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1.
We compared wintertime depth distributions of the mesozooplankton community and dominant copepods between the subtropical (S1) and subarctic (K2) Pacific Oceans to evaluate the relative importance of actively transported carbon by vertical migrants to sinking particulate organic carbon flux. Primary production was higher and the ratio of sinking particulate organic carbon flux to primary production was lower at S1 compared with those at K2. The mesozooplankton community was lower in abundance and biomass at S1 compared to K2. Copepods were the dominant group among both mesozooplankton abundance and biomass throughout the water column down to 1000 m at both sites. The depth distribution showed that diel vertical migration was obvious for the mesozooplankton abundance and biomass at S1 but was not apparent for the abundance at K2, because the dominant component was diurnally migrating species at S1 and overwintering species residing at mesopelagic depths at K2. The major components of diel migrants were copepods and euphausiids at S1 and only euphausiids at K2. Respiratory flux by the diurnally migrating mesozooplankton was estimated to be 2 mgC m−2 day−1 at S1 and 7 mgC m−2 day−1 at K2. The respiratory flux was equivalent to 131% of sedimentary fecal pellet flux at S1 and 136% of that at K2. Because pathways of downward carbon flux are facilitated by the mesozooplankton community, the actively transported carbon (respiration of dissolved inorganic carbon, excretion of dissolved organic carbon and egestion of fecal pellets at depth) might be larger during winter than the flux of sinking fecal pellets.  相似文献   

2.
The distribution, biomass, and assemblages of vertically migrating micronekton/macrozooplankton were studied in relation to oceanographic conditions around Guam and the adjacent Northern Mariana Islands during Spring 2010, using 3-m2 Isaacs-Kidd Midwater Trawl (IKMT). The study area was located within the oligotrophic waters of the westward flowing North Equatorial Current (NEC). However, southern stations of the survey were situated close to the northern boundary of the more productive North Equatorial Countercurrent (NECC), where we observed the highest biomass, abundance, species richness, and diversity of pelagic organisms. Overall, we recorded 85 species from 20 families of mostly mesopelagic species in the area, with lanternfishes (Myctophidae—40 species) and dragonfishes (Stomiidae—18 species) being the most taxonomically diverse groups. Three genera of mesopelagic shrimps, Sergestes, Janicella and Sergia, dominated the decapod crustacean component of the micronekton community numerically and by biomass, while the contribution from cephalopods was relatively minor. Assemblages of major micronekton/macrozooplankton groups, based on biomass and abundance showed principal changes with latitude. However, the classification and ordination analysis, based on taxonomically resolved taxa (fishes and decapod shrimps), indicated additional zonal variation, with areas east and west of the island chain showing different community structure. The mean total micronekton biomass for the area near the productive boundary region between the NEC and NECC was 5.8 mg/m3, with a mean biomass of 1.2 mg/m3 obtained for stations in the oligotrophic NEC area. The corresponding biomass of mesopelagic fishes was 0.88 mg/m3 and 0.24 mg/m3 for these two areas, respectively. We reviewed and compared the available information on the quantitative distribution of midwater fish biomass in the western tropical Pacific and outlined major patterns of variation in the equatorial Pacific in general.  相似文献   

3.
The biomass, species and chemical composition of the mesozooplankton and their impact on lower food levels were estimated along a transect across the Arctic Ocean. Mesozooplankton biomass in the upper 200 m of the water column was significantly higher (19–42 mg DW m-3) than has previously been reported for the Arctic Ocean, and it reached a maximum at ca. 87°N in the Amundsen Basin. The lowest values were recorded in the Chukchi Sea and Nansen Basin, where ice cover was lower (50–80%) than in the central Arctic Ocean. In the deeper strata (200–500 m) of the Canadian and Eurasian Basins, the biomass was always much lower (4.35–16.44 mg DW m-3). The C/N (g/g) ratio for the mesozooplankton population was high (6.5–8.5) but within the documented range. These high values (when compared to 4.5 at lower latitudes) may be explained by the high lipid content. Mesozooplankton accounted for approximately 40% of the total particulate organic carbon in the upper 100 m of the water column. Mesozooplankton species composition was homogeneous along the transect, consisting mainly of copepods (70–90% of the total number). It was dominated by four large copepod species (Calanus hyperboreus, C. glacialis, C. finmarchicus and Metridia longa), which together accounted for more than 80% of the total biomass. According to measurements of gut pigment and gut turnover rates, the mesozooplankton on average ingested between 6 and 30% of their body carbon per day as phytoplankton. Microzooplankton may have provided an additional source of energy for the mesozooplankton community. These data emphasize the importance of mesozooplankton in the arctic food web and reinforce the idea that the Arctic Ocean should no longer be considered to be a “biological desert”.  相似文献   

