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1.
An observation on differences in the vertical migration patternbetween Meganyctiphanes norvegica (M. Sars) and Thysanoessaraschii (M. Sars) caused by the presence of a thermocline isreported. Generally, M.norvegica did not migrate through a thermoclineat 50–60 m depth while T.raschii did. This observationis discussed in the context of how the combination of differencesin vertical migration and advection over sill depth may affectthe abundance of fjord stocks of these two euphausiid species.  相似文献   

2.
Discrete-depth, hourly mesozooplankton samples were collectedover a 92 h period in May 1992 at an anchor station within theSeine Region of Freshwater Influence (ROFI) (English Channel).The mesozooplankton community defined as a euryhaline marineassemblage was dominated by the calanoid copepods Acartia spp.,Temora longicornis and Centropages hamatus, the cladoceran Evadnenordmanni and the appendicularian Oikopleura dioica. The semi-diurnaltidal current was the dominant factor in determining the short-termtemporal changes in the community in terms of density and speciescomposition so that zooplankton patches displayed oscillatingmotion in relation to tidal advection. Although a few species(e.g. Pleurobrachia pileus) exhibited higher densities aroundlow tide, maximum densities were observed for most species (e.g.T.longicornis and E.nordmanni) around high tide, according tosalinity variations. Diurnal changes were only reported forcyclopoid copepods (i.e. Halicyclops sp. and Cyclopina sp.)which wer$$$ mainly endobenthic during the day and moved intothe water column at night. Besides temporal changes in depth-averageddensities, most species exhibited vertical migrations at dieland/or tidal periods. Tidal vertical migrations were reportedonly for a few taxa and could be the result of passive mechanisms(e.g. vertical mixing) rather than of active behaviour. Dielvertical migrations were observed in most of the abundant taxa.While this migration pattern did not appear to be an adaptationto predator avoidance within the Seine ROFI, it could regulatehorizontal transport of organisms and promote their retention.The consequences of the short-term mesozooplankton fluctuationsfor sampling designs are discussed.  相似文献   

3.
Due to hydro-electric power production, the freshwater inputto the Sandsfjord system has increased during winter, whilethe freshwater input to Jøsenfjorden has been removed.In studies conducted during autumn and winter, both Meganyctiphanesnorvegica and Thysanoessa raschii were most common in Jøsenfjorden,and T.raschii was almost wanting in the Sandsfjord system. Theeuphausiids generally swam to the surface layer at night, butavoided low salinity water in the uppermost metres of the Sandsfjordsystem. Still, in most locations 50% of M.norvegica was foundin the upper 10 m. Their vertical distribution overlapped withfreshwater-driven currents, and estuarine circulation was apparentlyprominent in governing the horizontal distribution within theSandsfjord system. At the entrance to the fjord, M. norvegicawas distributed deeper than within the fjord, reducing the importanceof transport with estuarine circulation relative to currentsdriven by wind at the coast. Estimates of advective rates ofM.norvegica suggest that exchange with coastal waters was moreimportant for the standing stock than local growth within thefjords. In Jtfsenfjorden, maxima of T.raschii were always foundin a zone of shallowing bottom topography (bottom depth 130m), where this species possibly maintained its horizontal positionby means of vertical migrations.  相似文献   

4.
Marine cladocerans in the zooplankton samples collected vertically(0–500 m) with Norpac nets at 2–4 week intervalsfrom February 1990 through January 1991 at a station in ToyamaBay, southern Japan Sea, were examined. From mid-March throughearly December, cladoceran populations exhibited five distinctpeaks. in which seven species occurred and showed a species-specific pattern of seasonal distribution. In mid-July. Peniliaavirostris exhibited a prominent peak in population density.Three species of Evadne, E.nordmanni, E.spinifera and E.tergestina,showed a clear ecological separation in time. The first twospecies had a peak density in April and July. respectively,but the last species demonstrated three distinct peaks betweenJune and September. Two congeneric species of the genus Podon,P.leuckarti and P.schmackeri, also did not co-exist in time.Simultaneous horizontal hauls with MTD nets made in June andSeptember 1986 revealed a strikingly marked surface distributionof all species. Evadne nordmanni occurred in June and demonstrateda clear ‘reverse’ did vertical migration, residingin the 30 m depth during the night time and ascending up tothe uppermost water column of 0–10 m during the daytime.Among the four species which occurred in September, E.spinifera,E.tergestina and P.schmackeri also showed a reverse migrationbetween the 30 m depth during the night and the very surfaceduring the day. A different pattern was observed in P.avirostrzs,which was distributed at a depth of 30 m during both the dayand the night with no sign of diel vertical migration. Amongthe more prevalent parthenogenetic individuals, some gamogeneticforms were found to exist in most species occurring in ToyamaBay.  相似文献   

