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1.
Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depth decreases from ca. 2400 m in the Rainbow vent field (36°13′N) to ca. 850 m in the Menez Gwen vent field (37°35′N). In this area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along with populations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical and chemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trap moorings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculata living in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-endemic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher for species living there. The main faunal differences observed between Lucky Strike and Menez Gwen hydrothermal fields are due to an impoverishment in the hydrothermal endemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests the existence of a succession of several biogeographic islands rather than a single province.  相似文献   

2.
The structure of the assemblages associated with the mussel aggregations of Bathymodiolus azoricus was investigated. The mussel beds were found on the hydrothermal vent fields on the Mid-Atlantic Ridge (the Menez Gwen, Lucky Strike, and Rainbow areas) at the depths of 850–2400 m. The community structure of the mussel bed assemblages varied between the studied areas. Large number of species was unique to Menez Gwen mussel beds; the most observed taxa were not specialized hydrothermal species. All the other, nonunique species were found for the Lucky Strike region. The lowest mussel assemblage structure evenness was observed in the shallowest area, the Menez Gwen area (850 m depth). We assume that two types of mussel assemblages—nematode-dominated and copepod-dominated ones—exist in the Lucky Strike field. The assemblages of B. azoricus differ significantly from the assemblages of B. thermophilus inhabiting the Pacific hydrothermal vents.  相似文献   

3.
Cockle (Cerastoderma edule) population dynamics were studied at the southern limit of the distribution of this marine bivalve in Merja Zerga, Morocco. Parameters such as growth, mortality, and production were compared with those of a population at Arcachon Bay (France) a site in the center of the cockle's range. At each sampling period between two and three cohorts were simultaneously observed at each site and the average total abundance was usually higher at Merja Zerga. Recruitment occurred at both sites in spring when temperature rose above 19 °C, independently of the month. In Merja Zerga, winter recruitment was also observed at one occasion, following high sediment disturbance. The first year (2005–06) at Merja Zerga, the mortality rate was close to nil for juveniles and was Z = 1.5 yr? 1 for adults, providing a high production (64 g dry weight m? 2 yr? 1). At Arcachon during the same period, the juvenile mortality rate was Z = 10.9 yr? 1, the adult mortality rate was 3.4 yr? 1 and production was 26 gDW m?2 yr? 1. The second year (2006–07), mortality after recruitment was much higher (Z = 8.6 yr? 1, for juveniles) and similar to what was observed at Arcachon (Z = 8.4 yr? 1). Mortality rate of adults was higher at Merja Zerga (Z = 3.0 yr? 1) than at Arcachon (Z = 1.5 yr? 1). Production was lower at Arcachon than at Merja Zerga although growth performances were higher at Arcachon. The higher growth performance at Arcachon (Φ′ = 3.3) was mainly due to high asymptotic length (L∞ = 38 mm) and was related to low intraspecific competition compared to Merja Zerga where cockle abundance was higher (Φ′ = 3.1, L∞ = 31 mm). P/B was low in both sites and slightly higher at Arcachon (1.1–1.5 against 1.0–1.1 yr? 1). At Arcachon, recruitment was correlated with temperature, a peak occurring when temperature rose above 19 °C (June–July). At Merja Zerga, recruitment was already 2–3 months earlier but was not significantly correlated to temperature.This study showed that population dynamics of cockles at the southern limit of this distribution fell in the range of what was observed elsewhere in the North-Eastern Atlantic coast. Most factors that were involved in population regulation (intraspecific competition, predation and sediment dynamics) were not strictly dependent on latitude. The direct role of temperature (latitude dependent factor) was not obvious. Variation in temperature could explain the recruitment delay between Arcachon and Merja Zerga and the low maximum shell length at Merja Zerga.  相似文献   

