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1.
A total of 67 samples from the upper and lower sediment traps in the central South China Sea were analyzed, which were collected during 1993~1996. It is indicated that the distribution of stable isotope values, surface primary productivity, fluxes of total particulate matter, carbonate, biogenic opal, organic carbon, planktonic foraminiferal species and their total amount exhibit obviously seasonal and annual fluctuations. High values of the fluxes occurred in the prevailing periods of the northeastern and southwestern monsoons, and the low values occurred during the periods between the two monsoons. The fluxes of some planktonic foraminiferal species (Globigerinoides sacculifer, G. ruber, Globigerinita glutinata, Neogloboquadrina dutertrei) and their percentages also exhibit two prominent peaks during the prevailing periods of the northeastern and southwestern monsoons respectively, while those of Globigerina bulloides, Globorotalia menardii and Pulleniatina obliquiloculata only exhibit one peak in the prevailing periods of the northeastern monsoon. In addition, fluxes and percentages of Globigerinoides sacculifer and Globorotalia menardii as well as the fluxes of carbonate and total amount of planktonic foraminifera decrease gradually from 1993 to 1996, and those of Globigerina bulloides, Globigerinita glutinata and biogenic opal increase gradually from 1993 to 1996. The fluxes of carbonate and organic carbon in the upper trap are higher than those in the lower one. The study indicates that the seasonal and annual variations of the sediment fluxes and planktonic foraminiferal species are mainly controlled by the changes of surface primary productivity and hydrological conditions related to the East Asian monsoon. The lower carbonate and organic carbon fluxes in the lower trap are related to the dissolution.  相似文献   

2.
1993~1996年南海中部海洋沉降颗粒通量的季节和年际变化   总被引：7，自引：0，他引：7  
通过对南海中部1993~1996年获得的浅层和深层时间系列沉积物捕获器的样品分析,发现了稳定同位素值、颗粒总通量、碳酸盐、生物蛋白石、有机碳、表层初级生产力、浮游有孔虫总通量和属种的分布存在明显的季节性变化,其通量都是在东北季风和西南季风盛行期出现高值,在季风转向期出现低值.浮游有孔虫Globigerinoides sacculifer,G.ruber,Globigerinita glutinata,Neo-globoquadrina dutertrei等种的通量和相对百分含量也是在东北季风和西南季风盛行期都出现高值,但Globigerina bulloides,Globorotalia menardii和Pulleniatina obliquiloculata等种则在东北季风盛行期出现高值.分析还发现Globigerinoides sacculifer和Globorotalia menardii等种的通量和百分含量以及有孔虫总通量和碳酸盐通量等从1993到1996年存在下降趋势,而Globigerina.bulloides和Globigerinita glutinata的通量和百分含量以及生物蛋白石通量等在该期间显示为上升趋势.浅层捕获器样品中的碳酸盐和有机碳的通量比深层的高.研究表明海洋沉降通量和浮游有孔虫属种的季节和年际变化主要受与东亚季风相关的表层初级生产力和海洋水文条件变化所控制,深层捕获器样品中的碳酸盐和有机碳的通量低应与碳酸盐溶解作用有关.  相似文献   

3.
The first recorded specimens of Scaeurgus unicirrhus from the South-East Atlantic are described. Five males and two females were captured on the Valdivia Bank, Walvis Ridge, between 24 and 27°S, 400 miles off the Namibian coast. They are compared with specimens caught from other areas, and historical and geographic surveys of the species are made.  相似文献   

