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

Natural populations of oceanic prochlorophytes were sampled from the northwestern Mediterranean Sea in winter, stained with the DNA-specific fluorochrome DAPI, and analysed by flow cytometry. DNA histograms exhibited two peaks (G₁ and G₂ cells, containing one and two genome copies, respectively), separated by a trough of DNA-synthesizing cells (S cells). This suggested a cell cycle with a discrete S phase similar to that observed in eucaryotes or slow-growing procaryotes. Nitrogen and light were the two key environmental factors controlling the in situ cell cycle distributions of this procaryote. When nitrate levels were below 0.4 μM or light below 0.1% of the surface intensity, most of the cells were found in G₁, suggesting that they were not actively cycling. Cells arrested in G₁ could be induced to cycle into S + G₂ by incubating them with added nitrogen. Response was a function of initial nitrate concentration and decreased with depth, indicating that it was modulated by available light. These findings strongly suggest that prochlorophytes, which are one of the key components of the picoplankton community, may grow slowly in nitrogen-depleted waters, but still have the potential to respond quickly to nitrogen pulses.

Based on CTD data sets obtained in 1981–1984, XBT profiles, and long-term current meter moorings, the large-scale circulation field of the northeastern Atlantic north of the Azores was investigated. The mean volume transport through a standard meridional CTD section between 40°N and 52°N along the eastern flank of the Mid-Atlantic Ridge (MAR) was estimated to be 30 ± 9 Sv, with the North Atlantic Current (NAC) transporting 26 Sv. The NAC was found to be composed of clearly defined current branches (jets), that appear in temperature-salinity diagrams as a modal structure of the Central Water. Whereas the northernmost current branch (subarctic front) was found to be topographically fixed at the Gibbs Fracture Zone, the number, intensity and T-S structure of the remaining current branches, as well as their path over the MAR, are subject to intense variability. From 2 years of observations the branches were found to continue into the basins east of the MAR. They appeared as mesoscale features in a region of increased eddy kinetic energy and are interpreted to result from baroclinic instability. No indications of a branch of the NAC moving south, i.e. a recirculation as part of the North Atlantic subtropical gyre, were found.

The size distribution of chlorophyll a (Chl a) was comprehensively investigated during four cruises along 165°E from 20°S to 6°N, with cell counts by epifluorescence microscopy and nutrient analysis being performed at the same stations. Observations tool place in two contrasting periods, an El Niño Southern Oscillation event in 1987 and a non ENSO period in 1988 and 1989. One micrometre Nuclepore filters proved to separate efficiently cyanobacteria from eucaryotic microalgae, in nutrient-rich water masses as well as in poor ones, and whatever the depth or the cell abundance. The Chl a distribution in the <1 μm and >1 μm fractions resulted from the relative contribution of procaryotic and eucaryotic cells to the total Chl a. In a stratified system, the euphotic zone was found to be divided into two parts: (1) an upper nitrate-depleted layer in which cyanobacteria were always numerically predominant, closely linked with Chl a in the <1 μm fraction which accounted for 60% of total Chl a on average; (2) a lower nutrient-rich layer in which Chl a > 1 μm dominated, belonging mainly to eucaryotic microalgae, as confirmed by the correlation between Chl a > 1 μm and the number of eucaryotes. The rapid change of the Chl a size pattern repeatedly observed at the top of the nitracline, whatever the depth, clearly demonstrated the major effect of nutrient increase on the size structure of phytoplankton. In systems such as the equatorial upwelling, where there is no oligotrophic mixed layer, Chl a > 1 μm predominated from the top to the bottom of the euphotic layer, in spite of very numerous cyanobacteria in the surface waters. Below the deep Chl a maximum, relatively large amounts of Chl b in the <1 μm fraction can be attributed to minute cells such as prochlorophytes. These results were compared with Chl a fractionations previously performed in the tropical Atlantic Ocean. Analysis of 230 profiles of Chl a in the <0.6, <0.8, <1, <2, <3, <10 and <20 μm fractions did not reveal any significant difference between the two areas. Since the Chl a size structure properties with respect to nutrient are common to both oceanic systems, the relationship evidenced in the western Pacific between the Chl a size pattern and the distribution of procaryotic and eucaryotic algal cells is likely to be similar in the tropical Atlantic: phytoplankton over wide areas has certain well-defined size distribution properties, probably typical of the whole of the tropical open ocean.

Nitrogen content and isotopic composition for particles collected by standard water-sampling bottles and in situ large-volume pumps are compared. Data are presented for stations in warm-core ring 82-B in April and June 1982, newly formed warm-core ring 82-H in September/ October 1982, and the OFP site off Bermuda. For warm-core rings, relatively large differences in γ¹⁵N and PN concentration were observed between the two techniques. For the OFP site, except for PN concentration in the upper 200 m, good agreement was observed. Consideration of the effects of different retention sizes for the filter types used by the two techniques and whether or not large particles were included in the samples brought the OFP results into even closer agreement. However, only a fraction of the discrepancies in the warm-core ring results could be accounted for. Possible explanations include artifacts associated with differences in the details of the sample collection and handling techniques or differences in the size distributions of submicron particles and their γ¹⁵N values in rings. The much better agreement in γ¹⁵N between pumps at the OFP site, bottles at the OFP site, and pumps in ring 82-H suggests that minimal sample handling is desired. In addition, choice of filter material appears to be critical since significant quantities of PN can pass through commonly used varieties. These difficulties underscore the need to evaluate carefully the limitations of any particle-sampling technique regarding the specific measurements to be made.