5.
6.
A study was performed to test the hypothesis that Atlantic cod Gadus morhua captured within fjords do not migrate across the sill into outer fjord segments or coastal waters. An 18 month long telemetry experiment, corroborated by recapture patterns, showed that 40% of the tagged fish crossed the sill. The majority of the fish that crossed the sill, however, returned after short excursions of only a few days. Many fish remained stationary at or near the release location, and migrations up the fjord into brackish water appeared more frequent than migration out of the fjord. The stable isotope analyses showed significant differences between the inner and the outer fjord segments, and this was seen for both adults and juveniles. This suggests that the Atlantic cod in the two areas have different diets and probably utilize different foraging ranges. Thus, both the isotope studies and the telemetry experiment suggest limited cross-sill migration and rather high site fidelity. Marketing restrictions only apply to fish caught in the fjord proper, not the comparatively clean outer fjord system beyond the sill. Based on this study of migratory tendencies, it is concluded that contamination levels in Atlantic cod in both the Frierfjord proper and the outer fjords probably result from local exposure and appear little affected by inter-fjord migration.  相似文献   

7.
We examined the vertical migration behavior of scallop (Placopectenmagellanicus) veligers in mesocosms and in previously reportedfield studies. Evidence suggests that these bivalve veligersmigTate in response to both tidal and diurnal stimuli in a mannersimilar to a proposed tidal/diel model. Both populations havea diurnal response to solar cues. The response to tidal cuesdiffers between the Georges Bank and Passamaquoddy Bay populations.Georges Bank veligers appear to utilize the differences in tidalphase that occur with depth to transport them in a northeasterlydirection, thus maintaining the population on the bank. PassamaquoddyBay veligers respond by swimming up at slack water (high andlow tides) and down when currents are strongest. Such behaviorwould minimize dispersal on the strong tidal currents in theBay of Fundy and thus also tend to maintain a population withinan area. Horizontal transport resulting from vertical migrationis the most likely selective pressure to CTeate and maintainthese different behaviors against the homogenizing effects ofmigration between the two populations. The implications of inheriteddifferences in behavior probably require consideration in themanagement of both wild and cultured populations. Common samplingpractices that obscure the tidal part of tidal/diel migration,including averaging the results from several days of sampling,sampling too infrequently to perceive a tidal periodicity, and assuming that only behavior that changes at high and lowtides will affect horizontal transport, need to be avoided instudies of vertical migration.  相似文献   

8.
The Eastern English Channel is known for its strong hydrodynamics.Tidal and residual currents are reinforced by the south-westerndominant wind and drift waters from the English Channel to theNorth Sea. Previous spatial studies have shown that the advectionof larvae could differ from one species to another. Flounder(Pleuronectes flesus) larvae were found offshore, drifting tothe north until the fins were formed; then they were found nearthe coast. However, sole (Solea solea) larvae remained in coastalwaters during their development. The difference in larval spatialdistribution is assumed to be related to the interaction betweenvertical migration and advection by alternative tidal currents,leading to a selective tidal stream transport. To describe thevertical distribution of these larvae, two strategies were used.First, a Eulerian study was carried out with samples taken atthe same geographical location every 1.5 h for 41 h. Ichthyoplanktonwere collected in the water mass using a Bongo net and witha suprabenthic multi-net sledge, at four layers above the seabed, between 0.1 and 1.4 m. Secondly, to enable water movementto be disregarded, a Lagrangian study was carried out by usinga Bongo net every 3 h, following a drifting buoy for 3 days.The results show that even during the youngest stages, solelarvae are able to perform tidal and diel vertical migration.We assume that they may limit their advection to the North Seabecause of their upward migration during ebb and at night, whichmay enable them to remain in the same area dealing with thecurrents. Flounder larvae begin their vertical migration atthe stage of notochord flexion, which ends their drift to thenorth. The larvae reach the bottom of the water column, particularlyduring ebb when they are concentrated in the first 40 cm abovethe bottom. This behaviour favours their advection during flow,leading to efficient and fast transport towards the coast.  相似文献   