4.
Several authors have argued that lunar reproductive cycling controls the shell fluxes of planktic foraminifera, one of the major carbonate-producing groups in the global pelagic ocean. A time-series sediment trap at 2700 m depth on the central Walvis Ridge below the South Atlantic central gyre demonstrate for the first time that shell deposition fluxes of Hastigerina pelagica are synchronous with lunar periodicity. Spectral analysis of the 6-month time-series with 8-day resolution showed a strong 30-day cyclicity in the flux maxima of H. pelagica arriving at the ocean floor on average 12.5 days after each full moon. Given a shell settling velocity of about 400 m day−1, which implies about 7 days for settling, this coincides with the pronounced endogenous reproduction rhythm of 5±2 days after full moon as originally observed in laboratory-cultured isolates from off Bermuda in the North Atlantic. By contrast, no endogenous or exogenous lunar periodicity was observed in the deposition flux or size distribution of any of the 27 other shell species from austral winter (August 2000) to austral summer (February 2001). Instead, the deposition fluxes of shell species, the bulk carbonate and the total mass were dominated by a seasonal maximum during austral spring, without any periodicity in the 16–90-day domain of this study. Since H. pelagica exhibits low fluxes with a low burial efficiency, and continuous (re)production is shown by the deposition fluxes of other species, lunar reproductive cycling appears not to affect pelagic carbonate productivity and deep ocean sedimentation fluxes.  相似文献   

5.
In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.  相似文献   

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

7.
The foraminiferal (planktonic and benthic) and nannofosssil assemblages have been analyzed in the sediments of Core ACB-17-1447 taken from the South America continental slope north of the Rio Grande Rise piedmont during Cruise 17 of the R/V Akademik Sergey Vavilov. The core section is largely composed of carbonate and marly hemipelagic mud. The Quaternary age of the host sediments is evident from the occurrence of the planktonic foraminiferal index species Globorotalia truncatulinoides. Based on the nannofossil assemblages, the core sediments are attributed to the upper Pleistocene-Holocene. They contain abundant reworked Pliocene, Miocene, and Paleogene taxa transported from the slopes of the underwater Rio Grande Rise. The paleotemperature analysis of the planktonic foraminifers provided data for constructing the temperature curve that demonstrates two warm peaks. During the first warm period (Interval of 7–9 cm), the surface water temperature was as high as 26°C (Holocene optimum), which exceeds by 3–4°C its presentday values and implies the more intense warm Brazil Current. The earlier warm peak with temperatures up to 24°C recorded in the upper Pleistocene sediments (Interval of 69–71 cm) most likely reflects the 3rd oxygenisotope stage (MIS 3), which corresponds to the interstadial phase of the last glaciation (30–40 ka ago). Based on the abundances, taxonomic diversity, and proportions of the characteristic species of benthic foraminifers, the Core ASV-17-1447 section is divided into six intervals correlated with the marine isotopic stages defined by both the planktonic foraminifers and climatic changes evident from the variations in the bottom water circulation along the southwestern slope of the Brazilian Basin during the Late Quaternary.  相似文献   

8.
波弗特环流(Beaufort Gyre)是 决定北冰洋海冰运移方向和滞留时间的主要因素。依据沉积物的颜色旋回、Mn元素含量和有孔虫丰度,本文建立了阿尔法洋脊(Alpha Ridge)B85-D孔轨道尺度上的年龄框架,并深入分析了该孔晚第四纪以来冰筏碎屑(IRD>154 μm)含量、组分及其源区的变化特点。结果表明,350ka以来最显著的变化发生在末次冰期(MIS2~4期),期间沉积物中Ca元素和白云石含量都接近于零,大部分来自班克斯岛、维多利亚岛和麦肯齐地区的碳酸盐岩碎屑没有在阿尔法洋脊沉积;随着波弗特环流的消亡,搬运陆源碎屑的海冰很可能沿着北美海岸线直接进入了欧亚海盆。而在末次间冰期(MIS5期),波弗特环流却十分流畅,并将大量碳酸盐岩碎屑限制在美亚海盆内,导致阿尔法洋脊沉积物中Ca元素含量急剧升高,与之对应的白云石含量高达16.4%。通过类比可知,在MIS6期波弗特环流的状态可能与MIS2~4期相似,而在MIS8和MIS10这两大冰期,波弗特环流却类似于MIS5期。显然,波弗特环流的存在与否并不严格遵循“冰期-间冰期”旋回,这种变化很可能是风场强度和海冰浓度双重作用的结果。  相似文献   