4.
The venerid clam Ruditapes philippinarum is the most prominent suspension-feeding bivalve inhabiting muddy intertidal seagrass beds in Arcachon Bay (SW France). It is exploited by fishermen, and Arcachon Bay ranks number one in France in terms of production and total biomass of this species. Previous studies revealed a decrease in the standing stock of R. philippinarum since 2003 and unbalanced length–frequency distributions with a lack of juveniles and of adults > 40 mm. Consequently, the population dynamics of this bivalve were studied at four intertidal sites and one oceanic site in Arcachon Bay. As clam size structure did not allow classical dynamics computations, field monitoring was coupled with field experiments (tagging–recapture) over two years. Monitoring of condition index and gonadal maturation stages highlighted a high variability in spawning number and intensity between sites. Recruitment events in the exploited area varied spatially but with uniformly low values. Von Bertalanffy Growth Function (VBGF) parameters (K, L∞) were determined using Appeldoorn and ELEFAN methods. In the exploited sites in the inner lagoon, K was relatively high (mean = 0.72 yr? 1) but L∞ was low (mean = 41.1 mm) resulting in a moderate growth performance index (Φ′ = 2.99). Growth parameters were not correlated with immersion time and L∞ was different between sites. Comparison of mortality coefficients (Z) between cage experiments and field monitoring suggested that fishing accounts for 65–75% of total adult mortality. Low recruitment, a low growth rate and a normal mortality rate led to low somatic production (4.1 and 8.7 g Shell-Free Dry Weight (SFDW) m? 2 yr? 1) and an annual P/B ratio from 0.44 to 0.92 yr? 1. Under current conditions, the possibility of a sustainable population in Arcachon Bay will strongly depend on recruitment success and fishing management.  相似文献   

5.
The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO3) and long-chain C37 alkenones (ΣAlk37) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r2=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO3 and ΣAlk37 were strongly correlated (0.6<r2<0.8) with those of OC. The major exception was the lower than expected ΣAlk37 fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m−2 yr−1 at 275 m) and diminished ΣAlk37 fluxes (e.g., 5 mg m−2 yr−1 at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m−2 yr−1) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m−2 yr−1) over the 1996–1999 study period. CaCO3 burial fluxes (30–40 g m−2 yr−1), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m−2 yr−1) but comparable to those obtained from the shallowest trap (i.e. 38 g m−2 yr−1 at 275 m). In contrast, the burial rates of ΣAlk37 (0.4–1 mg m−2 yr−1) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m−2 yr−1), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m−2 yr−1) and elevated ΣAlk37 fluxes (∼2 mg m−2 yr−1) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.  相似文献   

6.
Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15 yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20 yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4 mol O2 m−2 yr−1 and of oxygen consumption was −2.3±0.5 mol O2 m−2 yr−1 at ESTOC, and 4 mol O2 m−2 yr−1 and −4.4±1 mol m−2 yr−1 at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.  相似文献   

7.
The life-histories and the secondary production of four dominant peracarid crustaceans (the mysids Boreomysis arctica and Parapseudomma calloplura, the amphipod Rhachotropis caeca, and the isopod Ilyarachna longicornis) in bathyal depths of the Bay of Biscay (NE Atlantic; between 383 and 420 m) and the Catalan Sea (Northwestern Mediterranean; between 389 and 1355 m) were established. Both the Atlantic and the Mediterranean populations of the major part of the target-species had two generations/year with mean cohort-production intervals (CPI) ranging from 5.5 mo for Ilyarachna longicornis to 6.3 mo for Parapseudomma calloplura. The Hynes method showed secondary production to vary in the Bay of Biscay between 0.113 mg DW m−2 yr−1 for I. longirostris and 3.069 mg DW m−2 yr−1 for P. calloplura, with P/B ratios between 4.57 (I. longirostris) and 7.93 (Boreomysis arctica). In the Catalan Sea, production varied between 0.286 mg DW m−2 yr−1 for I. longirostris and 1.096 mg DW m−2 yr−1 for P. calloplura, with P/B between 5.72 (I. longirostris) and 6.66 (P. calloplura). Application of two different empiric models to the whole peracarid assemblage gave similar levels of secondary production in both study areas (between 29.26 and 32.14 mgDWm−2 yr−1 in the Bay of Biscay; between 26.23 and 26.54 mg DW m−2 yr−1 in the Catalan Sea). From the analysis of gut contents of 22 species the dominant species in each study area were assigned to two basic trophic levels, detritus feeders and predators. Also, cumulative curves of dominance showed high diversity (low dominance) for peracarid assemblages distributed at mid-bathyal depths (524–693 m) both in the Bay of Biscay off Arcachon and in the Catalan Sea off Barcelona. We also discuss and compare, both within and between areas, how environmental features may explain the observed diversity patterns, the trophic structure, and the production results obtained for the suprabenthos assemblages.  相似文献   