4.
IronEx I demonstrated a rapid and marked response by grazers to Fe-induced increases in phytoplankton stocks, which was thought to be due, in part, to arrested vertical migration by mesozooplankton. These observations prompted an investigation of the relative roles of Fe enrichment and grazing pressure in controlling the magnitude of phytoplankton stocks in the NE subarctic Pacific. The grazing impact of increased mesozooplankton abundance in response to a localised Fe-induced enhancement of algal biomass was simulated by performing in vitro (6 d) grazer perturbation experiments in May 1994 and September 1995 at Ocean Station Papa (OSP), when pelagic mesozooplankton stocks are usually at their annual maximum and submaximal, respectively. Manipulations were designed to increase mesozooplankton stocks in 25L carboys after various lag-times corresponding to grazing pressure greater or equal to that in situ, and to monitor changes in chlorophyll a levels as a proxy for grazing pressure. At the onset of the experiments, in vitro mesozooplankton abundances were comparable to those in situ. Despite the addition of mesozooplankton to selected Fe-enriched carboys in May after 24, 48 and 72 h, corresponding to ca. two-fold increases in their abundances, chlorophyll a increased to ca. 2 μg l−1 in all treatments. In September, chlorophyll a levels increased five-fold to 2 μg l−1 after 4 days – but little thereafter – in the presence of up to ten-fold higher animal abundances (added at t=0) than observed in situ. Thus, Fe-induced increases in diatom growth rates were sufficiently high to escape both initial and additional grazing pressure. If and when Fe is supplied to this region, it is unlikely that mesozooplankton can respond and graze down the resulting elevated algal abundance. Theoretical calculations, based on algal growth and grazing rate data from May in this study, suggested that a greater than five-fold increase in mesozooplankton abundance, after a 48-h lag, is required to exert sufficient grazing pressure to prevent Fe-mediated increases in algal biomass. These findings are discussed in relation to the scale dependency of such events, and the pelagic ecology of other High Nitrate Low Chlorophyll regions.  相似文献   

5.
The results of two oceanographic surveys designed to delineate the flow response near Cato Island (155°32′E, 23°15′S) in the Western Coral Sea are presented. The surveys were conducted in October 1992 and February 1993 and coincided with conditions of strong, steady incident currents and relatively weak, variable currents, respectively. For the strong inflow case study, a surface-intensified cyclonic eddy observed in the wake of the island was co-incident with a zone of strong upwelling. Isotherm displacements within the eddy were in excess of 90 m. The lee side response was strongly depth dependent, with recirculation confined to the upper 120 m. A dynamical systems approach incorporating ADCP data was used to compute Lagrangian trajectories numerically for particles released at various locations in the wake zone. There was no evidence of enhanced chlorophyll concentrations downstream of the island. Comparisons with other dynamically similar studies indicate that eddy shedding is likely during periods of steady incident currents. During the second survey, weaker incident currents resulted in a less pronounced flow disturbance. Small isothermal displacements were capped beneath the strong seasonal thermocline. Lee side currents were weak and variable, with recirculation confined to the upper 50 m. A strong biological response was observed downstream, with increased integrated chlorophyll content and zooplankton biomass in the lee providing evidence of the island mass effect.  相似文献   