9.
Kaartvedt distinguished between drifting and resident planktonand hypothesized that the latter were distinguished by theirability to maintain their horizontal position in desired habitats(Kaartvedt, 1993). In this study, we examined the populationgenetic consequences of these two lifestyles for copepods infour fjords of western Norway (Lurefjorden, Masfjorden, Sognefjordenand Sørfjorden) and one fjord in eastern Norway (Oslofjorden).Based on DNA sequence variation of a region of mitochondrial16S rRNA, we contrasted population genetic diversity and structurein drifting populations of Calanus spp. with that of residentpopulations of Acartia clausi. With the exception of Sørfjorden(where Calanus spp. were rare), two or three species of Calanusco-occurred in significantly different proportions in the fjords.Based on a 350 base-pair region of mitochondrial 16S rRNA, Calanusspp. varied in molecular genetic diversity, with the highestvalues for C.helgolandicus. There was no evidence of significantgenetic structure of fjord populations for either C.finmarchicusor C.helgolandicus; the population structure of C.glacialiscould not be evaluated as the species was only abundant in Lurefjorden.Acartia clausi was abundant in all five fjords sampled for thisstudy. Molecular genetic diversity of A.clausi, based on a 220bp region of mt 16S rRNA, was within the range of Calanus spp.values. Populations of A.clausi showed significant genetic structure(i.e. haplotype frequencies differed markedly) among the fjords.The results of this study indicated that little exchange (geneflow) occurs between populations of A.clausi in different fjords,and suggested that the populations are long-term residents ofa fjord. In contrast, most Calanus spp. fjord populations maybe replaced periodically, as they drift with currents flowingto and from coastal and fjord environments.  相似文献   

10.
11.
The exchange of zooplankton between two basins separated bya sill in the upper St. Lawrence estuary, Québec (Canada),was investigated using simultaneous times series of currentvelocity at various depths and zooplankton vertical samplesat an anchor station in the vicinity of a sill. Though strictlyaccurate estimates of organisms exchange could not be obtainedin this manner, it was possible to break down the exchange intoa mean flow mode, the magnitude and direction of which dependson the mean flow of water at the depth where the organisms areconcentrated, and a turbulent mode which is independent of waterexchange and varies considerably on a species basis. The secondmode of exchange is caused by differences in the concentrationof organisms between ebb and flood tides. The opposition ofthe two modes of exchange, which is presumably a feature ofsill areas, could explain the maintenance of large copepod populationsin the vicinity of the sill. 1Contribution to GIROQ program (Groupe interuniversitaire derecherches océanographiques du Québec). 2Present address: BIO-CONSEIL Inc., 105 Côte de la Montagne,suite 501, Québec, Canada.  相似文献   

12.
Research into the vertical migration behavior of scallop (Placopectenmagellanicus) veligers has led us to examine whether these,and possibly other small zooplankters, may migrate in responseto a combination of tidal and diel stimuli. This paper usesHill's (1991) model to evaluate the horizontal transport effectsof such migrations. We demonstrate that most types of verticalmigration behavior reported in the literature (e.g. nocturnal,twilight, midnight sink) appear at different phases of the lunarcycle. Moreover, migrating in response to both of these cuesmay provide horizontal transport advantages if the zooplankteris very small (unable to migrate the full water column depth)and/or has difficulty determining its position in the watercolumn (especially if the behavior also holds it in regionsof increased shear). Such behavior need not interfere with otheradvantages of vertical migration, including avoiding predation,avoiding UV light, searching for patchy food, etc. Tidal/dielmigration may have distinct advantages for occupying new habitatsor coping with local changes associated with altered currentregimes. Because averaging the results of several days, samplingless frequently than every 2 h or sequential sampling of differentsites is likely to obscure the tidal portion of a tidal/dielmigration, such behavior could be common without being obviousto researchers. Aliasing of the lunar and solar cycles (a 14.8day period) may allow the detection of tidal period migrationsin long-term records with lower sampling frequencies.  相似文献   