8.
Thermocline ventilation rates for the subtropical North Pacific are determined using a 1-dimensional (meridional) along-isopycnal advective–diffusive model tuned to chlorofluorocarbon (CFC) concentrations measured along 152°W in 1991 during WOCE P16. Mean southward advection rates in the subtropics range from 1.03 to 0.56 cm s-1 between σθ=25.5 and 26.6. Model-derived ventilation times for the subtropical gyre increase from about 10 to 27 years for that isopycnal range. Oxygen utilization rates (OURs) determined using the advective-diffusive model decrease with depth from 6.6 to 3.2 μmol kg-1 yr-1 between σθ=25.5 and 26.6. Extrapolation of the OUR versus depth trend to the base of the euphotic zone with the 1/Z power function of Martin et al. (1987) and integration from 500 to 100 m depth implies a carbon export rate from the overlying euphotic zone of 2.2±0.5 moles C m-2 yr-1 at 30°N, 152°W. Analysis of the WOCE radiocarbon and salinity distributions indicates that zonal and cross-isopycnal transport terms would have to be considered in modeling these tracers in the subtropical North Pacific.  相似文献   

9.
Fish farming impact on the seasonal biomass, carbon and nutrient (nitrogen and phosphorus) balance of the endemic Mediterranean seagrass Posidonia oceanica was assessed in the Aegean Sea (Greece) in order to detect changes in magnitude and fate of seagrass production and nutrient incorporation with organic loading of the meadows. Phosphorus concentration in the leaves, rhizomes and roots was enhanced under the cages throughout the study. Standing biomass was diminished by 64% and carbon, nitrogen and phosphorus standing stock by 64%, 61% and 48%, respectively, under the cages in relation to those at the control. Seagrass production decreased by 68% and element (C, N, P) incorporation by 67%, 58% and 58%, respectively, under the cages. Leaf shedding was reduced by 81% and loss of elements (C, N, and P) through shedding by 82%, 74% and 72%, respectively, under the cages. Leaf and element (C, N, P) residual loss rate, accounting for grazing and mechanical breakage of leaves, was decreased by 79%, 85%, 100% and 96%, respectively, at the control station. At the control station, 13.98 g C m?2 yr?1, 1.91 g N m?2 yr?1 and 0.05 g P m?2 yr?1 were produced in excess of export and loss. In contrast, under the cages 12.69 g C m?2 yr?1, 0.31 g N m?2 yr?1 and 0.04 g P m?2 yr?1 were released from the meadow. Organic loading due to fish farm discharges transformed the seagrass meadow under the cages from a typical sink to a source of organic carbon and nutrients.  相似文献   

