A method is developed whereby the vertical fluxes of heat and salt can be deduced from oceanic microstructure measurements of temperature and shear variance. This method is appropriate when both turbulent mixing and double-diffusive convection contribute to the vertical property fluxes. Previous methods that deduce property fluxes from microstructure measurements have assumed that either turbulent mixing or double-diffusive convection is the cause of the observed microstructure; here we present a method suitable for the more general situation where both mixing processes contribute to the property fluxes in the region of interest. The key assumptions are that the mixing efficiency of the turbulence and the flux ratio of the salt fingers are unchanged by the presence of the other process. It is found that sufficiently accurate total heat and salt fluxes can be deduced from the microstructure data, but our present imprecise knowledge of the salt-finger buoyancy flux ratio places large error bars on the total buoyancy flux.
A three-dimensional time-dependent model of the circulation in the Bransfield Strait-South Shetland Islands region and a physiologically-based, temperature-dependent model of the descent-ascent behavior of the embryos and larvae of Euphausia superba were combined in a Lagrangian particle tracing model to simulate trajectories of krill embryos and larvae. The Lagrangian calculations show that: (1) surface flow is the primary factor influencing the final location of the embryo-larva particle; and (2) timing of krill spawning affects the eventual position of the feeding larvae. Seasonal changes in the wind stress field result in variability in direction and velocity of surface currents, which affects the embryo-larva trajectories. Conditions favourable for the transport of larvae to Bransfield Strait occur early in the spawning season. East of the Antarctic Peninsula larvae have a greater probability of entering Bransfield Strait if the krill embryos are released in mid-summer, January to February. Embryos released to the north of the South Shetland Islands, west of 62°W are transported into Drake Passage. Embryos released to the north of the South Shetland Islands and east of Livingston Island are transported westward where they can eventually enter Bransfield Strait. Krill larvae also are transported into Bransfield Strait from the Bellingshausen and Weddell Seas. The Lagrangian trajectories show that the western Bransfield Strait is a region of potentially high larval concentration due to transport from surrounding areas as well as local production. This is in agreement with observed krill larvae distributions, which show higher concentrations in this region.
A method for estimating the diffuse attenuation coefficient for downward irradiance, Kd(λ), from time-series data of irradiance and sea-surface wave elevation is presented. Data acquisition is accomplished with a fixed-depth, single moored instrument system. Kd(λ) is obtained as a ratio of the standard deviation of natural logarithms of irradiance to the standard deviation of wave elevation. This statistical estimation assumes that the irradiance fluctuations are caused only by light attenuation over a fluctuating vertical path length associated with oscillations of surface wave elevation. The limitations and the possible range of applicability of the method depend on environmental conditions, including surface wave characteristics, water clarity, meteorological conditions and solar elevation. The probability density and power spectral analysis are useful for interpreting the validity of statistical estimates of Kd(λ). Based on this preliminary concept, some recommendations for an improved experimental design are proposed.
In order to compare the collection efficiency of settling particles among sediment traps in a variety of design concepts, 28 sediment traps of 11 different designs were deployed at six depths ranging from 665 to 3769 m along five rigid moorings anchored in a sill-protected marginal basin about 3865 m deep for about 4 months from August to November 1979. The traps represented three basic designs: (1) cylinders with an aspect ratio between 2 and 3; (2) funnels with a large opening covered by a baffle with a small grid and (3) open boxes whose openings were covered by a baffle. All but two of these types of participating traps had a mechanism to isolate the collected sample. Monitoring instruments indicate that all moorings provided a stable platform throughout the duration of Sediment Trap Intercomparison Experiment (STIE) with relatively low current velocity at the middle layers and verv low velocity at the deep layers. Total mass flux, fluxes of three size fractions after water sieving, carbonate, combustible and noncombustible fractions, organic carbon, nitrogen and other sedimentary constituents in the individual samples were determined and evaluated with regard to the relative consistency in terms of depth and statistical tests on the similarity of the constituents.Under the conditions tested, the trapping efficiency of settling particles between a large funnel trap with baffle and an intermediate-sized cylinder trap was nearly identical considering the laboratory analytical errors. This conclusion might be extended to cylinder traps with diameters as small as 7 cm and a large aspect ratio when deployed rigidly in a low energy ocean environment. A funnel-type trap with a more effective baffle had a higher collecting efficiency than the other traps. Because of mechanical problems, comparison of the box-type traps to the other types was inconclusive.