6.
Microzooplankton herbivory in the Arabian Sea was measured using dilution experiments towards the end of the SW monsoon in September and during the intermonsoon to NE monsoon period in November–December 1994. Microzooplankton grazing resulted in a turnover of phytoplankton stocks that ranged from 11 to 49% per day. This was equivalent to grazing fluxes of between 1 and 17 mg C m-3 d-1. Depth-integrated microzooplankton herbivory ranged between 161 and 415 mg C m-2 d-1 during the SW monsoon cruise, and between 110 and 407 mg C m-2 d-1 during the intermonsoon period. Microzooplankton grazed between 4 and 60% of daily primary production, with higher percentages found during the intermonsoon season. Phytoplankton growth rates during the SW monsoon ranged from 0.3 to 1.8 d-1, with lower values in upwelling waters and higher values in downwelling and oligotrophic areas. During the intermonsoon period, phytoplankton growth was more uniform across the basin and averaged 0.68±0.15 d-1. Microzooplankton abundance in experimental samples varied between 2800 and 16 162 cells l-1, equivalent to a biomass of between 1.1 and 7.2 mg C m-3. The mean cell carbon content of microzooplankton was similar in both periods and ranged from 0.33 to 0.55 ng C cell-1. Microzooplankton were smallest in downwelling waters and largest in oligotrophic waters. Average clearance rates in those taxa that took up fluorescently-labelled algae ranged from 0.2 to 14 μl ind-1 hr-1. Average mesozooplankton grazing rates, derived from biomass data, varied from 19 to 92 mg C m-2 d-1; these rates accounted for removal of between 4 and 12% of the daily primary production. Mesozooplankton herbivory was most pronounced in upwelling and downwelling waters and reduced in stratified oligotrophic waters during the SW monsoon period. Microzooplankton herbivory was greater than the average mesozooplankton herbivory at all stations, during both the SW monsoon and intermonsoon periods.  相似文献   

7.
Zooplankton communities, studied in the surface mixed layer on a 1600 m transect across the Arabian Sea, were found to differ in their temporal and spatial response to seasonal forcing. The transect studied, spanned seasonally eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic waters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozooplankton showed a clear response to both upwelling and season. The heterotrophic nanoflagellates were concentrated in the surface mixed layer, except in the eutrophic upwelling waters of the SW monsoon. They reached maximum cell concentrations of 855 ml-1 during the SW monsoon and a maximum biomass of 8.4 mg C m-3 during the intermonsoon. Nanozooplankton standing stocks, in the surface mixed layer, ranged between 7 and 333 mg C m-2, with highest stocks found during the intermonsoon. The microzooplankton community was dominated by Protozoa, particularly aloricate ciliates and heterotrophic dinoflagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower numbers (<400 l-1). The microzooplankton were also concentrated in the surface mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found below the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg C m-2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total mesozooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 1248 to 238 mg C m-2 during the late SW monsoon and 656–89 mg C m-2 during the following intermonsoon. The largest size class, of 1000–2000 μm sized organisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200–500 μm, were more important. The shift in size structure from large to small zooplankton occurred in response to a shift in dominance from large to small phytoplankton cells both spatially, along a eutrophic–oligotrophic gradient, and seasonally. These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea.  相似文献   

8.
The vertical distribution (0–900 m) of zooplankton biomass and indices of feeding (gut fluorescence, GF) and metabolism (electron transfer system, ETS) were studied across an anticyclonic eddy south of Gran Canaria Island (Canary Islands). Two dense layers of organisms were clearly observed during the day, one above 200 m and the other at about 500 m, coincident with the deep scattering layer (DSL). The biomass displacement due to interzonal migrants in the euphotic zone was more than 2-fold higher than that previously reported for the southern area of this archipelago. The gut flux estimated (0.14–0.44 mgC m−2 d−1) was similar to the values previously found in the Canaries. The respiratory flux outside the eddy (1.85 mgC m−2 d−1) was in the lower range of values reported for this area. Inside the eddy, migrant biomass and respiration rates were 2- and 4- fold higher than in the surrounding waters. Active flux mediated by diel vertical migrants inside the eddy (8.28 mgC m−2 d−1) was up to 53% of the passive carbon flux to the mesopelagic zone (15.8 mgC m−2 d−1). It is, therefore, suggested that the anticyclonic eddy enhanced both migration from deep waters and active flux.  相似文献   