9.
Rose Bengal stained benthic foraminifera were studied from 11 cores collected along two depth transects off southern Portugal: one in the Lisbon-Setúbal Canyon and the other along the canyon edge. The total standing stocks and distribution of foraminifera were investigated in relation to sediment and pore water geochemistry. Nitrate was used as a redox indicator, sedimentary chlorophyll a and CPE (chloroplastic pigment equivalents) contents as a measure of labile organic matter, and total organic carbon as a measure of bulk organic matter availability.The canyon sediments were enriched in organic carbon and phytopigments at all water depths in comparison with the canyon edge. Water depth seemed to control sedimentary phytopigment content, but not total organic carbon. No significant correlation was seen between pigment and total organic carbon content.The abundance of calcareous foraminifera correlated with the phytodetritus content, whereas a weaker correlation was observed for the agglutinated taxa. Therefore, calcareous foraminifera appear to require a fresher food input than agglutinated taxa. The foraminiferal species composition also varied with pigment content and nitrate penetration depth in the sediment, in line with the TROX concept. Phytopigment-rich (surficial CPE content >20 μg/cm3) sediments with a shallow nitrate penetration depth (∼1 cm depth) were inhabited by generally infaunal species such as Chilostomella oolina, Melonis barleeanus and Globobulimina spp. As the nitrate penetration increased to ∼2 cm depth in sediment and the pigment content remained relatively high (>15 μg/cm3), Uvigerina mediterranea and Uvigerina elongatastriata became dominant species. With declining CPE content and increasing nitrate penetration depth, the foraminiferal assemblages changed from the mesotrophic Cibicides kullenbergi-Uvigerina peregrina assemblage to the oligotrophic abyssal assemblage, mainly consisting of agglutinated taxa.  相似文献   

10.
Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ18Oshell) of five planktic foraminiferal species. Both the δ18Oshell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ18Oshell of G. ruber (w) from surface sediments is similar to the δ18Oshell values measured from the sediment-trap samples during winter. However, the δ18Oshell of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ18Oshell values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (～180 m) and deep surface water (～350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ18Oshell.  相似文献   

11.
The continental margin of SW Africa is typical of a volcanic rifted margin associated with a hotspot trail characterized by a large volcanic ridge, the Walvis Ridge, defining the hotspot migration, and extensive extrusive volcanism that produced seaward-dipping reflectors (SDR). Previously unpublished seismic data show two significant anomalies of the SW African Margin when compared to other typical volcanic rifted margins: (1) Hyaloclastitic outer highs are rare, and (2) the SDR in the North dip towards the Walvis Ridge. We explain these anomalies by a major transform segment close to the centre of volcanism combined with pulsed volcanism. The Walvis Ridge represents an east-west striking extrusive centre which produced a SDR sequence. Following break-up the northern boundary of the Walvis Ridge became a left lateral transform fault. Our data support the idea that a transform fault system interacting with a ridge jump were responsible for the accretion of the São Paulo Plateau to the American plate.  相似文献   

12.
The southern coasts of Africa are influenced by two major oceanic currents, leading to biogeographic patterns in inshore and offshore species assemblages, and in the stable isotope signatures of suspended particulate matter and filter-feeding mussels. We used the stable isotope ratios of carbon (13C/12C) and nitrogen (15N/14N) from the blood and feathers of adult and chick Cape gannets (Morus capensis) to investigate whether the geographic differences observed at the lower levels in the marine communities are deep penetrating effects that reach top predators. Additionally, we evaluated whether trophic segregation occurs between adult and reared chick gannets, and whether a shift to wintering habitat occurs in adults. The study was conducted during the 2006 breeding season on Bird Island in the Agulhas system, and on Malgas and Ichaboe Islands, in the south and north Benguela respectively. Our results showed significant differences in the isotope ratios of members of different colonies, but no intra-colony differences between tissues or age groups. These results indicate that there is neither age-related nor temporal segregation in the diet of members of the same colony. Feather isotopic values suggest that adults remain all year round in the same habitats, and do not undertake long migration after reproduction. Since all gannets tend to target similar prey, we attributed among-colony differences in isotope signatures mostly to the oceanic conditions experienced by the main prey of birds rather than substantial differences in diet composition. Overall, isotopic signatures segregate the two current systems, with depleted carbon values in the Agulhas and enriched nitrogen values in the upwelled waters of the Benguela. Within the Benguela birds from Ichaboe in the north had higher δ15N values than those from Malgas in the south, which we attributed to differences in the functioning of the upwelling cells in the vicinity of the two colonies. Finally, slight variation in the proportion of main prey and discards from fisheries may contribute to the variation in the stable-isotope signatures between colonies in the Benguela.  相似文献   