Estimates of currents from velocities measured in a surface-following reference frame are biased due to the correlation between the sensor motion and wave orbital velocity. We demonstrate a technique that reduces this bias by an order of magnitude in cases where the motion of the sensor is known. As an example, we applied the technique to data obtained from a fixed tower with a miniature acoustic current meter attached to a mechanical wave-follower.
In this study we investigate the utility of GEOSAT altimetry for monitoring the Iceland-Faeroe frontal zone. Since an expected dynamic topography relief of 10–20 cm over the Iceland-Faeroe Front (IFF) was not much above the 10 cm uncertainty in GEOSAT observations, validation by AVHRR imagery and satellite-tracked drifters constituted an important part of the experiment. Sea Surface Height (SSH) relief of greater than 20 cm occurred a long the western side of the IFF and along the eastern side, north of the Shetland Islands. However, with SSH relief of only 10–15 cm in the central region of the IFF, substantial difficulties were encountered in the ability to unambiguously monitor the location of the front. In contrast, frontal meanders with 20–30 cm SSH relief, current speeds up to 50 cm s−1 and radii of curvature of 25 km, were clearly observed on three occasions during the 2 year study. These meanders first appeared north of the Faeroe Islands, in the region from 6 to 8°W, and propagated southeastward at speeds of about 3.3 km day−1, being lost from view in the Faeroe-Shetland Channel. Their strong signals and lifetimes of 2–3 months would appear to make them important constituents of IFF dynamics.
A series of synchronous, 24-h experiments using sensor-equipped sediment traps revealed that higher particle collection rates were associated with higher approach velocieties. Surface-tethered traps with variable drag configurations provided distinct differences in approach velocities for paired 400 m deployments and paired 1500 m deployments. Small-scale hot-film hydrodynamics sensors located both inside and outside the sediment traps detected flow cells within the traps with velocities between 50 and 100% of the external fluid approach velocities. In conjunction with laboratory flume simulations, these observations reveal that particles do not settle gravitationally across trap apertures. Intead, particles are swept advectively into traps at the downstream portion of trap apertures, and most are then expelled at the upstream portions of trap apertures. Fluid flows detected inside the drifting traps, which ranged from 1.2 to 31 cm s−1,l probably overwhelm all but the strongest “swimmers” that interact with these sampling divices. At our two sampling horizons (400 and 1500 m), tether-line motions generated trap depth oscillations with a period of the order of 10 s and an amplitude of about 0.5 m. Such effects have not been accounted for in flume simulated of sediment traps collection experiments.
The nutrient dynamics of a warm-core eddy from the Eastern Mediterranean was studied through a complete annual cycle. Winter mixing of the upper 450 m of the eddy core resulted in phytoplankton biomass build-up (70 mg Chl a m−2) of stock well above that present in the boundary (44 mg m−2). During this biomass build-up phosphate was reduced to undetectable levels while excess nitrate (0.6 μM) remained which, taken with other evidence, showed that this area of the Eastern Mediterranean is phosphorus limited. After stratification in late March, nitrate and phosphate concentrations were at undetectable levels in the upper 120 m, a deep chlorophyll maximum had developed (90–120 m), and a gradual systematic accumulation of nutrients occurred in the zone from 120 to 450 m. The permanent nutricline lay between 450 and 650 m. The net supply of nutrients to the euphotic zone (0–120 m), which is the annual new production, was 300 mmoles N m−2 y−1 and 14.9 mmoles P m−2 y−1. Most of this new production (∼90%) was supplied by deep winter mixing and occurred over a limited period of time (days to a few weeks). Previous estimates for new production in the Eastern Mediterranean by Dugdale and Wilkerson (1988, Oceanologica Acta, 9, 170–184), based on bottle incubations, were low because the major input of nutrients by deep winter mixing was not included in the calculation. Most of the nutrients supplied to the euphotic zone in the eddy (60–70%) were derived from the decomposition of the previous year's productivity.
The role of the density ratio during double diffusive interleaving is investigated experimentally. A sharp vertical front is created by lifting a barrier separating two stratified compartments of different T-S composition but similar density. The environment is either stably stratified in both components or double diffusively stratified in the diffusive sense. Interleaving layers driven entirely by double diffusive processes start to form and to propagate horizontally. The observed thickness of the intrusions agrees well with the predicted vertical scale of Ruddick and Turner [Deep-Sea Research, 26, 903–913 (1979)] when the stratification is doubly stable, and is found to be smaller in the diffusive case, as can be predicted from Linden's [Deep-Sea Research, 23, 895–908 (1976)] formula. The cross-frontal flux is independent of the stability of the environment except when the environment is double diffusively stratified in the diffusive sense and the gradient of the unstable component is at least 66% that of the stable component. In this latter case the cross-frontal flux is larger than for a double stable environment, as an extra source of potential energy is already present in the unstably stratified component. A parametrization of the cross-frontal flux is given in terms of the vertical finger flux. These results are applied to oceanic Meddies and to the survival of the core region of a Meddy against lateral intrusions.
