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1.
《Journal of Sea Research》2000,43(2):113-119
The distribution of phytoplankton was measured in vertical profiles at three stations in Limfjorden (Denmark) within and above Mytilus edulis beds (−5 to 200 cm above the bed surface) and related to the filtration capacity of the mussels and hydrodynamic conditions. The density of phytoplankton was 2690 to 10 600 cells cm−3 at 200 cm above the bottom (∼1.4 to 3.2 mg Chl-a m−3), declining towards the bottom to about 1000 cells cm−3 at two of the stations. This is a threshold of minimum algal concentration below which mussels are reported to stop feeding. The results of the present study further demonstrate that the near-bed phytoplankton concentrations observed in the field often remained below the concentration level at which mussels reduce their maximal filtration activity.  相似文献   

2.
Under present-day conditions, rivers are the main source of fine sediments dispersed to the Bay of Biscay. They deliver about 2.5×106 t yr−1 of continental fine sediments, 60% of which is derived from the Gironde estuary. Of this flux, 65% is believed stored on the shelf. Two kinds of mud fields can be found in the Bay of Biscay: coastal mud and shelf mud belts. The total mass of fine sediments stored during the past 2000 years is 3.2×109 t. Consequently, about 0.9×106 t yr−1 could reach the shelf edge and eventually the open sea. From this amount of displaced material and the deposition surface areas, an evaluation of sediment fluxes across the margin during the late Holocene period is discussed. This evaluation is compared with results obtained from ECOsystéme du canyon du cap-FERret (ECOFER) data from sediment traps and surficial box cores.  相似文献   

3.
We investigated the impact of sediment reworking fauna and hydrodynamics on mobilization and transport of organic matter and fine particles in marine sediments. Experiments were conducted in an annular flume using lugworms (Arenicola marina) as model organisms. The impact of lugworms on sediment characteristics and particle transport was followed through time in sediments experimentally enriched with fine particles (< 63 μm) and organic matter. Parallel experiments were run at low and high water current velocity (11 and 25 cm s− 1) to evaluate the importance of sediment erosion at the sediment–water interface. There was no impact of fauna on sediment composition and particle transport at current velocity below the sediment erosion threshold. At current velocity above the erosion threshold, sediment reworking by lugworms resulted in dramatic particle transport (12 kg dry matter m− 2) to an adjacent particle trap within 56 days. The transported matter was enriched 6–8 times in fine particles and organic matter when compared to the initial sediment. This study suggests that sediment reworking fauna is an important controlling factor for the particle composition of marine sediments. A. marina mediated sediment reworking greatly increases the sediment volume exposed to hydrodynamic forcing at the sediment–water interface, and through sediment resuspension control the content of fine particles and organic matter in the entire reworked sediment layer (> 20 cm depth).  相似文献   

4.
Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean Corg input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m−2 d−1. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m−2 d−1 (mean 3.7 mg C m−2 d−1), whereas fluxes on the East Greenland shelf are considerably higher, 9.1–22.5 mg C m−2 d−1. On the Norwegian continental slope Corg fluxes of 3.3–13.9 mg C m−2 d−1 (mean 6.5 mg C m−2 d−1) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O2 fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004–1.1 mg C cm−3 a−1 at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from Corg degradation rates and sediment organic carbon content, are in the range 0.03–0.6 a−1. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in Corg flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.  相似文献   