10.
Data from the first systematic survey of inorganic carbon parameters on a global scale, the GEOSECS program, are compared with those collected during WOCE/JGOFS to study the changes in carbon and other geochemical properties, and anthropogenic CO2 increase in the Atlantic Ocean from the 1970s to the early 1990s. This first data-based estimate of CO2 increase over this period was accomplished by adjusting the GEOSECS data set to be consistent with recent high-quality carbon data. Multiple Linear Regression (MLR) and extended Multiple Linear Regression (eMLR) analyses to these carbon data are applied by regressing DIC with potential temperature, salinity, AOU, silica, and PO4 in three latitudinal regions for the western and eastern basins in the Atlantic Ocean. The results from MLR (and eMLR provided in parentheses) indicate that the mean anthropogenic CO2 uptake rate in the western basin is 0.70 (0.53) mol m?2 yr?1 for the region north of 15°N; 0.53 (0.36) mol m?2 yr?1 for the equatorial region between 15°N and 15°S; and 0.83 (0.35) mol m?2 yr?1 in the South Atlantic south of 15°S. For the eastern basin an estimate of 0.57 (0.45) mol m?2 yr?1 is obtained for the equatorial region, and 0.28 (0.34) mol m?2 yr?1 for the South Atlantic south of 15°S. The results of using eMLR are systematically lower than those from MLR method in the western basin. The anthropogenic CO2 increase is also estimated in the upper thermocline from salinity normalized DIC after correction for AOU along the isopycnal surfaces. For these depths the results are consistent with the CO2 uptake rates derived from both MLR and eMLR methods.  相似文献   

11.
Mass fluxes of diatom opal, planktonic foraminifera carbonate and coccolithophorid carbonate were measured with time-series sediment traps at six sites in the Arabian Sea, Bay of Bengal and Equatorial Indian Ocean (EIOT). The above fluxes were related to regional variations in salinity, temperature and nutrient distribution. Annual fluxes of diatom opal range between 3 and 28 g m−2 yr−1, while planktonic foraminifera carbonate fluxes range between 6 and 23 g m−2 yr−1 and coccolithophorid carbonate fluxes range between 4 and 24 g m−2 yr−1. Annual planktonic foraminifera carbonate to coccolithophorid carbonate ratios range between 0.8 and 2.2 and coccolithophorid carbonate to diatom opal ratios range between 0.5 and 3.3.In the western Arabian Sea, coccolithophorids are the major contributors to biogenic flux during periods of low nutrient concentrations. Coccolithophorid carbonate fluxes decrease and planktonic foraminiferal carbonate and diatom opal fluxes increase when nutrient-rich upwelled waters are advected over the trap site. In the oligotropic eastern Arabian Sea, coccolithophorid carbonate fluxes are high throughout the year. Planktonic foraminiferal carbonate fluxes are the major contributors to biogenic flux in the EIOT. In the northern and central Bay of Bengal, when surface salinity values drop sharply during the SW monsoon, there is a drastic reduction in planktonic foraminiferal carbonate fluxes, but coccolithophorid carbonate and diatom opal fluxes remain steady or continue to increase. Distinctly higher annual molar Sibio/Cinorg (>1) and Corg/Cinorg (>1.5) ratios are observed in the northern and central Bay of Bengal mainly due to lower foraminiferal carbonate production as a result of sharp salinity variations. We can thus infer that the enhanced freshwater supply from rivers should increase oceanic CO2 uptake. Its silicate supply favours the production of diatoms while the salinity drop produces conditions unfavourable for most planktonic foraminifera species.  相似文献   

