Deep-sea research. Part A, Oceanographic research papers最新文献

We measured bacterial production and estimated the carbon consumption by bacteria in the mesopelagic zone (80–600 m) in the subarctic Pacific during May and August. Bacterial production was measured by leucine and thymidine incorporation. The two methods gave similar results. Bacterial production in the euphotic zone accounted for about 13% of primary production and in the whole water column for 20% (0–600 m). To bracket bacterial carbon consumption we made a lowest and highest estimate of bacterial production. The lowest estimate assumes zero isotope dilution for converting ¹⁴C-leucine incorporation rates into bacterial production and a 50% growth efficiency. In the mesopelagic zone, this estimate implies that bacterial account for 52 and 41% of the POC sinking flux as measured by sediment traps in May and August, respectively. The highest estimate, assuming two-fold isotope dilution of ¹⁴C-leucine and a 30% growth efficiency, yields bacterial carbon consumption values of 172 and 137% of the POC downward flux in both months. This indicates that bacteria are important, if not the major consumers of organic matter in the mesopelagic zone of the subarctic Pacific.

Previous work (by us and other) has shown that sediment trap fluxes do not correlate well with the total particulate mass concentration as determined with a transmissometer. Sediment traps are thought to collect the setting particles in the marine snow size range (d 〉 0.5mm). Cameras have been developed to quantitatively image particles in the marine snow size range but a correlation between measured flux in sediment traps and large-particle camera (LPC) profiles has not been established. In this study, LPC total particulate volume data are correlated with fluxes measured in sediment traps, indicating that sediment traps sample the large aggregate size range and that the flux is proportional to the concentration and size distribution of large aggregates. Partitioning of the major components of the bulk chemistry indicates that rebound aggregates (particles which do not undergo an appreciable change in their bulk chemistry before resuspension from the seafloor) contribute to aggregate nepheloid layers and increase measured trap fluxes. The bulk chemical composition of material from the deepest sediment traps indicates that downslope advection as well as cross-slope advection and subsequent settling may be amportant pathways for biogenic material to the deep ocean.

Sub-micrometer particles have recently been shown to exist in marine water at concentrations exceeding 10⁷ particles ml⁻¹. Their presence has important implications for ocean optics, global biogeochemical models and trophic relationships in the microbial food web. Small particles that were stainable by Acridine Orange (AO) and 4′,6-diamidino-2-phenylindole (DAPI) were enumerated and sized using a quantitative fluorescence microscopy imaging system along an onshore-offshore transect from the mouth of Chesapeake Bay to the Sargasso Sea. The particles were characterized by staining with DAPI, a stain specific for double-stranded DNA and generally indicative of a living cell or viral particle, and AO, a more general bio-polymer stain indicative of organic matter. Two distinct particle populations were measured in the 0.2–1.0 μm size range: (1) typical bacteria; and (2) abundant small, dimly fluorescing (SD) particles. Surface concentrations of organic (AO-staining), SD particles ranged from 3×10⁷ ml⁻¹ near the mouth of Chesapeake Bay to 4×10⁶ ml⁻¹ in the Sargasso Sea. A variable proportion of the SD particles were DAPI-positive, probably very small bacteria and viruses. The DAPI-positive SD particles constituted 9–29% of the total organic SD particles at coastal and shelf stations, and 25–61% in a vertical profile in oligotrophic waters. The vertical distribution of SD particles in oligotrophic waters showed higher numbers in the surface layer and lower numbers below the sub-surface chlorophyll maximum, suggesting an association of the particles with biological productivity. Our carbon estimates, based on measured particle size spectra and abundances, and reasonable values for particle carbon density, agree with recent measurements of bulk elemental particulate carbon in the 0.2–0.7 μm size fraction in the Sargasso Sea. The particle volume ml⁻¹ of the total SD particles ranged from equal to twice the bacterial biovolume ml⁻¹, indicating a significant carbon pool.