A 2000 m deep section of total attenuation and chlorophyll and phycoerythrin fluorescence from 26° to 59°N latitude in the northeast Pacific is discussed in terms of inferred biological processes. Photic zone distributions of these quantities vary from nutrient-limited conditions in the subtropics to light-limited conditions in the subarctic. Phycoerythrin-containing organisms, probably Synechococcus, contribute to a strong, near-surface orange fluorescence signal in the Gulf of Alaska. We now recognize that the fluorescence minimum (about 300 m) between the photic zone and the tertiary fluorescence maximum may be related to secondary producers that "repackage" organic matter produced in the photic zone. The tertiary fluorescence maximum (about 1000 m) is a continuous feature of the oxygen minimum zone in the North Pacific. The presence of phycoerythrin in the tertiary maximum is consistent with heterotrophic cyanobacteria and other unidentified microbial assemblages in the oxygen minimum, though there is no strong biological evidence that this is true.
The concept of the recurvature of the Agulhas Current has gone through a number of historical phases. The perception that a major part of the current returns to the Indian Ocean was first put forward in 1832 by Rennell. In time this idea became increasingly diluted with a bifurcation of the current being more generally accepted up until at least 1933, even though work by Dutch researchers such as Andrau in 1857 had supported the findings of Rennell. Obfuscation by various subsequent portrayals only came to an end by 1970 when Bang put forward the concept of a retroflection. All current work uses this conceptual interpretation.
We describe a modified (GARSIDE, 1982. Marine Chemistry, 11, 159-167) nitrite method that permits measurements down to subnanomolar concentrations and present data from Atlantic and Caribbean deepwater profiles for comparison with a published Pacific section.This important intermediate in the nitrogen cycle was detected in all samples. Concentrations were consistently lowest (0.1-0.4 nM) in oligotrophic surface waters. Below 1 km. Caribbean and Southwest Sargasso sea nitrite concentrations were 0.4-1 nM, decreasing with increasing depth; reported Pacific [NO2-] averages are several times higher. Profiles in the upper kilometer beneath the classical primary nitrite maximum (PNM) were qualitatively similar, exhibiting a smooth supra-exponential drop with depth to values of approximately 1-4 nM at 1 km.The nitrite inventory in this "tail" of the PNM above 1 km with 1 nM less-than-or-equal-to [NO2-] less-than-or-equal-to 50 nM roughly equals that in the classical PNM. Significant differences among profiles in the 0.1-1 km region are observed, consistent with nitrite pool turnover times of 3-7 days estimated from Redfield stoichiometry and tritium-helium ages. Thus seasonal and/or regional variations in factors altering the nitrite production-consumption balance, rather than transport, seem to be responsible for nitrite variability.Nitrite profiles with anomalous midwater or near-bottom fine structure, including multi-point maxima and minima, were found along the Venezuelan continental margin and at approximately 13-degrees-N. These features are tentatively ascribed to boundary effects, as hydrographic and circumstantial evidence suggests that these waters interacted previously with the bottom.
The occurrence of particulate proteins and the molecular characteristics of such proteins are reported here for the first time from analyses of particulate matter samples from a water column profile taken in the North Pacific Ocean and the Bering Sea. Particulate proteins were solubilized and separated by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE). Two characteristic groups of proteins were identified from the vertical changes in their molecular distribution. The first group, derived directly from organisms, mainly phytoplankton, was made up of a large number of proteins, each present at relatively low levels, and with widely ranging molecular weights. The concentrations of proteins in this group decreased rapidly with depth, generally to zero in deep waters, indicating that these proteins are very susceptible to biological degradation. The second group included specific proteins distributed over a limited range of molecular weights between 45,000 and 66,000 daltons. These were the most abundant protein constituents in particulate organic matter (POM) from the intermediate and deep waters, and may be resistant to biological attack, although the source and the chemical nature of the specific proteins are not clear.