12.
JGOFS-KERFIX (KERguelen point FIXe) time-series station, located south of the polar front in the Indian sector of the Antarctic Ocean, was occupied monthly between January 1990 and March 1995. Annual cycles of dissolved inorganic carbon (DIC), total alkalinity (TALK), oxygen (O2) and nutrients (nitrate, silicate, phosphate and ammonia) in the upper ocean are presented for this site. From seasonal drawdown of nutrients and DIC, we estimate a spring–summer net community production of 3.2±0.5 mol m−2 and C/N/P ratios of 100/16/1. The Si/N ratio varies between 1.8 and 3, suggesting low iron concentrations. The spring–summer biogenic silicon export derived from silicate drawdown is 1.18 mol m−2, consistent with model estimates of silicate export at this site. Seasonal and interannual variations of oxygen, nitrate and DIC due to physical and biological processes are quantified using a simple month-to-month budget formulation. From these budgets, an annual net community production of 5.7±3.3 mol m−2 yr−1 is estimated, about twice the averaged spring–summer production, indicating that, at KERFIX, there is a positive net community production throughout the year. Air–sea CO2 fluxes show that KERFIX is a strong CO2 sink for the atmosphere of 2.4–5.1 mol m−2 yr−1 in 1993, depending on the gas exchange formulation used. A 2.1–3.3 mol m−2 yr−1 outgassing of O2 is observed at KERFIX except in 1993 and 1994 where a decreasing trend of temperature induces an increase of O2 solubility.  相似文献   

13.
The total organic carbon (TOC) and total inorganic carbon (CT) exchange between the Atlantic Ocean and the Mediterranean Sea was studied in the Strait of Gibraltar in September 1997. Samples were taken at eight stations from western and eastern entrances of the Strait and at the middle of the Strait (Tarifa Narrows). TOC was analyzed by a high-temperature catalytic oxidation method, and CT was calculated from alkalinity–pHT pairs and appropriate thermodynamic relationships. The results are used in a two-layer model of water mass exchange through the Strait, which includes the Atlantic inflow, the Mediterranean outflow and the interface layer in between. Our observations show a decrease of TOC and an increase of CT concentrations from the surface to the bottom: 71–132 μM C and 2068–2150 μmol kg−1 in the Surface Atlantic Water, 74–95 μM C and 2119–2148 μmol kg−1 in the North Atlantic Central Water, 63–116 μM C and 2123–2312 μmol kg−1 in the interface layer, and 61–78 μM C and 2307–2325 μmol kg−1 in the Mediterranean waters. However, within the Mediterranean outflow, we found that the concentrations of carbon were higher at the western side of the Strait (75–78 μM C, 2068–2318 μmol kg−1) than at the eastern side (61–69 μM C, 2082–2324 μmol kg−1). This difference is due to the mixing between the Atlantic inflow and the Mediterranean outflow on the west of the Strait, which results in a flux of organic carbon from the inflow to the outflow and an opposite flux of inorganic carbon. We estimate that the TOC input from the Atlantic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies from (0.97±0.8)104 to (1.81±0.90)104 mol C s−1 (0.3×1012 to 0.56×1012 mol C yr−1), while outflow of inorganic carbon ranges from (12.5±0.4)104 to (15.6±0.4)104 mol C s−1 (3.99–4.90×1012 mol C yr−1). The high variability of carbon exchange within the Strait is due to the variability of vertical mixing between inflow and outflow along the Strait. The prevalence of organic carbon inflow and inorganic carbon outflow shows the Mediterranean Sea to be a basin of active remineralization of organic material.  相似文献   

14.
Interannual variability of nutrients and plankton cycles were studied at the time-series station KERFIX (50°40′S, 68°25′E) using a 1-D coupled physical-biogeochemical model that is descended from that of Pondaven et al. (1998). At KERFIX, a high half saturation constant for silicic acid uptake (KSi) and a high Si/N uptake ratio are required to reproduce the Si and N cycles. Although very high in comparison with most data from temperate systems, these values are consistent with KSi and Si/N uptake ratios measured in the Indian sector of the Southern Ocean. Past and recent finding on the role of light and iron limitation on nutrient consumption ratios might explain these “unusual” silicon uptake kinetic parameters. Comparison of model results with observations show that the model correctly reproduces the observed interannual variability of nutrients and plankton cycles at KERFIX between 1992 and 1995. Characteristic features of this region are a spring phytoplankton bloom of 1.0–1.5 mg Chlorophyll a m−3 and a net excess of silicic acid utilisation over that of nitrate. This high silicic acid utilisation leads to low Si concentrations in late summer and subsequent Si limitation of diatom growth. The interannual variability of production of silicon and nitrogen predicted by the model is 1.93±0.04 mol Si m−2 yr−1 and 1.35±0.07 mol N m−2 yr−1 (±SD). In parallel, the predicted export is 1.12±0.04 mol Si m−2 yr−1 and 0.06±0.01 mol N m−2 yr−1. It is shown that diatoms may contribute significantly to export if diatom sinking is taken into account. An interannual variability of the predicted Si and N cycles is detected. This variability is associated with changes in the mixed layer properties, which have been documented to be linked to the Pacific El Niño Southern Oscillation or displacement of the Polar Front.  相似文献   