5.
Sulfate reduction rate measurements by the 35SO42− core injection method were carried out in situ with a benthic lander, LUISE, and in parallel by shipboard incubations in sediments of the Black Sea. Eight stations were studied along a transect from the Romanian shelf to the deep western anoxic basin. The highest rates measured on an areal basis for the upper 0–15 cm were 1.97 mmol m−2 d−1 on the shelf and 1.54 mmol m−2 d−1 at 181 m water depth just below the chemocline. At all stations sulfate reduction rates decreased to values <3 nmol cm−3 d−1 below 15 cm depth in the sediment. The importance of sulfate reduction relative to the total mineralization of organic matter was very low, 6%, on the inner shelf, which was paved with mussels, and increased to 47% on the outer shelf at 100 m depth. Where the oxic–anoxic interface of the water column impinged on the sea floor at around 150 m depth, the contribution of sulfate reduction increased from >50% just above the chemocline to 100% just below. In the deep sea, mean sulfate reduction rates were 0.6 mmol m−2 d−1 corresponding to an organic carbon oxidation of 1.3 mmol m−2 d−1. This is close to the mean sedimentation rate of organic carbon over the year in the western basin. A comparison with published data on sulfate reduction in Black Sea sediments showed that the present results tend to be higher in shelf sediments and lower in the deep-sea than most other data. Based on the present water column H2S inventory and the H2S flux out of the sediment, the calculated turnover time of H2S below the chemocline is 2100 years.  相似文献   

6.
Between November 2001 and March 2002 an Australian/Japanese collaborative study completed six passes of a transect line in the Seasonal-Ice Zone (south of 62°S) along 140°E. Zooplankton samples were collected with a NORPAC net on 22–28 November, and a Continuous Plankton Recorder on 10–15 January, 11–12 February, 19–22 February, 25–26 February, and 10–11 March. Zooplankton densities were lowest on 22–28 November (ave=61 individuals (ind) m−3), when almost the entire transect was covered by sea ice. By 10–15 January sea surface temperature had increased by ∼2 °C across the transect line, and the study area was ice-free. Total zooplankton abundance had increased to maximum levels for the season (ave=1301 ind m−3; max=1979 ind m−3), dominated by a “Peak Community” comprising Oithona similis, Ctenocalanus citer, Clausocalanus laticeps, foraminiferans, Limacina spp., appendicularians, Rhincalanus gigas and large calanoid copepodites (C1–3). Total densities declined on each subsequent transect, returning to an average of 169 ind m−3 on 10–11 March. The seasonal density decline was due to the decline in densities of “Peak Community” taxa, but coincided with the rise of Euphausia superba larvae into the surface waters, increased densities of Salpa thompsoni, and an increased contribution of C4 to adult stages to the populations of Calanoides acutus, Calanus propinquus and Calanus simillimus. The seasonal community succession appeared to be influenced by the low sea ice extent and southward projection of the ACC in this region. The relatively warm ACC waters, together with low krill biomass, favoured high densities of small grazers during the January/February bloom conditions. The persistence of relatively warm surface waters in March and the seasonal decrease in chlorophyll a biomass provided favorable conditions for salps, which were able to penetrate south of the Southern Boundary.  相似文献   

7.
This study focuses on sediment exchange in the degraded Mwache mangrove forest wetland located in southern Kenya. It involved measurement of total and particulate organic suspended sediment concentrations (TSSC and POSC), tidal water elevation and current velocities. Results showed that in the heavily degraded backwater zone mangrove forest, the ebb and flood tide total sediment fluxes were of same order of magnitude, however, flood tide sediment fluxes were slightly higher than the ebb ones. In the moderately degraded frontwater zone mangrove forest, the flood tide sediment fluxes were more than 50% higher than the ebb tide fluxes. The peak net sedimentation in the highly degraded backwater zone was 4 g m−2 tide−1 but that in the moderately degraded frontwater zone was 63 g m−2 tide−1. In the frontwater zone of the mangrove forest, the peak instantaneous ebb tide sediment flux was 3206 kg tide−1 equivalent to 35.6 g m−2 tide−1 and the flood one 8574 kg tide−1 (95 g m−2 tide−1). The peak instantaneous flood and ebb tide particulate organic sediment (POS) fluxes in the frontwater zone mangrove forest were 1316 kg tide−1 (15 g m−2 tide−1) and 587 kg tide−1 (6.5 g m−2 tide−1), respectively. The peak ebb and flood tide sediment fluxes in the backwater mangrove forest were 3206 kg tide−1 (36 g m−2 tide−1) and 3305 kg tide−1 (36.7 g m−2 tide−1), respectively. In case of POS fluxes in the backwater zone mangrove forest, the peak flood period POS flux was 969 kg tide−1 (10.7 g m−2 tide−1) while the ebb period one was 484 kg tide−1 (5.4 g m−2 tide−1). In both highly degraded backwater and moderately degraded frontwater zone of the mangrove forest, there is net import of sediments. However, the net import is relatively lower in the backwater zone forest where the trapping efficiency is 27%. In the moderately degraded frontwater zone of the mangrove forest, the sediment trapping efficiency is 65%. The net sediment import occurs mainly in periods of high river discharge in both neap and spring tides, but occurs only in spring tides during dry season. The net accretion rates in the backwater and frontwater zone mangrove forests are 0.25 and 3.5 cm year−1, respectively.  相似文献   