9.
During two cruises to the Greenland Sea, we studied the abundance and biomass of the sea ice biota in summer and late autumn. The mean calculated biomass of the sympagic community was 0.2 g C m−2 ice. Algae contributed on average 43% to total biomass, followed by bacteria (31%), heterotrophic flagellates (20%), and meiofauna (4%). Diatoms were the main primary producers (60% of total algal biomass), but flagellated cells contributed significantly to the algal biomass. Among the meiofauna, ciliates, nematodes, acoel turbellarians and crustaceans were dominant. Calculated potential ingestion rates of meiofauna (0.6 g C m−2 (120 d)−1) are on the same order of magnitude as annual primary production estimates for Arctic multi-year sea ice. We therefore assume that grazing can control biomass accumulation of primary producers inside the sea ice.  相似文献   

10.
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11.
Zooplankton biomass, gut fluorescence and electron transfer system (ETS) activity were measured in vertical profiles (0–900 m) in two different size classes (<1 and >1 mm) in Canary Island waters. Both size fractions displayed a typical pattern of distribution with higher biomass, gut fluorescence and ETS in the shallower layers at night. By day, however, the vertical distribution varied between the size fractions, with higher biomass of the small fraction in the 0–200 m and a layer of large organisms at depth (∼500 m). For both size fractions, average ETS activity was higher by day than at night at depths between 200 and 600 m. Similarly, gut fluorescence was slightly higher by day below 200 m. The downward export of respiratory carbon was 1.92 and 4.29 mg C m−2 d−1 for samples obtained southwest of Gran Canaria Island and west of Tenerife Island respectively, being 2.68 mg C m−2 d−1 for the whole area. These values represented 16–45% (22–28% for the area) of the calculated passive particulate export production resulting from primary production. The estimated “gut flux” accounted for 0.35 (western zone) and 2.37 mg C m−2 d−1 (southwest of Gran Canaria), being 1.28 mg C m−2 d−1 for the whole area and represented between 3 and 25% (11–14% for the whole area) of the estimated passive particle export flux. These results agree with previous estimates and suggest that diel-migrant zooplankton can play an important role in the downward flux of carbon.  相似文献   

12.
Using simultaneous sampling with a commercial-sized trawl, a zooplankton net, and a sediment trap, we evaluated the contribution of vertically migrating micronekton to vertical material transport (biological pump) at two stations (3°00′N, 146°00′E and 3°30′N, 145°20′E) in the western equatorial North Pacific. The gravitational sinking particulate organic carbon flux out of the euphotic zone was 54.8 mg C m−2 day−1. The downward active carbon flux by diel migrant mesozooplankton was 23.53 and 9.97 mg C m−2 day−1, and by micronekton 4.40 and 2.26mg C m−2 day−1 at the two stations. Assuming that the micronekton sampling efficiency of the trawl was 14%, we corrected the downward carbon flux due to micronekton respiration to 29.9 and 15.2mg C m−2 day−1, or 54.6 and 27.7% of the sinking particle flux at the two stations. The corrected micronekton gut fluxes were 1.53 and 0.97mg C m−2 day−1. The role of myctophid fish fecal matter as a possible food resource for deep-sea organisms, based on its fatty acid and amino acid analysis, is discussed.  相似文献   

13.
Despite the fact that marine viruses have been increasingly investigated in the last decade, knowledge on virus abundance, biomass and distribution in mesopelagic and bathypelagic waters is limited. We report here the results of a large-spatial-scale study (covering more than 3000 km) on the virioplankton distribution in epi-, meso- and bathypelagic waters in 19 areas of the Mediterranean Sea, from the Alboran Sea and Western Mediterranean, to the Tyrrhenian Sea, Sicily Channel and Ionian Sea. Integrated viral abundance in epipelagic waters was significantly higher than in deep-sea waters (on average, 2.4 vs. 0.5×1012 viruses m−3). However, abundance of viruses in the deep-Mediterranean waters was the highest reported so far for deep seas worldwide (7.0 and 3.1×1011 viruses m−3 in mesopelagic and bathypelagic waters, respectively) and their biomass accounted for 13–18% of total prokaryotic C biomass. The significant relationship between viral abundance and prokaryotic abundance and production in deep waters suggests that also deep-sea viruses are closely dependent on the abundance and metabolism of their hosts. Moreover, virus to prokaryote (and nucleoid-containing cell (NuCC)) abundance ratio increased with increasing depths suggesting that deep waters may represent optimal environments for viral survival or proliferation. Overall, our results indicate that deep waters may represent a significant reservoir of viruses and open new perspectives for future investigations of viral impact on the functioning of meso-bathypelagic ecosystems.  相似文献   