15.
Bathymodiolus brevior [von Cosel, R., Métivier, B., Hashimoto, J., 1994. Three new species of Bathymodiolus (Bivalvia: Mytilidae) from hydrothermal vents in the Lau Basin and the North Fiji Basin, western Pacific, and the Snake Pit Area, mid-Atlantic ridge. Veliger 37, 374–392] a bivalve mollusk living at deep-sea hydrothermal vents, exhibits daily microgrowth structures in its shell. This interpretation is substantiated by various lines of evidence: (1) similar shell portions of contemporaneous specimens from the same locality contain almost the same number of microincrements; (2) the number of microincrements coincides with the expected number of days in which shell portions of Bathymodiolus spp. form; (3) the width of such microincrements compares well with daily growth rates estimated for the close relative B. thermophilus [Kenk, V.C., Wilson, B.R., 1985. A new mussel (Bivalvia, Mytilidae) from hydrothermal vents in the Galapagos rift-zone. Malacologia 26, 253–271]; (4) different specimens from the same site show similar microgrowth curves. In addition, we found support for tide-controlled shell growth. Daily shell growth rates fluctuate on a fortnightly basis. Some shell portions also revealed the typical tide-controlled microgrowth pattern commonly observed in intertidal bivalves. Based on the analyses of lunar daily growth increments, a growth curve for B. brevior was computed: Xt=14 cm−(14–0.04 cm) e−0.26t. This curve enables estimation of ontogenetic age from shell length. According to this equation, B. brevior reaches its maximum shell length of 14 cm at about age 18. Shell isotope analyses suggest that some major shell growth interruptions or retardations are related to extremely active hydrothermal venting activity. However, shell growth also stopped during periods of low venting implying physiological controls on shell formation. Results of the present study demonstrate that shells of B. brevior provide calendars and environmental data loggers that can complement or partly substitute for long-term observations of venting systems.  相似文献   

16.
The first carbon budget constructed for the Barents Sea to study the fluxes of carbon into, out of, and within the region is presented. The budget is based on modelled volume flows, measured dissolved inorganic carbon (DIC) concentration, and literature values for dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations. The results of the budget show that ～5600±660×106 t C yr?1 is exchanged through the boundaries of the Barents Sea. If a 40% uncertainty in the volume flows is included in the error calculation it resulted in a total uncertainty of ±1600×106 t C yr?1. The largest part of the total budget flux consists of DIC advection (～95% of the inflow and ～97% of the outflow). The other sources and sinks are, in order of importance, advection of organic carbon (DOC+POC; ～3% of both in- and outflow), total uptake of atmospheric CO2 (～1% of the inflow), river and land sources (～0.2% of the inflow), and burial of organic carbon in the sediments (～0.2% of the outflow). The Barents Sea is a net exporter of carbon to the Arctic Ocean; the net DIC export is ～2500±660×106 t C yr?1 of which ～1700±650×106 t C yr?1 (～70%) is in subsurface water masses and thus sequestered from the atmosphere. The net total organic carbon export to the Arctic Ocean is ～80±20×106 t C yr?1. Shelf pumping in the Barents Sea results in an uptake of ～22±11×106 t C yr?1 from the atmosphere which is exported out of the area in the dense modified Atlantic Waters. The main part of this carbon was channelled through export production (～16±10×106 t C yr?1).  相似文献   