13.
The present study determined whether puffer Takifugu rubripes and T. xanthopterus larvae use selective tidal stream transport (STST) for migration into the nursery area. The influence of the tidal cycle on the vertical distribution of Thkifugu larvae was investigated during a 24 h sampling period at one location off Shimabara Peninsula in Ariake Bay. Samples were collected in three depth layers, from near the sea floor to near the surface (5, 20 and 30 m depth). The change in vertical distribution in relation to tidal phase was not observed. This data did not support STST hypothesis. Diel vertical migration was observed irrespective of tidal phase, where larvae migrated to the middle layer during the night, and sank to the bottom layer during the day, however, larvae hardly emerged into the surface layer during the study period. In Ariake Bay, the residual current leads to a layered vertically stratified structure, in which surface water flows towards the mouth and the middle-bottom water flows toward inner part of the Bay. It is suggested that Takifugu larvae use not STST but residual currents for transport into the nursery ground, namely, undergoing nocturnal diel vertical migration in the water column between the middle layer and the bottom layer where the net flow is northward.  相似文献   

14.
The abundance and distribution of Candea larvae was studiedin Ísafjord-deep, north-west Iceland, at approximatelymonthly intervals from February 1987 to February 1988 Zooplanktonsampling was made at nine stations along the length of the fjord,while temperature and chlorophyll a measurements from one ofthe stations are also presented Larvae of six species occurredin the samples, Eualus pusiolus and Pandalus borealis were mostnumerous, constituting 62 8 and 25 9% of the larvae respectively.The other species were, in declining order of abundance, Pandalusmontagui, Spirontocaris spp. (S spinus and s lilljeborgii) andSabinea septemcarinata. Eualus pusiolus was of highest abundancein the outer and middle parts of the fjord, while P.borealiswas most common in the middle and inner parts The onset of hatchingof all species in April–May appeared closely linked tothe phytoplankton spring bloom, while the temperature in thefjord was by then near the annual low (2–3°C). Exceptfor E pusiolus, of which a small part of the population produceda second brood during the summer, most of the larvae had disappearedfrom the plankton by the middle of August The monthly carapacegrowth of P.borealis larvae during the summer months was estimatedto be 1.0 mm.  相似文献   

15.
Svalbard bryozoan communities were investigated along a depth range from the surface to 296 m between the inner glacial fronts and fjord mouths during 2001 and 2002. The main study area was Kongsfjorden (79°N, 12°E). A total of 137 taxa of bryozoans were identified: 108 to species, 24 to genus, 3 to family, 1 to order and 1 to phylum level. Cluster and multidimensional scaling analyses revealed four distinct assemblages of bryozoans: shallow (0–40 m; 68 taxa), deep (40–296 m; 80 taxa), inner fjordic (three taxa) and an assemblage found on small stones in shallow waters (nine taxa). The inner fjordic assemblage was recorded from the front of tidal glaciers extending about 10 km out into the fjord. In terms of abundance, Celleporella hyalina Linnaeus dominated in shallow areas (18%), Hippothoa arctica Kluge (55%) in deep water, Alcyonidium disciforme Smitt (86%) proximate to glaciers fronts and Electra arctica Borg on small stones (98%). The species were classified according to their depth range as a stenobathic-shallow (46 taxa), stenobathic-deep (57 taxa) and eurybathic-generalist (21 taxa). Mean diversity measures did not show any significant differences between the shallow and deep communities. The bryozoan assemblages seem to be structured primarily by processes related to depth and sediment characteristics.  相似文献   

16.
The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.  相似文献   

17.
The activity of photosynthesis and mitochondrial respiration,nocturnal organic acid accumulation and water relations wereinvestigated in Prenia sladeniana L. Bol. [malic enzyme (ME)-type]andCrassula lycopodioides Lam. [phosphoenolpyruvate carboxykinase(PEPCK)-type] to compare the physiological responses to waterdeficit in crassulacean acid metabolism (CAM) plants differingin their decarboxylating enzyme systems. Withholding water inhibiteddaytime gas exchange within 2 d while night time CO2gain andmalic acid accumulation remained relatively unchanged in bothspecies. In P. sladeniana, maximum photochemical efficiency(Fv/Fm) and photosynthetic electron transport declined to nearlythe same degree as CO2supply was restricted during drought.Despite limited CO2availability, photosynthetic activity waslargely unaffected in C. lycopodioides, as were mitochondrialproperties. There is no indication of a drought-induced increasein the capability to totally oxidize malate, yielding 4 CO2, in either species. Nevertheless, the enhanced ratio of malateto glycine oxidation may have increased the in vivo capabilityfor malate oxidation in P. sladeniana. Although pressure potentialwas maintained throughout the experiment in both species, activeosmotic adaptation occurred only inP. sladeniana. The observeddecrease in photosynthetic and mitochondrial activity may haveresulted from the large increase in osmotic concentration inthis species. Copyright 2000 Annals of Botany Company Chlorophyll fluorescence analysis, Crassula lycopodioides Lam., crassulacean acid metabolism, citric acid, gas exchange, malic acid, mitochondria, photosynthetic electron transport, Prenia sladeniana L. Bol., water relations  相似文献   