8.
Beyond the shelf break at ca. 150 m water depth, sulfate reduction is the only important process of organic matter oxidation in Black Sea sediments from the surface down to the sulfate–methane transition at 2–4 m depth. Sulfate reduction rates were measured experimentally with 35SO42−, and the rates were compared with results of two diffusion-reaction models. The results showed that, even in these non-bioirrigated sediments without sulfide reoxidation, modeling strongly underestimated the high reduction rates near the sediment surface. A hybrid modeling approach, in which experimentally measured rates in the upper sediment layers force a model that includes also the deeper layers, probably provides the most realistic estimate of sulfate reduction rates. Areal rates of sulfate reduction were 0.65–1.43 mmol SO42− m−2 d−1, highest in sediments just below the chemocline. Anaerobic methane oxidation accounted for 7–11% of the total sulfate reduction in slope and deep-sea sediments. Although this methane-driven sulfate reduction shaped the entire sulfate gradient, it was only equivalent to the sulfate reduction in the uppermost 1.5 cm of surface sediment. Methane oxidation was complete, yet the process was very sluggish with turnover times of methane within the sulfate–methane transition zone of 20 yr or more.  相似文献   

9.
The three-dimensional structure of two potential mesoscale upwelling areas that are located in the external waters of the Philippine archipelago (i.e. northwest of Luzon and east of Mindanao) were constructed by analysis of historical data. A unique characteristic of both upwelling sites is that they can be identified by their anomalously cold subsurface temperatures rather than sea surface temperature distributions. As such, they cannot be observed in sea surface temperature fields derived by satellite imagery. The data used in the analysis were obtained from the National Oceanographic Data Center hydrographic database. Objective analysis was performed to produce monthly temperature fields at several standard depths within the upper 500 m of the region 0–30°N and 100–140°E with a horizontal grid resolution of 0.5°. The extent and timing of these upwelling areas are described. A review of existing hypothesis on the mechanisms for their evolution and seasonal modulation are presented. The change in heat content during the upwelling season is greater than 300 W m−2 in both areas. Based on the excursion of isotherms, vertical velocities of 83 cm day−1 and 26 cm day−1 were obtained for upwelling northwest of Luzon and east of Mindanao, respectively.  相似文献   

10.
Between October 2001 and March 2002 six transects were completed at monthly intervals in the Sub-Antarctic Zone (SAZ) and Inter-Sub-Antarctic Front Zone (ISAFZ)/Polar Frontal Zone (PFZ) in the Southern Ocean south of Australia. Zooplankton were collected with a Continuous Plankton Recorder and NORPAC net and multivariate analysis was used to analyse the seasonal succession of communities. Despite strong, seasonally consistent, biogeographic differences between the SAZ and ISAFZ/PFZ, community structure in all zones was dominated by a suite of common taxa. These included the ubiquitous Oithona similis, foraminiferans and appendicularians (Core taxa), occurring in >97% of samples and contributing an average of 75% to total sample abundance, and Calanus simillimus, Rhincalanus gigas, Ctenocalanus citer, Clausocalanus brevipes, Clausocalanus laticeps, Oithona frigida, Limacina spp. and chaetognaths (Summer taxa), present in >57% of samples and occurring at seasonally high densities. Because of the dominance of the Core and Summer taxa, the seasonal succession was most clearly evident as a change in zooplankton densities. In October densities averaged <15 ind m−3, rising to 52 ind m−3 (max=92 ind m−3) in November, and subsequently increasing slowly through to January (ave=115 ind m−3; max=255 ind m−3). Densities peaked abruptly in February (ave=634 ind m−3; max=1593 ind m−3), and remained relatively high in March (ave=193 ind m−3; max=789 ind m−3). A latitudinal lag in seasonal development was observed with peak densities occurring first in the SAZ (February) and then in the ISAFZ/PFZ (March). The seasonal community succession was strongly influenced by species population cycles. The role of zooplankton in biogeochemical cycling in the SAZ and ISAFZ/PFZ was discussed in the light of past sediment trap data collected from the study area.  相似文献   