A long CTD/hydrographic section with closely-spaced stations was occupied in July–August 1988 from Iceland southward to 3°S along a nominal longitude of 20°W. The section extends from the surface down to the bottom, and spans the entire mid-ocean circulation regime of the North Atlantic from subpolar gyre through the subtropical gyre and the equatorial currents. Vertical sections of potential temperature, salinity and potential density from CTD measurements and of oxygen, silica, phosphate and nitrate, based on discrete water-sample measurements are presented and discussed in the context of the large-scale circulation of the North Atlantic Ocean. The close spacing of high-quality stations reveals some features not described previously. The more important findings include: (1) possible recirculation of the lightest Subpolar Mode Water into the tropics; (2) a thermostad at temperatures of 8–9°C, lying below that of the Equatorial 13°C Water; (3) the nutrient distribution in the low-salinity water above the Mediterranean Outflow Water that supports the previous conjecture of northern influence of the Antactic Intermediate Water; (4) a great deal of lateral structure of the Mediterranean Outflow Water, with a number of lobes of high salinity; (5) an abrupt southern boundary of the Labrador Sea Water at the Azores-Biscay Rise and a vertically well-mixed region to its south; (6) a sharp demarcation in the central Iceland Basin between the newest Iceland-Scotland Overflow Water and older bottom water, which has a significant component of southern water; (7) evidence that the Northeast Atlantic Deep Water is a mixture of the Mediterranean Outflow Water and the Northwest Atlantic Bottom Water with very little input from the Iceland-Scotland Overflow Water; (8) an isolated core of the high-salinity, low-silica Upper North Atlantic Deep Water at the equator; (9) a core of the high-oxygen, low-nutrient Lower North Atlantic Deep Water pressed against the southern flank of the Mid-Atlantic Ridge just south of the equator; (10) a weak minimum of salinity, and well-defined maxima of nutrients associated with the oxygen minimum that separates the Middle and Lower North Atlantic Deep Waters south of the equator; (11) a large body of nearly homogeneous water beneath the Middle North Atlantic Deep Water between 20°N and the Azores-Biscay Rise; and (12) a deep westward boundary undercurrent on the southern slope of the Rockall Plateau.

The NOAMP field program, carried out in the deep Northeast Atlantic from 1983 to 1986, was designed to investigate the local deep flow and the dispersion of suspended material by means of current meter, SOFAR float, turbidity and CTD data. The bottom topography of the NOAMP area (45–49°N, 17–23°W) represents a highland with hills and ridges rising up to 1000 m above the deep-sea floor (approx. 4500 m). The deep mean flow (3–5 cm s⁻¹) runs fairly parallel to the depth contours and consists of a system of small topographically induced gyres that can trap any kind of passive admixture.

During the first 50 days after release, a cluster of 14 deep-sea floats drifting at depths of around 3500 m spread steadily over an area with a diameter of about 300 km. During the following roughly 200 days a further drift of the centre of mass of the cluster—as well as a further spreading relative to the centre of mass—was hardly noticeable.

Occasionally, high energetic events (benthic storms) with durations between 3 and 27 days, and maximum velocities up to 27 cm s⁻¹ occurred directly above the deep-sea floor.

We examine the effects of light and temperature on the relationship between photosynthesis and natural fluorescence in oceanic and coastal waters. While a moderately-sized database suggests a strong correlation, there is evidence that light and temperature alter the ratio of the quantum yields of photosynthesis and natural fluorescence. Specifically, we find a reduction of this ratio as light intensity increases and temperature decreases. In this paper, we review the effects of these factors and present empirical equations to account for their behavior. Although these equations significantly improve our ability to predict photosynthetic rate from natural fluorescence, the biophysical and biochemical mechanisms undelying these effects are not understood sufficiently.