18.
Temporal patterns of larval occurrence of brachyuran taxa weredescribed from Saco mangrove creek, Mozambique, based on planktonsamples. Brachyuran larvae were collected hourly in four 24h cycles during neap and spring tidal periods at a fixed station,in November 1997 and February 1998. Results indicate a semilunarcycle of larval release activity for most species. Newly-hatchedstages of sesarmids showed a peak occurrence in post-crepuscularebbing tides, Macrophthalmus spp. and Uca spp. showed highestdensities during ebbing tides at night. Dotilla fenestrata andPinnotheridae, in spite of showing a semilunar pattern, didnot display significant differences between day and night. Mostmangrove taxa showed thus a clear larval exportation behaviourduring ebb tides, with little return during the following floods,with varying degrees of dependence on the diel period. Megalopaewere restricted to the night period, and highest values wereobtained during spring tidal periods. However, maximum valuesof megalopae were obtained both during flood and ebb periods,contrary to most previous studies. This is interpreted as abi-directional transport of non-competent megalopae into andfrom the mangrove area.  相似文献   

19.
Summary This study presents sedimentological and micropaleontological data on ostracods from a cold-temperate inner shelf setting in the Troms District, northern Norway. The coarse fraction analyses carried out on sediment surface samples from coastal platforms and adjacent outer fjord troughs reveal a considerable contribution from ostracods to the accumulation of skeletal carbonates in distinct depositional settings. Ostracod accumulation is highest along wave-sheltered areas of coastal platforms where fleshy seaweed and coralline algal communities thrive in 10 to 30 m water depth. Current-exposed slopes of outer fjord troughs show a highly mixed ostracod assemblage consisting of imported species from the shallow coastal platform which is mixed with the trough assemblage. The hydrodynamic transport of ostracods into deeper areas results from the strong tidal current regime. Because of this mixing process, only the end members, the coastal platform and the fjord trough assemblages can be defined. The former is characterized byBaffinicythere emarginata, Cythere lutea, Finmarchinella angulata, Hemicytherura clathrata, Robertsonites tuberculatus, Sclerochilus rudjakovi, Semicytherura undata andXestoleberis cf.depressa. The outer fjord trough assemblage is characterized byCluthia cluthae, Cytherella abyssorum, Cytheropteron alatum, Krithe cf.adelspergi, Muellerina abyssicola, Cytherella cf.vulgatella andCytheropteron testudo. Members of the northern Norwegian trough assemblage are known to occur in deeper open shelf environments of the NE-Atlantic.  相似文献   

20.
A coupled physical–chemical–biological ocean model was applied to study primary production in an idealized 60-km long and 4-km wide fjord. Three different scenarios were simulated: (a) Without fresh water runoff; (b) A river at the head of the fjord adds 100 m3 s–1 fresh water to the surface layer; (c) The river adds 90 m3 s–1 fresh water to the surface layer and 10 m3 s–1 enters the fjord through a pipe at 50 m depth. The average productions in the inner 40 km of the fjord for the three scenarios are 68, 70 and 233 g C m–2 year–1 respectively. It is thus shown that there is a considerable potential for increasing the primary production in many fjords and coastal areas by submerging some of the river runoff. Because of the higher silicate content of the deeper water, artificial upwelling tends to stimulate diatom more than flagellate growth. This may be beneficial to mariculture developments. The sensitivity of the simulated primary production to horizontal and vertical resolution, horizontal viscosity, vertical diffusivity and viscosity, discharge depth, wind forcing, sill depth and pulsating the discharge is also studied. A simulation where a large river flux (90 m3 s–1) was submerged, showed that primary production was significantly enhanced in the outer parts of the fjord as well as along the coastline.  相似文献   

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