13.
A regime shift is considered to be a sudden shift in structure and functioning of a marine ecosystem, affecting several living components and resulting in an alternate state. According to this definition, regime shifts differ from species replacement or alternation of species at similar trophic levels, whereby the ecosystem is not necessarily significantly altered in terms of its structure and function; only its species composition changes. This paper provides an overview of regime shifts, species replacements and alternations that have been observed in the northern and southern Benguela ecosystems over the past few decades. Bottom-up control, initiating and sustaining regime shifts or species replacements via environmental forcing, is documented for both the southern and the northern Benguela ecosystems. Fishing (a case of top-down control) appears to have played an important role in regime shift processes in the Namibian ecosystem. Very low biomass levels of exploited fish stocks associated with less efficient energy transfer in the northern Benguela are indicative of a regime shift. Very high biomass levels have been reached in the southern Benguela in the 2000s. However the alternation between sardine and anchovy that has been observed in the southern Benguela over the last two decades appears not to have had major effects on the overall functioning of the ecosystem. The consequences of regime shifts for exploitation are highlighted, suggesting that fisheries managers should move towards a more effective ecosystem approach to fisheries.  相似文献   

14.
Transport between shelf and offshore environments supports a significant proportion of ocean primary productivity and is critical to the life cycle of many marine species. While fundamental differences in the underlying dynamics of eastern and western boundary currents have been recognized and studied for more than half a century, the implications for physical dispersal rates have received much less attention. In this study we explore how Australia’s two major boundary current systems, the East Australian Current and the Leeuwin Current, differ in their local retention and cross-shore transports in the upper water column and how these differences favor contrasting life histories of small pelagic fishes. The results suggest that the East Australian Current forms a partial barrier to onshore transport, but is effective in entraining shelf waters and transporting them offshore, particularly in the region where the current separates from the coast. Blue mackerel (Scomber australasicus) spawn on the outer-shelf in this separation region and may thereby maximize the dispersion of eggs and larvae in the mainly oligotrophic waters of the southern Coral Sea. In contrast, the Leeuwin Current system promotes onshore transport through the combined effects of mean onshore flow and eddy-induced mixing. In the Great Australian Bight, sardine (Sardinops sagax) and anchovy (Engraulis australis) may exploit the high coastal retention of the Leeuwin Current system by spawning on the inner-shelf during summer when the current is weakest and winds assist retention and enhance production through local upwelling.  相似文献   

15.
Pigment patterns and associated absorption properties of phytoplankton were investigated in the euphotic zone along two meridional transects in the Atlantic Ocean, between the UK and the Falkland Islands, and between South Africa and the UK. Total chlorophyll a (TChla=MVChla+DVChla+chlorophyllide a) concentrations and the biomarker pigments for diatoms (fucoxanthin), nanoflagellates and cyanobacteria (zeaxanthin) appeared to have similar distribution patterns in the spring and in the autumn in the temperate NE Atlantic and the northern oligotrophic gyre. Divinyl chlorophyll a levels (prochlorophytes) were greater in spring at the deep chlorophyll maximum in the oligotrophic gyre, however. Marked seasonal differences were observed in the NW African upwelling region. TChla concentrations were twice as high in the upper mixed layer in the spring, with the community dominated by diatoms and prymnesiophytes (19′-hexanoyloxyfucoxanthin). A layered structure was prevalent in the autumn where cyanobacteria, diatoms and prymnesiophytes were located in the upper water column and diatoms and mixed nanoflagellates at the sub-surface maximum. In the South Atlantic, the Benguela upwelling ecosystem and the Brazil-Falklands Current Confluence Zone (BFCCZ) were the most productive regions with the TChla levels being twice as high in the Benguela. Diatoms dominated the Benguela system, while nanoflagellates were the most ubiquitous group in the BFCCZ. Pigment concentrations were greater along the eastern boundary of the southern oligotrophic gyre and distributed at shallower depths. Deep chlorophyll maxima were a feature of the western boundary oligotrophic waters, and cyanobacteria tended to dominate the upper water column along both transects with a mixed group of nanoflagellates at the chlorophyll maximum.Absorption coefficients were estimated from spectra reconstructed from pigment data. Although absorption was greater in the productive areas, the TChla-specific coefficients were higher in oligotrophic regions. In communities that were dominated by diatoms or nanoflagellates, pigment absorption was generally uniform with depth and attenuating irradiance, with TChla being the major absorbing pigment at 440 nm and photosynthetic carotenoids (PSC) at 490 nm. Absorption by chlorophyll c and photoprotective carotenoids (PPC) was much lower. Populations where cyanobacteria were prevalent were characterized by high PPC absorption, particularly at 490 nm, throughout most of the euphotic zone. The data suggested that the effect of pigments on the variability of phytoplankton absorption was due primarily to the variations in absorption by PPC.  相似文献   