17.
Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)4 and biogenic silica (bSiO2) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m? 2 yr? 1. This supply must be equivalent to the net annual bSiO2 production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m? 2 yr? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m? 2 yr? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?30Si) probably reflects (1) a significant impact of bSiO2 dissolution, enriching the bSiO2 pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)4 from below when the surface Si(OH)4 pool is significantly depleted. We estimated a net integrated bSiO2 production of 10.5 ± 1.4 mol Si m? 2 yr? 1 in the AASW above the plateau, which includes a significant contribution of bSiO2 production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.  相似文献   

18.
A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m−3, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m−3. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m−2 yr−1) and export flux (0.79 and 0.61 mol Si m−2 yr−1), compared to the North Atlantic (production: 0.23 mol Si m−2 yr−1, export: 0.12 mol Si m−2 yr−1). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.  相似文献   

19.
Previous work has shown that methane anomalies frequently occur within the rift valley of the Mid-Atlantic Ridge (MAR). The plumes appear confined within the high, steep walls of the valley, and it is not known whether methane may escape to the open ocean outside. In order to investigate this question, the concentration and 13C/12C ratio of methane together with CCl3F concentration were measured in the northeastern Atlantic including the rift valley near 50°N. This segment contained methane plumes centered several 100 m above the valley floor with δ13C values mostly between –15‰ and –10‰. A limited number of helium isotope measurements showed that δ3He increased to 17% at the bottom of the valley, which suggests the helium and methane sources may be spatially separated. In the eastern Atlantic away from the ridge (48°N, 20°W), the methane concentration decreased monotonically from the surface to the bottom, but the methane δ13C exhibited a mid-water maximum of about –25‰. The bottom water methane contained a significantly lower δ13C of about –36‰. Thus, it appears that isotopically heavy methane escapes from the MAR into North Atlantic Deep Water (NADW) that contacts the ridge crest while circulating to the east. The formation of NADW supplies isotopically light methane that dilutes the input of heavy carbon from the ridge. We employed a time-dependent box model to calculate the extent of isotope dilution and thereby the flux of MAR methane into the NADW circulation. The degree of methane oxidation, which affects the 13C/12C of methane through kinetic isotope fractionation, was estimated by comparing methane and CFC-11 model results with observations. The model calculations indicate a MAR methane source of about 0.06×10−9 mol L−1 yr−1 to waters at the depth of the ridge crest. Assuming this extends to a 500 m thick layer over half of the entire Atlantic, the amount of methane escaping from the MAR to the open ocean is estimated to be about 1×109 mol yr−1. The total production of methane within the rift valley is likely much greater than the flux from the valley to the outside because of local oxidation. This implies that serpentinization of ultramafic rocks supports much of methane production in the rift valley because the amount expected from basalt degassing in association with mantle helium (<0.6×109 mol CH4 yr−1) is less than even the net amount escaping from the valley. The model results also indicate the methane specific oxidation rate is about 0.05 yr−1 in open waters of the northern Atlantic.  相似文献   

20.
The macrotidal bay of Marennes-Oléron is the most important French site for shellfish production (oysters and mussels); yet the primary productivity of the phytoplankton compartment in this system is not well known. In this study, photosynthetic parameters were determined using 14C incubations of bottom and surface water samples, during fall, winter and summer (2001–2002), along a north–south transect in the bay. Estimates of primary productivity showed that water column primary production is light-limited in the bay and that a BZpI0 type model can be applied. Spatial differences existed in the bay, with a more productive northern zone and less productive river area. With a water column primary production of 185 g C m−2 yr−1, Marennes-Oléron Bay lies in the mean range for phytoplankton primary production capacity among European and North American estuaries.  相似文献   

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