11.
Recent constructions of the global nitrogen budget estimate that at least half of the ocean's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. The Arctic contains approximately 20% of the world's continental shelf, suggesting it is a substantial contributor to the global sedimentary denitrification rate. During two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the Chukchi Sea in the western Arctic. Additionally, in the spring of 2004, denitrification rates were determined by whole-core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along three transects crossing the shelf and slope (50–3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1.6 mmol N m−2 d−1 for the shallow-water sediments to undetectable in deep-water sediments. Rates showed little variation between the two seasons. However, rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes. An extrapolation to the whole Arctic yielded an average Arctic sedimentary denitrification rate of 13 Tg N yr−1. Taken in the context of the global nitrogen budget, it is about 4–13% of the total sink of fixed nitrogen in the ocean.  相似文献   

12.
Biochemical and productivity measurements and nutrient enrichment experiments were conducted on three cruises in summer and two cruises in winter on the shelf and the basin of the northern South China Sea (SCS) between 2001 and 2004. Phytoplankton production, in terms of depth-integrated new production (INP) or depth-integrated primary production (IPP), was higher in winter than in summer and on the shelf than in the basin. In winter, with deepening of the mixed layer, nitrate from the shallow nitracline that characterized the SCS waters was made available in the surface and supported the highest production of the year. Averaged INP measured in winter (0.25 g C m−2 d−1) was about twice the summer average (0.12 g C m−2 d−1) and was 0.19 g C m−2 d−1 on the shelf compared with 0.15 g C m−2 d−1 in the basin. In winter, average INP on the shelf was higher than the basin (0.34 versus 0.21 g C m−2 d−1); whereas in summer, averaged INP on the shelf (0.13 g C m−2 d−1) and the basin (0.11 g C m−2 d−1) were similar. While averaged IPP measured in the basin was higher in winter than in summer (0.53 versus 0.35 g C m−2 d−1), IPP on the shelf showed little temporal variation (0.82 in winter versus 0.84 g C m−2 d−1 in summer). Considerable spatial and inter-annual variation in production was measured in the shelf waters during summer, which could be linked to discharge volume and plume flow direction of the Zhujiang River. While the shelf waters in summer were mostly nitrogen starved or nitrogen and phosphorus co-limited, excessive river runoff may cause the nutritive state to shift to phosphorus deficiency. Waters with low surface salinities and high fluorescence from riverine mixing could be found extending from the Zhujiang mouth to as far as offshore southern Taiwan after a typhoon passed the northern SCS and brought heavy rainfall. Overall, both nutrient advection in winter and river discharge from the China coast in summer made new nitrogen available and shaped the dynamics of phytoplankton production in these oligotrophic waters.  相似文献   

13.
Dense communities of shallow-water suspension feeders are known to sidestep the microbial loop by grazing on ultraplankton at its base. We quantified the diet, rates of water processing, and abundance of the deep-sea hexactinellid sponge Sericolophus hawaiicus, and found that, like their demosponge relatives in shallow water, hexactinellids are a significant sink for ultraplankton. S. hawaiicus forms a dense bed of sponges on the Big Island of Hawaii between 360 and 460 m depth, with a mean density of 4.7 sponges m−2. Grazing of S. hawaiicus on ultraplankton was quantified from in situ samples using flow cytometry, and was found to be unselective. Rates of water processing were determined with dye visualization and ranged from 1.62 to 3.57 cm s−1, resulting in a processing rate of 7.9±2.4 ml sponge−1 s−1. The large amount of water processed by these benthic suspension feeders results in the transfer of approximately 55 mg carbon and 7.3 mg N d−1 m−2 from the water column to the benthos. The magnitude of this flux places S. hawaiicus squarely within the functional group of organisms that link the pelagic microbial food web to the benthos.  相似文献   