We report on studies of the carbon balance of the upper water column, done as part of the JGOFS North Atlantic Bloom Experiment, over a 13-day period, at 47°N, 20°W, during the 1989 spring phytoplankton bloom. Gross carbon production was calculated from data on ¹⁸O gross O₂ production and from ¹⁴C production as well. Net carbon production was calculated from net O₂ production rates measured in vitro, as well as from changes in the inventories of nutrients and O₂ along with O₂ evasion rates by gas exchange. Gross carbon production during this period was measured to be 1.83 mol m⁻², and net production was 0.68 mol m⁻². Of this net carbon production, 0.30 mol m⁻² was stored in the euphotic zone as particulate organic carbon, and 0.09 mol m⁻² rained out to depths >150 m. The remainder was remineralized to DIC in the 50–150 m depth interval, with perhaps some DOC storage in the upper 150 m.

Flows due to point sources and sinks of mass are considered on both f- and β-planes. The fluid is assumed to be continuously stratified with constant buoyancy frequency, N, and has no boundaries. Solutions to the flow field giving both horizontal and vertical structure are obtained on large timescales compared with an inertial period for the case when the source/sink flow is started from rest. Steady solutions are obtained through the inclusion of simple Rayleigh damping. On an f-plane the flow is radially symmetric, and a sink (source) generates cyclonic (anticyclonic) swirl due to angular momentum conservation. The nature of the flow of fluid toward the sink depends critically on the ratio f/N. On a β-plane a sink (source) also generates cyclonic (anticyclonic) swirl, but the velocity field is no longer radially symmetric and the circulation about the source/sink drifts westward. Also, the flow of fluid toward the sink intensifies to an eastward flowing jet. The behaviour is described both in terms of vorticity arguments and in terms of the westward propagation of energy by very low frequency Rossby waves. The solution is then used to calculate the surface circulation and vertical density structure in the vicinity of deep ocean convection events where the process is modelled by a simple mass transfer over depth.

Annual new production resulting from winter nitrate has been estimated for the North Atlantic using data from the Coastal Zone Color Scanner (CZCS) between 1979 and 1983. Twelve monthly mean surface chlorophyll images, based on 5-year averages of CZCS data, were used to identify three zones with distinct seasonal patterns. A mid-latitude zone, with an area of 7 × 10⁶ km², exhibited a spring bloom followed by oligotrophic conditions at the surface throughout the summer. The decline in surface chlorophyll following the spring bloom was assumed to indicate that winter nitrate in the mixed layer was exhausted and that a nitracline had formed at depth. Based on observations by Strass and Woods (Deep-Sea Research, 38, 35–56, 1991) in the North Atlantic, we estimate that the nitracline deepened at a rate of 10 m per month, starting at the base of the mixed layer. By determining the timing of the onset of oligotrophic conditions from the satellite data, we can estimate the volume of nitrate-depleted water lying above the nitracline in late summer. This was combined with an estimate of the nitrate concentration at the start of the growing season to derive new production. The model for the initial nitrate concentration is based on an empirical relationship between winter nitrate from Glover and Brewer (Deep-Sea Research, 35, 1525–1546, 1988) and the maximum chlorophyll at the end of the spring bloom.

The resulting new production was 24 g C m⁻² y⁻¹ (4.2 g N m⁻² y⁻¹) in this mid-latitude transitional zone. Applying the same method to subtropical zone of approximately equal area yields an estimate of new production of 18 g C m⁻² y⁻¹ (3.1 g N m⁻² y⁻¹); the area-weighted average for both zones was 21 g C m⁻² y⁻¹ (3.7 g N m⁻² y⁻¹). In the subtropical zone, the maximum chlorophyll occurred in the winter and the minimum in late summer, suggesting that production was nutrient-limited throughout the year. The third smaller zone (2.5 × 10⁶ km²) was located in subpolar regions to the north, where surface chlorophyll was minimum in winter and maximum in late summer. Assuming that all winter nitrate had been assimilated by the end of the summer within the upper 40 m of the water column in this zone, new production is estimated to be 43 g C m⁻² y⁻¹ (7.6 g N m⁻² y⁻¹). Our results underestimate new production because they are based solely on winter nitrate assimilated by phytoplankton between winter and late summer. Nevertheless, the values are comparable in magnitude to previous estimates of total productivity in oligotrophic oceanic regions (Koblentz-Mishkeet al., in Scientific exploration of the South Pacific