flowing southward along the east coast of Korea and, known as the North Korean Cold Water (NKCW), becomes noticeable in April near the Sokcho coast. The zonal temperature gradient there is largest around June. The width of the NKCW becomes larger from April to August. After October, the NKCW retreats back toward the coast. The southward movement of the NKCW is thus strong over a period of six to seven months and weak in winter, especially in February. The NKCW flows southward relatively quickly along the coast in April to October and arrives at the Ulleung Basin within one to two months. Because of the sill between the Ulleung Basin and Korea Strait, this water cannot continue to flow to south, but piles up for about two to three months before it moves over the sill. The convergence of the subsurface cold water in the Ulleung Basin displaces the isopycnals upward and this water then intrudes over the sill along the isopycnals. This explains why in April or May, when this water appears noticeably at the Sokcho coast, the near-bottom water in Korea Strait is warmest and in August or September when the NKCW, which is piled up enough at the southern end of the Ulleung Basin, intrudes to Korea Strait, the near-bottom temperatures there are at their lowest.The origin of the NKCW seems to be the water of salinity less han 34.1 psu and surface density of 27σ_θ or higher, which sinks in the northwestern East Sea in January-March. The sinking of the water results from surface cooling in winter and is intensified due to the strong negative windstress curl. The cold and relatively fresh water, formed in the northwestern East Sea, is hypothesized to flow to the Ulleung Basin along three major paths, along the east coast of Korea, through the channel north of Ulleung-do Island, and through the channel between Ulleung-do and Dok-do Islands.     

Dissolved gases of the near-bottom water layer in the area of Epi island,Vanuatu

The distribution of dissolved gases in the near-bottom water layer over the submarine volcanoes of the Epi caldera in the New Hebrides off northeastern Australia is widespread. Maximum contents of methane and carbon dioxide in the near-bottom layer were as high as 440 nl/liter and 1.1 ml/liter, respectively. Composition and anomalous concentrations of hydrocarbon gases (C₁–C₅) suggest a thermogenic source and an inflow from surrounding sediment. Accumulation of carbon dioxide is connected with fumarolic activity of the Epi-b Volcano.     

Extending the versatility of the Bullard heat probe

A versatile probe for simultaneous studies of heat flow and near-bottom water parameters has evolved through modifications of the Bullard heat probe frame. Suitable sensor arrays have been used with this instrument to study (1) heat flow through the ocean floor, (2) water column temperature structure, (3) near-bottom current speeds, and (4) the differential cooling of water-column temperature sensors placed in a current speed gradient.Some of the advantages of such a modified Bullard probe are: (1) several parameters, including heat flow, can be measured across the sediment-water interface simultaneously, (2) the instrument frame is rigidly pinned to the ocean floor during measurement, permitting true Eulerian measurement in the water column with no effects of ship movement, swaying moorings or cable oscillation, and (3) the device is inexpensive and simple.     

Bottom currents,nepheloid layers and sedimentary features under the Gulf Stream near Cape Hatteras

The velocities of near-bottom currents were measured at six locations on a 180-km transect of the Gulf Stream adjacent to Cape Hatteras. The average velocities indicate a southwesterly flow - the Western Boundary Undercurrent. Maximum recorded velocities at each of the six locations ranged from 15 to 47 cm/sec. Depth distributions of suspended particulate matter over the transect indicated that near-bottom nepheloid layers were present and that relatively large amounts of suspended matter were being carried to the southwest. Bottom photographs taken over the same transect, however, showed no evidence that the sediment surface was being affected by the active bottom currents. These results indicate that swift bottom currents do not always leave a record of their work on deep ocean sediment.     

Temperature variability caused by internal tides in the coastal waters of east coast of Peninsular Malaysia

The effects of tidal currents (i.e., barotropic and internal tides) are important in the biogeochemistry of a coastal shelf sea. The high-frequency of currents and near-bottom temperatures collected in three consecutive southwest monsoon seasons (May, June, July and August of 2013 until 2015) is presented to reveal the role of the tidal currents to the temperature variability in the coastal shelf sea of the east coast of Peninsular Malaysia (ECPM), south of the South China Sea (SCS). The results of a spectral density and harmonic analysis demonstrate that the near-bottom temperature variability and the tidal currents are influenced by diurnal (O₁ and K₁) and semidiurnal (M₂) tidal currents. The spectral density of residual currents (detided data) at 5, 10 and 16 m depth also shows significant peaks at the diurnal tidal frequency (K₁) and small peaks at the semidiurnal tidal frequency (M₂) indicating the existence of internal tides. The result of the horizontal kinetic energy (HKE) shows a strong intermittent energy of internal tides in the ECPM with the strongest energy is found at 16 m depth during a sporadic cooling event in June and July. A high horizontal cross-shore heat flux (16 m) also indicates strong intrusions of cooler water into the ECPM in June and July. During the short duration of cold pulse water observed in June and July, a cross-wavelet analysis also reveals the strong relationship between the near-bottom temperatures and the internal tidal currents at the diurnal tidal frequency. The intrusion of this cooler water is probably related to the monsoon-induced upwelling in June. It is loosely interpreted that the interaction between the strong barotropic tides and the steep slope in the central basin of the SCS under the stratified condition in southwest monsoon has generated these internal tides. The dissipation of internal tides from the slope area probably has driven the cold-upwelled water into the ECPM coastal shelf sea when the upwelling intensity is the highest in June and July.     

Freshening of near-bottom waters in Lake Baikal triggered by the Mw6.2 Kultuk earthquake of August 2008

In September 2008, freshening of near-bottom water and an increase in concentration of suspended particles were observed in the western part of southern Lake Baikal. The reduction in the content of total dissolved solids was about 0.4–0.7?mg/kg (0.7?%), and average suspended particle concentration increased strongly to 6–9?mg/l, the background value being 0.2?mg/l. The spatial distribution of these waters was virtually identical to the focal area of the M_w6.2 Kultuk earthquake that occurred on 27 August 2008. It is suggested that there was a causal relationship between these two phenomena. Freshening of a significant amount (about 20?km³) of near-bottom waters was plausibly caused by an input of poorly mineralized pore waters from bottom sediments as a result of dissociation of methane gas hydrates suspected to occur in the area. The energy radiated by the earthquake source was four orders of magnitude smaller than that needed to explain the observed freshening of near-bottom waters. This points to other mechanisms leading to seismic-induced sediment failure and possible subsequent hydrate dissociation in the case of the Kultuk earthquake.     

Zooplankton distribution above the Lost City (Atlantis massif) and broken spur hydrothermal fields in the North Atlantic according to the data of visual observations

Three dives of Mir manned submersibles with plankton counts and two vertical plankton hauls with a BR net were carried out above the Lost City (Atlantis underwater massif) and the Broken Spur hydrothermal fields during cruise 50 of R/V Akademik Mstislav Keldysh. Above the Atlantis seamount, no significant increase in the plankton concentration was found. Above the Lost City field, the horizontal heterogeneity of the plankton distribution in the near-bottom layer and in the overlying water layers was shown. The near-bottom aggregations of euphausiids and amphipods previously reported by other scientists seem to be related to the attraction of the animals by the submersible’s headlights rather than represent a natural phenomenon.