14.
Abundance distribution and cellular characteristics of picophytoplankton were studied in two distinct regions of the equatorial Pacific: the western warm pool (0°, 167°E), where oligotrophic conditions prevail, and the equatorial upwelling at 150°W characterized by high-nutrient low-chlorophyll (HNLC) conditions. The study was done in September–October 1994 during abnormally warm conditions. Populations of Prochlorococcus, orange fluorescing Synechococcus and picoeukaryotes were enumerated by flow cytometry. Pigment concentrations were studied by spectrofluorometry. In the warm pool, Prochlorococcus were clearly the dominant organisms in terms of cell abundance, estimated carbon biomass and measured pigment concentration. Integrated concentrations of Prochlorococcus, Synechococcus and picoeukaryotes were 1.5×1013, 1.3×1011 and 1.5×1011 cells m−2, respectively. Integrated estimated carbon biomass of picophytoplankton was 1 g m−2, and the respective contributions of each group to the biomass were 69, 3 and 28%. In the HNLC waters, Prochlorococcus cells were slightly less numerous than in the warm pool, whereas the other groups were several times more abundant (from 3 to 5 times). Abundance of Prochlorococcus, Synechococcus and picoeukaryotes were 1.2×1013, 6.2×1011 and 5.1×1011 cells m−2, respectively. The integrated biomass was 1.9 g C m−2. Prochlorococcus was again the dominant group in terms of abundance and biomass (chlorophyll, carbon); the respective contributions of each group to the carbon biomass were 58, 7 and 35%. In the warm pool the total chlorophyll biomass was 28 mg m−2, 57% of which was divinyl chlorophyll a. In the HNLC waters, the total chlorophyll biomass was 38 mg m−2, 44% of which was divinyl chlorophyll a. Estimates of Prochlorococcus, Synechococcus and picoeukaryotes cell size were made in both hydrological conditions.  相似文献   

15.
During the ECOFER experiment (French ECOMARGE program), surficial sediments were sampled on the Aquitanian margin with box corers and analyzed to determine the quantity and quality of organic matter. Sediments from the margin are enriched in organic carbon (mean value 1.35%) in comparison to deep-sea and shelf sediments, due to a fine grain-size sedimentation. As sedimentation rates are high, the margin appears to be an organic depocenter. Some preferential organic enrichment zones were identified in the Cap-Ferret Canyon. There is a supply of continental material from the Gironde estuary, but marine contribution seems more possible than Adour or spanish rivers. No seasonal variations of organic matter were observed at the surface of sediments, suggesting mineralization processes of labile organic matter: average organic carbon consumption was evaluated to 9.0 g C m−2 yr−1. Rapid biological mineralization processes are lower than on the Mediterranean margin, mainly related to significant differences in water temperature. The great width of the canyon, its distance from the continent, and the current circulation pattern prevent any precise recording of the variable organic inputs to the sediment and favor nepheloı̈d transport, resuspension and shelf break processes, which wipe out any print of fresh material input. An organic carbon budget indicates that an equilibrium between organic inputs and organic mineralization+accumulation is not obtainable. The supply of suspended matter could have been minor during the year in question, and sedimentation rates are still imprecise.  相似文献   