16.
A 9-year study of planktonic foraminifer fluxes was conducted in the Bering Sea (Station AB) and in the central subarctic Pacific (Station SA). Results clearly reflected variations of the water mass characteristics in the upper layers. The 9-year means of total foraminifer fluxes were the same (1400 shells m−2 d−1) at both stations. However, total foraminifer flux at Station AB tended to show its primary maximum during fall (October–December) and its secondary maximum in spring (April–June), whereas the primary maximum appeared in spring and the secondary maximum in fall at Station SA. Seasonal variation was more apparent at hemipelagic Station AB than at pelagic Station SA. Planktonic foraminifers found at both stations were of six species: Neogloboquadrina pachyderma, Globigerina umbilicata, Globigerinita glutinata, Globigerina quinqueloba, Globorotalia wilesi, and Orbulina universa. The foraminifer assemblages at the two stations reflected the temperature difference in the surface waters. The variable %G. umbilicata tended to be high in the warm surface waters during the summers. The temporal and geographical variation of %G. quinqueloba indicated that this taxon prefers regions with relatively low diatom fluxes. A notable appearance of O. universa occurred in 1997 at Station SA. During this period, other measured biogenic particle fluxes, such as those of diatoms, were low. This unusual 1997 event may be a reflection of global climatic change that happened to be observed in the central subarctic Pacific Ocean.  相似文献   

17.
Rose-Bengal-stained benthic foraminifera in six pilot-core samples and one multicore sample collected from the Hess Rise and Suiko Seamount in August 1994 were studied in order to understand foraminiferal distributions between two areas divided by an oceanic front in the central North Pacific. Samples from the Hess Rise were collected in depths of 2167–3354 m under the warm, saline Kuroshio Extension, while samples from Suiko Seamount came from depths of 1811–1955 m under the cold, less-saline subarctic current. Sediment-trap results for the year prior to our sediment sampling show that organic matter fluxes were about 2.5 times greater at Suiko Seamount than at the Hess Rise. However, the hydrographic structure between 1800 and 3400 m, based on CTD observations, is almost the same at both sites. Temperature decreases from 2.2 to 1.7°C over the depth range of 1800–3400 m, salinity increases from 34.5 to 34.7, and the dissolved oxygen content gradually increases from 1.5 to 3.0 ml l−1. The faunal populations at the Hess Rise are quite different from those at Suiko Seamount. The abundant species at the Hess Rise are Epistominella exigua, Brizalina pacifica, Fursenkoina cedrosensis and Alabaminella weddellensis. These species characteristically inhabit phytodetrital aggregates deposited on an oligotrophic seafloor. The populations at Suiko Seamount are dominated by Triloculina frigida, Lagenammina cf. arenulata, Reophax subfusiformis, and Reophax scorpiurus. The reason for differences between these populations is unclear. However, the typical phytodetritus-dwelling species E. exigua is dominant at the Hess Rise, which is located in a subtropical area that has a pulsed supply of settling organic matter in the spring. On the other hand, E. exigua is rare at Suiko Seamount, a subarctic site where there are more stable and greater fluxes of organic matter in summer and autumn. Occurrences of this species may be related to the seasonally short supply of organic matter that reaches the seafloor in the oceanic North Pacific.  相似文献   