¹³C uptake experiments were carried out in the Kuroshio warm-core ring and adjacent waters during the cruise of R.V. Hakuho-Maru. The depth-integrated primary productivity at the warm-core center ranged from 840 to 1200 mgC m⁻² day⁻¹. These rates were higher than those obtained in the surrounding water and the warm streamer existing in the southern part of the warm-core ring, and lower than that in the cold Oyashio water.

Production rates of 13 amino acids and eight monosaccharides at six stations were determined after hydrolysis of particulate matter by the ¹³C−GC−MS method. Protein and free amino acid (sum of 13 amino acids) carbon production rate accounted for 20–39% of organic carbon production at 10 m, whereas carbohydrate (sum of eight monosaccharides) was responsible for 11–21%. Composition of photosynthetic products and the spectrum of specific production rate (SPR) of organic compounds indicate that phytoplankton population in and around the warm-core ring was in a generally healthy state.

Vertical profiles of the natural abundances of ¹⁵N and ¹³C in suspended particulate organic materials (POM) were determined together with PON and POC concentrations to the depth of ca 4000 m at five stations in and out of a warm-core ring (WCR).

The ¹⁵N and ¹³C profiles showed generally, with one exception for a ¹⁵N profile at the northern edge of the ring, the same trend: an increasing abundance of ¹⁵N and ¹³C in the subsurface layer, followed by a decrease with depth. At the northern station, the ¹⁵N abundance of PON in the euphotic zone was unexpectedly high (∼20.4‰). PON with high ¹⁵N abundance was also observed at 30, 75 and 150 m at the southern periphery of the ring. This was not seen for the ¹³C of POC. T-S analysis indicated that POM with high ¹⁵N content was transported from the northern periphery by the ‘cold streamer’ of the WCR.

In the depth zone of 100–500 m, where thermostad appeared in the ring, the ¹⁵N of PON increased with depth while the ¹³C of POC remained constant in the ring; conversely, the ¹³C of POC decreased with depth while ¹⁵N of PON remained constant out of the ring. Temporal variation was noted for the ¹⁵N abundance of PON in the thermostad of the WCR, but little variation was observed for the ¹³C of POC.

The spatial and temporal differences between the ¹⁵N and the ¹³C profiles were explained assuming an intensified vertical mixing of the water column in the thermostad of the WCR, and an order of magnitude difference between the abundance of refractory fractions in POC and PON enriched with both ¹⁵N and ¹³C. It is suggested that the POM ¹⁵N and ¹³C isotopie ratios provide complementary information to study particle dynamics in the ocean.

A 1-m² MOCNESS with 20 nets was used to make a series of tows in Gulf Stream meander/ring 82-H (September/October 1982) including two 0–100 m undulating “towyos”. One towyo, made at dusk in the core of 82-H (of Sargasso Sea/Gulf Stream origin) permitted study of the effect of diel migration on the spatial variability of copepod and euphausiid species abundance, and species composition in a region of low physical variability. The other towyo taken across a front on the outer edge of 82-H (a mixture of Gulf Stream, Shelf and Slope Water), allowed comparison of spatial variability of the same biological properties in a region of strong physical variability. A sharp transition in euphausiid species composition occurred in the ring core after sunset as dielly migrating euphausiids moved into the surface waters. A similar, but less extreme change took place in copepod species composition because a smaller proportion of these migrated. All copepod migrants also entered surface waters after sunset with species living deeper in the water column during the day arriving in the surface waters later than those living shallower. Enright's (1977, Limnology and Oceanography, 22, 856–872) hypothesis for the metabolic advantages available through diel vertical migration does not account for the observed behavior of the migrating copepods and euphausiids at this time and place. Estimated swimming speeds (typically 50–200 m h⁻¹) of migrating copepods and euphausiids were similar in spite of large differences in body size between the two groups. Variations in species composition were substantially larger at the edge of the ring where species proportions changed radically in concert with changes in water-mass properties. There were also large differences in species composition between the samples from the ring core and the front which equaled those which occurred across the front. Hydrographic differences were stronger than diel changes due to migration for copepods but not for euphausiids. Streamers of surface water which originated within the frontal region and spiraled into the ring core could provide colonizers of many species not present at the time of ring formation.

Chcmiluminescent nitrate analysis was used in conjuction with ¹⁵N-labeled $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ to assess the rates of $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ uptake by phytoplankton in warm-core ring 82B. The relatively high precision of this method compared to conventional $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ analyses permits reliable estimates of $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ uptake in oligotrophic waters.

Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ turnover times ranging from about 4 h near the surface to 150 h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of 93 nmol kg⁻¹ for $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a $B=-B_0{e}^{-\frac{2\pi }{a_0}x}k$ threshold concentration of about 16 nmol kg⁻¹ was required for the initiation of uptake.

These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of $B=-B_0{e}^{-\frac{2\pi }{a_0}}$