16.
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   

17.
The often-rapid deposition of phytoplankton to sediments at the end of the spring phytoplankton bloom is an important component of benthic–pelagic coupling in temperate and high latitude estuaries and other aquatic systems. However, quantifying the flux is difficult, particularly in spatially heterogeneous environments. Surficial sediment chlorophyll-a, which can be measured quickly at many locations, has been used effectively by previous studies as an indicator of phytoplankton deposition to estuarine sediments. In this study, surficial sediment chlorophyll-a was quantified in late spring at 20–50 locations throughout Chesapeake Bay for 8 years (1993–2000). A model was developed to estimate chlorophyll-a deposition to sediments using these measurements, while accounting for chlorophyll-a degradation during the time between deposition and sampling. Carbon flux was derived from these estimates via C:chl-a = 75.Bay-wide, the accumulation of chlorophyll-a on sediments by late spring averaged 171 mg m−2, from which the chlorophyll-a and carbon sinking fluxes, respectively, were estimated to be 353 mg m−2 and 26.5 gC m−2. These deposition estimates were ∼50% of estimates based on a sediment trap study in the mid-Bay. During 1993–2000, the highest average chlorophyll-a flux was in the mid-Bay (248 mg m−2), while the lowest was in the lower Bay (191 mg m−2). Winter–spring average river flow was positively correlated with phytoplankton biomass in the lower Bay water column, while phytoplankton biomass in that same region of the Bay was correlated with increased chlorophyll-a deposition to sediments. Responses in other regions of the Bay were less clear and suggested that the concept that nutrient enrichment in high flow years leads to greater phytoplankton deposition to sediments may be an oversimplification. A comparison of the carbon flux associated with the deposition of the spring bloom with annual benthic carbon budgets indicated that the spring bloom did not contribute a disproportionately large fraction of annual carbon inputs to Chesapeake Bay sediments. Regional patterns in chlorophyll-a deposition did not correspond with the strong regional patterns that have been found for plankton net community metabolism during spring.  相似文献   

18.
Whereas diatoms (class Bacillariophyceae) often dominate phytoplankton taxa in the Amazon estuary and shelf, their contribution to phytoplankton dynamics and impacts on regional biogeochemistry are poorly understood further offshore in the western tropical Atlantic Ocean (WTAO). Thus, relative contribution of diatoms to phytoplankton biomass and primary production rates and associated environmental conditions were quantified during three month-long cruises in January–February 2001, July–August 2001, and April–May 2003. The upper water column was sampled at 6 light depths (100%, 50%, 25%, 10%, 1% and 0.1% of surface irradiance) at 64 stations between 3° and 14°N latitude and 41° and 58°W longitude. Each station was categorized as ‘oceanic’ or ‘plumewater’, based on principal component analysis of eight physical, chemical and biological variables. All stations were within the North Brazil Current, and plumewater stations were characterized by shallower mixed layers with lower surface salinities and higher dissolved silicon (dSi) concentrations than oceanic stations. The major finding was a much greater role of diatoms in phytoplankton biomass and productivity at plumewater stations relative to oceanic stations. Mean depth-integrated bSi concentrations at the plumewater and oceanic stations were 14.2 and 3.7 mmol m−2, respectively. Mean depth-integrated SiP rates at the plumewater and oceanic stations were 0.17 and 0.02 mmol m−2 h−1, respectively. Based on ratios of SiP and PP rates, and typical Si:C ratios, diatoms contributed on average 29% of primary productivity at plumewater stations and only 3% of primary productivity at oceanic stations. In contrast, phytoplankton biomass (as chlorophyll a concentrations) and primary production (PP) rates (as 14C uptake rates) integrated over the euphotic zone were not significantly different at plumewater and oceanic stations. Chlorophyll a concentrations ranged from 8.5 to 42.4 mg m−2 and 4.0 to 38.0 mg m−2 and PP rates ranged from 2.2 to 11.2 mmol m−2 h−2 and 1.8 to 10.8 mmol m−2 h−2 at plumewater and oceanic stations, respectively. A conservative estimate of annual integrated SiP in offshore waters of Amazon plume between April and August is 0.59 Tmol Si, based on mean SiP rates in plumewaters and satellite-derived estimates of the area of the Amazon plume. In conclusion, river plumewaters dramatically alter the silicon dynamics of the WTAO, forming extensive diatom-dominated phytoplankton blooms that may contribute significantly to the global Si budget as well as contributing to energy and matter flow off of the continental shelf.  相似文献   