18.
Benthic faunal activity and density play an important role in determining the rates of benthic nutrient fluxes, which enrich the water column and contribute to phytoplankton growth. The intensity of nutrient fluxes in the Bay of Brest depends on the density of the invasive gastropod, Crepidula fornicata. In order to study the impact of benthic fluxes on phytoplankton dynamics, realistic daily nutrient inputs simulating various densities of C. fornicata were added to six enclosures during three weeks. The increase in fertilization intensity influenced the phytoplankton biomass. A succession from Chaetoceros spp. to Pseudo-nitzschia spp. and Leptocylindrus danicus was observed in all enclosures, but the dynamics of successions were different. Pseudo-nitzschia spp. was favored in the three more fertilized enclosures, while Chaetoceros spp. persisted longer in less enriched enclosures. Despite an apparent nitrogen limitation, the quantum efficiency of PSII (Fv/Fm) was high (>0.5) and stable in all enclosures. The maximal photosynthetic capacity (PBmax) was also invariable and oscillated around an average value of 2.23 mg C (mg Chl a)−1 h−1. The stability of Fv/Fm and PBmax observed at different nutrient input intensities demonstrates that the daily inputs maintained the physiological balance of the microalgae. The maximal light utilization efficiency (α) and the light saturation parameter (Ek) were also quite stable after day 8, which reveals that photosynthetic parameters were driven by growth constraints due to nutrient availability and not by incident light or species successions. We suggest that our results correspond to an “Ek independent variation” regulation. We propose that such regulation of photosynthetic parameters appears when there are frequent nutrient additions which do not allow replete nutrient conditions to be reached but lead to physiological equilibrium.  相似文献   

19.
Planktic foraminiferal (PF) flux and faunal composition from three sediment trap time series of 2002–2004 in the northeastern Atlantic show pronounced year-to-year variations despite similar sea surface temperature (SST). The averaged fauna of the in 2002/2003 is dominated by the species Globigerinita glutinata, whereas in 2003/2004 the averaged fauna is dominated by Globigerinoides ruber. We show that PF species respond primarily to productivity, triggered by the seasonal dynamics of vertical stratification of the upper water column. Multivariate statistical analysis reveals three distinct species groups, linked to bulk particle flux, to chlorophyll concentrations and to summer/fall oligotrophy with high SST and stratification. We speculate that the distinct nutrition strategies of strictly asymbiontic, facultatively symbiontic, and symbiontic species may play a key role in explaining their abundances and temporal succession. Advection of water masses within the Azores Current and species expatriation result in a highly diverse PF assemblage. The Azores Frontal Zone may have influenced the trap site in 2002, indicated by subsurface water cooling, by highest PF flux and high flux of the deep-dwelling species Globorotalia scitula. Similarity analyses with core top samples from the global ocean including 746 sites from the Atlantic suggest that the trap faunas have only poor analogs in the surface sediments. These differences have to be taken into account when estimating past oceanic properties from sediment PF data in the eastern subtropical North Atlantic.  相似文献   

20.
A study of the mesoscale distribution of phytoplankton communities observed along the Portuguese upwelling coast in 1985–1986 made it possible to identify an assemblage composed by the coccolithophores Helicosphaera carteri, Syracosphaera pulchra and Coronosphaera mediterranea. The assemblage was consistently present in the coast in the following years (1991, 1992, 1994, 2003 and 2005) and from 1992 onwards, Rhabdosphaera clavigera was recognised as being part of it, although in low concentrations During late summer, this group of species showed higher affinities with upwelling waters of subtropical origin (ENACWst) on the SW coast. At the end of the upwelling season, in autumn, the assemblage was advected to the NW shelf due to the intensification of the poleward surface circulation or meridional seasonal changes of environmental conditions. The species maximum abundances were not coincident in space: H. carteri developed at the central part of the western coast while blooms of S. pulchra and C. mediterranea generally occurred further south, such as at the upwelling centre of cape S. Vicente. Syracosphaera pulchra blooms were characterised by lower concentrations of C. mediterranea and vice-versa. The above patterns were recurrent along the years but the relative abundance of each species appeared related to the interannual variability of physical conditions as the upwelling.  相似文献   

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