14.
Seawater samples were collected for microbial analyses between 55 and 235 m depth across the Arctic Ocean during the SCICEX 97 expedition (03 September–02 October 1997) using a nuclear submarine as a research platform. Abundances of prokaryotes (range 0.043–0.47×109 dm−3) and viruses (range 0.68–11×109 dm−3) were correlated (r=0.66, n=150) with an average virus:prokaryote ratio of 26 (range 5–70). Biomass of prokaryotes integrated from 55 to 235 m ranged from 0.27 to 0.85 g C m−2 exceeding that of phytoplankton (0.005–0.2 g C m−2) or viruses (0.02–0.05 g C m−2) over the same depth range by an order of magnitude on average. Using transmission electron microscopy (TEM), we estimated that 0.5% of the prokaryote community on average (range 0–1.4%) was visibly infected with viruses, which suggests that very little of prokaryotic secondary production was lost due to viral lysis. Intracellular viruses ranged from 5 to >200/cell, with an average apparent burst size of 45±38 (mean±s.d.; n=45). TEM also revealed the presence of putative metal-precipitating bacteria in 8 of 13 samples, which averaged 0.3% of the total prokaryote community (range 0–1%). If these prokaryotes are accessible to protistan grazers, the Fe and Mn associated with their capsules might be an important source of trace metals to the planktonic food web. After combining our abundance and mortality data with data from the literature, we conclude that the biomass of prokaryoplankton exceeds that of phytoplankton when averaged over the upper 250 m of the central Arctic Ocean and that the fate of this biomass is poorly understood.  相似文献   

15.
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   

16.
The bathymetric distribution, abundance and diel vertical migrations (DVM) of zooplankton were investigated along the axis of the Cap-Ferret Canyon (Bay of Biscay, French Atlantic coast) by a consecutive series of synchronous net hauls that sampled the whole water column (0–2000 m in depth) during a diel cycle. The distribution of appendicularians (maximum 189 individuals m−3), cladocerans (maximum 287 individuals m−3), copepods (copepods<4 mm, maximum 773 individuals m−3, copepods>4 mm, maximum 13 individuals m−3), ostracods (maximum 8 individuals m−3), siphonophores (maximum >2 individuals m−3) and peracarids (maximum >600 individuals 1000 m−3) were analysed and represented by isoline diagrams. The biomass of total zooplankton (maximum 18419 μg C m−3, 3780 μg N m−3) and large copepods (>4 mm maximum 2256 μg C m−3, 425 μg N m−3) also were determined. Vertical migration was absent or affected only the epipelagic zone for appendicularians, cladocerans, small copepods and siphonophores. Average amplitude of vertical migration was about 400–500 m for ostracods, some hyperiids and mysids, and large copepods, which were often present in the epipelagic, mesopelagic, and bathypelagic zones. Large copepods can constitute more than 80% of the biomass corresponding to total zooplankton. They may play an important role in the active vertical transfer of carbon and nitrogen.  相似文献   

17.
Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m−2 in the surface mixed layer, 0–30 m) was only 25% of the initial inventory, which was 1/3–2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus plumchrus. Feeding of MZ (247.2 mmol m−2 during Day 0–21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14–21), the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m−2), 89% (72.8 mmol N m−2) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m−2, 1392 mmol C m−2) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition.  相似文献   