In this paper we present the first data set for rare earth elements (REE) in the Arctic Ocean. The data are from unfiltered samples. The concentrations are La: 18.8–41.3 pM; Ce: 5.7–25; Pr: 1.4–9.9; Nd: 14.6–39.6; Sm: 2.7–7.3; Gd: 3.8–13.4; Dy: 3.7–12.3; Er: 3.6–9.6; and Yb: 2.9–8.1. The concentrations found in the deep water are lower than deep-water concentrations found elsewhere and are oceanographically consistent. Our data show that the distribution of REE are influenced by riverine input in the northern part of the investigated area. Transport of resuspended material into the water from the Barents Sea Shelf is suggested to influence the concentrations of REE in the southern part of the Nansen Basin. The covariation between REE and nutrients is opposite to that found in other oceans. The fresh water end member concentrations for the REE in the northern surface water have been calculated.
Measurements on krill from distinct swarms and echo-sounder measurements of the same swarms have been analysed using multivariate methods. The analysis has addressed the problems of whether krill swarms in a small area may be of a limited number of types, and whether the biological characteristics of the krill could be related to the echo-sounder measurements of the swarm. The main sources of biological variation between krill in different swarms seemed to be related to size and maturity. Krill swarms did not appear to form groups based on biological similarity, and swarms caught in the same haul were as different as swarms caught in different hauls. However, swarms could be divided into groups on the basis of time of day and swarm depth. The biological features (size, maturity, sex ratio and moult stage) of krill in the swarms were not related to echo-sounder observations (horizontal and vertical extent, depth and density of swarms), although some degree of separation between swarms containing large and small krill may be possible if the shape of the swarms is considered. Information on biological characteristics of krill in swarms must still be collected using nets.
An eddy-kinetic energy model for simulations of the oceanic vertical mixing (GASPAR et al., 1990, Journal of Geophysical Research, 95, 16179-16193) is used to investigate the seasonal cycle of dissolved total CO2 and CO2 partial pressure at Station P (Gulf of Alaska) during the years 1971 and 1972.The model simulates relatively weak seasonal variations of surface seawater pCO2. All processes appear to contribute to the same order of magnitude: interaction of gas exchange and biology tends to compensate for the thermodynamical effect on pCO2. The influence of various model parameters (total annual production of O2, P, depth of photic zone, z0, gas transfer velocity, K) on the seasonal variability of seawater PCO2 and air-Sea CO2 flux is examined. Total annual production, P, seems to be the most influential.The model also suggests a strong short-term variability of PCO2 during episodic events. Model results show that the area around Station P behaves as a weak or neutral CO2 source during summer and as a net sink during other seasons. The net annual CO2 flux is an invasion flux, with values of 0.88 and 0.70 mol m-2 y-1 for 1971 and 1972, respectively, in agreement with the flux of 0.7 mol m-2 y-1 derived from observations during 1973-1978 (WONG and CHAN, 1991, Tellus, 43B, 206-223).
Acoustic Doppler current profilers (ADCPs) are now commonplace on many ships and have had a considerable impact on modern oceanography. During a recent cruise on R.R.S. Discovery, however, the presence of a large spurious shear in the direction of the ship's motion was noted on a 150 kHz profiler, particularly when steaming into heavy seas. It is thought that this results from the trapping of bubbles near the ship's hull. Parameters representing the state of the wind and sea, and the ship's motions, are combined in a semi-empirical way to produce a function (E) that estimates the severity of this effect and so gives insights into the factors affecting the data quality. The results are compared with a further cruise upon which the transducers were extended beyond the bubble layer. The estimating function allows a meaningful intercomparison for the different weather conditions on the second cruise and shows that the spurious shear was effectively eliminated. Similar problems are now being observed on other vessels, and it is recommended that ADCP transducers should be mounted below the bubble layer whenever possible.