19.
In order to study temporal variations of the genetic material in the continental shelf and deep-sea sediments of the extremely oligotrophic Cretan Sea, samples were collected on seasonal basis from August 1994 to September 1995, with a multiple corer, at seven stations (from 40 to 1540 m depth). Surface sediments (0–1 cm) were sub-sampled and analyzed for nucleic acid content (DNA, RNA) and bacterial density. DNA concentrations in the sediments were high (on annual average, 25.0 μg g-1) and declined with increasing water depth, ranging from 3.5 to 55.2 μg g-1. DNA concentrations displayed wide temporal changes also at bathyal depths confirming the recent view of the large variability of the deep-sea environments. Also RNA concentrations decreased with increasing water depth (range: 0.4–29.9 μg g-1). The ratio of RNA to DNA did not show a clear spatial pattern but was characterized by significant changes between sampling periods. DNA concentrations were significantly correlated with protein and phytopigment concentrations in the sediment, indicating a possible relationship with the inputs of primary organic matter from the photic layer. Bacterial densities were generally high (range: 0.9–4.6×108 cells g-1) compared to other deep-sea environments and decreased with increasing water depth. Estimates of the bacterial contribution to the sedimentary genetic material indicated that bacterial-DNA accounted, on annual average, for a small fraction of the total DNA pool (4.3%) but that bacterial-RNA represented a significant fraction of the total sedimentary RNA (26%). Bacterial contribution to nucleic acids increased, even though irregularly, with increasing depth. In deep-sea sediments, changes in RNA concentrations appear to be largely dependent upon bacterial dynamics. Estimates of the overall living contribution to the DNA pools (i.e. microbial plus meiofaunal DNA) indicated that the large majority (about 90%) of the DNA in continental and deep-sea sediments of the eastern Mediterranean was detrital. The non-living DNA pools reach extremely high concentrations up to 0.41 g DNA m-2 cm-1. Thus, especially in deep benthic habitats, characterized by low inputs of labile organic compounds, detrital DNA could represent a suitable and high quality food source or a significant reservoir of nucleic acid precursors for benthic metabolism.  相似文献   

20.
Investigations of lithogenic and biogenic particle fluxes using long-term sediment traps are still very rare in the northern high latitudes and are restricted to the arctic marginal seas and sub-arctic regions. Here data on the variability of fluxes of lithogenic matter, CaCO3, opal, and organic carbon and biomarker composition from the central Arctic Ocean are presented for a 1-year period. The study was carried out on material obtained from a long-term mooring system equipped with two multi-sampling traps, at 150 and 1550 m depth, and deployed on the southern Lomonosov Ridge close to the Laptev Sea continental margin from September 1995 to August 1996. In addition, data from surface sediments were included in the study. Annual fluxes of lithogenic matter, CaCO3, opal, and particulate organic carbon were 3.9, 0.8, 2.6, and 1.5 g m−2 y−1, respectively, in the shallow trap and 11.3, 0.5, 2.9, and 1.05 g m−2 y−1, respectively, in the deep trap.Both the shallow and the deep trap showed significant variations in vertical flux over the year. Higher values were found from mid-July to the end of October (total mass flux of 75–130 mg m−2 d−1 in the shallow trap and 40–225 mg m−2 d−1 in the deep trap). During all other months, fluxes were fairly low in both traps (most total mass flux values <10 mg m−2 d−1). The interval of increased fluxes can be separated into (1) a mid-July/August maximum caused by increased primary production as documented in high abundances of marine biomarkers and diatoms and (2) a September/October maximum caused by increased influence of Lena River discharge indicated by maximum lithogenic flux and large amounts of terrigenous/fluvial biomarkers in both traps. During September/October, total mass fluxes in the deep trap were significantly higher than in the shallow trap, suggesting a lateral sediment flux at greater depth. The lithogenic flux data also support the importance of sediment input from the Laptev Sea for the sediment accumulation on the Lomonosov Ridge on geological time scales, as indicated in sedimentary records from this region.  相似文献   

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