18.
Mero- and holoplanktonic organisms from 23 large taxa have been detected in the coastal waters of Morocco. Seven Cladocera species and 164 Copepoda species were identified. Copepod fauna mostly consisted of oceanic epipelagic widely tropical species, but the constant species group (frequency of occurrence over 50%) included neritic and neritic–oceanic widely tropical species. The neritic community that formed a biotopic association with coastal upwelling waters and the distant-neritic community associated with Canary Current waters were the two major communities detected. The former community was characterized by a high abundance and biomass (5700 ind./m3 and 260 mg/m3) and predominance of neritic species. The trophic structure was dominated by thin filter feeders, mixed-food consumers, and small grabbers; the species structure was dominated by Paracalanus indicus, Acartia clausi, and Oncaea curta; the indices of species diversity (3.07 bit/ind.) and evenness (0.63) were relatively low. The latter community was characterized by low abundance and biomass (1150 ind./m3 and 90 mg/m3); variable biotopic, trophic, and species structure; and higher Shannon indices (3.99 bit/ind.) and Pielou (0.75). Seasonal variation of the abundance of organisms was not detected in the communities. Anomalous mesozooplankton states were observed in summer 1998 and winter 1998–1999.  相似文献   

19.
This paper addresses how large aggregations of fish found on many seamounts are sustained. We used a generic seamount ecosystem model from the Northeast Atlantic to examine the impact of a potential increase of local primary production on higher trophic levels, to quantify the immigration of allochthonous micronekton that would be required to maintain a “typical” seamount community, and to quantify if the necessary immigration ratios could be supported by local oceanographic conditions. Our simulation predictions indicate a lack of autochthonous resources in the system to support large amounts of seamount aggregating fish. In other words, autochthonous seamount production may be responsible for sustaining only a small amount of its total biomass. Additionally, our study supports the idea that enhancement of primary productivity also cannot sustain large aggregations of seamount fish. Our seamount model, which took into account high abundances of fish, marine mammals, seabirds and tuna, required a total immigration of allochthonous micronekton of 95.2 t km−2 yr−1 less than the potential available biomass after considering the immigration of prey based upon average current velocities and prey standing stocks in oceanic waters. Our model predicted that the horizontal flux of prey would be sufficient to sustain the rich communities living on seamounts.  相似文献   

20.
We use hydrographic and buoy data to compare the initial temperature fields and Lagrangian evolution of water parcels in two vortices generated by the southward flowing Canary Current passing around the island of Gran Canaria Island. One vortex is anticyclonic, shed in June 1998 as the result of an incident current of about 0.05 m s−1, and the second one is cyclonic, shed in June 2005 with the impinging current estimated as 0.03 m s−1. The two vortices exhibit contrasting characteristics yet display some important similarities. The isopycnals are depressed in the core of the anticyclonic vortex, at least down to a depth of 700 m, whilst they dome up in the core of the cyclonic vortex but only down to 450 m. In the top 300 m the depression/doming of the isotherms is similar for both vortices, with a maximum vertical displacement of the isotherm of about 80 m, which correspond to temperature anomalies of some 2.5 °C at a given depth. A simple method is developed to obtain the initial orbital velocity field from the temperature data, from which we estimate peak values of 0.7 and 0.5 m s−1 for the anticyclonic and cyclonic vortices, respectively. The buoys, three for the anticyclonic vortex and two for the cyclonic one, were drougued at 100 m depth, below the surface mixed layer, and their initial velocities are consistent with the above values. In both vortices, the buoys revolve either within a central core, where the rotation rate remains stable and large for several weeks, or in an outer ring, where the rotation rate is significantly smaller and displays large radial fluctuations. Within the inner core the anticyclonic vortex has significant inward radial velocity, while the cyclonic vortex has near-zero radial mean motions. The cyclonic vortex rotates more slowly than the anticyclonic, their initial periods being 4.5 and 2.5 days, respectively. A simple axisymmetric model with radial diffusion (coefficient Kh≅25 m2 s−1) and advection reproduces the observations reasonably well, the diffusive effect being more important than that resulting from the observed radial advection. The model also supports the hypothesis that the rotation rate of cyclonic vortices is less than that of anticyclonic vortices, as otherwise they would become inertially unstable. Both the buoys data and sea surface temperature images confirm that the vortices evolve from youth to maturity, as the cores shrink and the outer rings expands, and then to a decay stage, as the core rotation rates decrease, though frequent interactions with other mesoscale structures result in more accelerated aging. Despite these interaction they last many months as coherent structures south of the Canary Islands.  相似文献   

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