We analyse surface-to-bottom profiles of temperature, salinity, light attenuation coefficient and acoustic (150 kHz) backscatter intensity collected in June 1990 in the vicinity of the central hydrothermal vent field on Endeavour Ridge in the northeast Pacific. Data from coincident deep plankton net tows corroborate earlier speculation (THOMSON et al., 1991, Journal of Geophysical Research, 96, 4839-4844) that the 100 m thick acoustic scattering layer found near 1.9 km depth in this region consists of a dense concentration of macrozooplankton living near the top of the plume-contaminated bottom waters. Peak (almost-equal-to 10 dB) acoustic anomalies in the June 1990 backscatter layer were located near the top of the plume at depths of 1800-2000 m throughout the principal 60 km2 study area. The scattering layer contained a high zooplankton biomass of 21 mg m-3 and maximum species richness of 83 taxa. In all profiles, the backscatter intensity decreased to anomalously low values beneath the core of the plume. The presence of the backscatter layer 12 km to the east of the vent site is evidence for zooplankton layering beyond the immediate confines of the ridge.We conclude that pelagic and deep-sea zooplankton congregate near the top of the plume-contaminated bottom waters in the vicinity of the ridge to take advantage of increased concentrations of chemosynthetic bacteria, fine-grained particles and other nutrients carried vertically upward by the buoyant portions of the plume. The zooplankton depletion below the central core of the spreading hydrothermal plume indicates that zooplankton avoid elevated concentrations of hydrothermally-derived minerals and other chemicals inherent to the main body of the plume.
Biomasses, abundances and feeding ecology of larger (> 50 μm diameter) protozooplankton were studied in the upper 210 m in the ice edge zone of the Weddell/Scotia Sea area in the austral winter of 1988. Sixty-liter water samples were taken at five depths at 17 stations, and organisms were concentrated by reverse-flow filtration. Mean abundances of the total assemblage of larger protozooplankton (radiolarians, formaminiferans, acantharians, the heliozoan Sticholonche, tintinnid and aloricate ciliates, and thecate and athecate dinoflagellates) ranged from 2040 to 3745 M−3 in the upper 210 m. Biomass ranged from 33 to 48 μg C m−3 in the upper 85 m, and from 32 to 54 μg C m−3 from 115 to 210 m. Phaeodarian radiolarians larger than 1.6 mm (sampled with plummet nets) contributed an additional 3 μg C m−3 in the upper 100 m and an additional 7 μg C m−3 from 100 to 200 m. These abundances and biomasses are lower than for other seasons in the Antarctic, but are comparable to abundances reported for several of these groups in lower latitude waters. We attribute the low winter abundances to slower growth and reduced food, rather than to increased mortality. The large protozooplankton are trophically diverse; in addition to heterotrophy on a variety of organisms, we found apparent evidence of mixotrophy and symbiosis in some of the groups. The large protozooplankton fed on both autotrophic and heterotrophic organisms in winter, although the biomass of smaller forms is dominated by heterotrophs. Feeding on detrital particles also was indicated by the presence of siliceous fragments in vacuoles. The larger protozooplankton in the winter ice edge zone may be important in reducing particle flux to the deep sea and as a food source for larger zooplankton, especially from the base of the euphotic zone to 210 m.
Hydrography and satellite-tracked drifters from the Gulf of Alaska Recirculation Study (GARS) were used to describe the regional circulation from 1986 to 1989 in the northwest Gulf of Alaska. The average baroclinic transport (0/1000 db) from six occupations of a section across the Alaska Stream near the Shumagin Islands (55-degrees-N, 160-degrees-W) was 7.4 Sv. The seasonal variation in the transport of the Alaska Stream was negligible relative to the seasonal variation of the Sverdrup transport in the Gulf of Alaska as calculated from the wind-stress curl. However, the mean transport agreed with mean annual Sverdrup transport. Anticyclonic mesoscale eddies frequently appeared in the dynamic topography and hydrography along the easternmost sections of the cruise grid (140-degrees-W).Drifters released in the Alaska Stream offshore of Kodiak Island usually moved southwestward following the isobaths. However, the trajectories of four drifters from 1988 to 1989 described an anticyclonic meander in the Alaska Stream that propagated southwestward at about 0.022 m s-1. The hydrography confirmed the existence of the meander off Kodiak Island in April 1988. The temperature and salinity characteristics of the anticyclonic meanders and eddies indicated that the water masses at the center of the features were derived from Alaska Current water.Two mechanisms of enhanced vertical mixing in the Gulf of Alaska are suggested by the hydrography. The first one is due to wind mixing of the water column by intense winter storms and subsequent outcropping of the 26.8 sigma(theta) isopycnal surface in the center of the Alaska Gyre: a mechanism originally proposed by VAN SCOY et al. (Journal of Geophysical Research, 96, 16,801-16,810, 1991). The second one is associated with fine structure in the temperature and salinity profiles centered at a density of 26.8 sigma(theta), the approximate density of North Pacific Intermediate Water. These mechanisms freshen the water on the 26.8 sigma(theta) isopycnal surface. Subsequent lateral mixing on isopycnal surfaces by mesoscale eddy activity may contribute to the low salinity signature of the North Pacific Intermediate Water.
