Nepheloid layer dynamics in the northern Portuguese shelf   总被引：1，自引：0，他引：1  

A. Oliveira  J. Vitorino  A. Rodrigues  J. M. Jouanneau  J. A. Dias  O. Weber 《Progress in Oceanography》2002,52(2-4)

A general hydrographic and nephelometric survey of the NW Portuguese continental shelf and upper slope was undertaken, under winter and spring conditions. The nepheloid layer dynamics along the shelf during three cruises were controlled, principally by the following factors: (a) the hydrography of the shelf-slope waters, i.e. the nepheloid layers followed isopycnals and water masses; (b) prevalence of upwelling or downwelling circulation over the shelf; (c) dispersion of material by river discharge (mainly the Douro river); (d) resuspension of mid-shelf fine deposits induced by swell; (e) peculiar morphology with the presence of Porto Canyon and outer shelf rock outcrops.The general circulation controls the seaward extension of the nepheloid layers. In winter, under dominant downwelling conditions, an intense bottom nepheloid layer (BNL) was observed on the shelf, due to river borne particle supply and remobilization of mid-shelf muddy sediments (depth ≈100 m). In this period the BNL increased in thickness to the top of the slope, following the isopycnals. Near the shelf-break the BNL detached to form intermediate nepheloid layers (INL). Small INLs appear deeper in some areas of the slope. A surface nepheloid layer (SNL) appears in the surface water over the shelf and slope. In spring, biological particles mainly contribute to the SNL, which is separated from the BNL by a zone of clear water. In winter the SNL is restricted to the inner shelf where there are high inputs of fluvial particles.Downwelling circulation probably induces transport of shelf particles to deeper waters in the BNL. In spring, the predominant circulation was southward (upwelling), the water column was highly stratified, and dispersion of particles in the SNL was offshore.  相似文献   

台湾海峡南部夏季的颗粒有机碳   总被引：5，自引：3，他引：2       下载免费PDF全文

叶翔  李炎  黄邦钦  陈坚 《海洋学报》2009,31(6):87-99

根据2004年夏季、2005年夏季和2006年夏季3个航次观测的颗粒有机碳(POC)数据,表层POC含量的分布表现出近岸高、远岸低的特点,表层和次表层POC含量的高值区出现在东山以东以及南澳附近上升流区。沿岸流较强的2005年和2006年航次具有相对丰富的总悬浮颗粒物(TSM)含量。浮游植物是该区POC的主要来源。各航次的POC含量与叶绿素a(Chl a)含量正相关,所有航次统计的Chl a/POC与Chl a呈正相关,上升流区Chl a/POC偏小,上升流较强的航次高Chl a区的Chl a/POC也偏小,反映了上升流与高营养转换效率浮游生物生态在空间和时间上的联系。根据Chl a/POC~Chl a关系图判断,台湾海峡南部海域1988年、1998年和2004年夏季航次的浮游生物营养转换效率受到相对抑制,而在上升流信号较强的2005年和2006年夏季航次,上升流区的浮游生物高营养转换效率状态得以强化。各航次的POC含量和Chl a/POC比值与TSM含量正相关,POC/TSM比值与TSM负相关,沿岸流带来的陆源悬浮颗粒物通量波动,并没明显干扰台湾海峡南部以海洋浮游生物生态主导的颗粒有机碳生物地球化学循环格局。  相似文献   

Late Holocene estuarine–inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida?     

B. T. Donahue  A. C. Hine  S. Tebbens  S. D. Locker  D. C. Twichell   《Marine Geology》2003,200(1-4):219-241

The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sea-level rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped – one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebb-tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf.  相似文献   

HF radar-derived origin and destination of surface waters off Bodega Bay, California   总被引：1，自引：0，他引：1  

David M. Kaplan  John Largier 《Deep Sea Research Part II: Topical Studies in Oceanography》2006,53(25-26):2906

As an integral part of the WEST study of the role of wind-driven transport in shelf productivity, HF radar currents are analyzed to determine typical surface flow patterns off Bodega Bay in northern California. Radar-derived surface trajectories and surface velocity divergences are used to determine the proximal origins and destinations of surface waters in the area. Surface trajectory results show a strong bimodality, with water over the entire shelf originating in the north under upwelling conditions and waters over the inner/mid-shelf originating in the south during relaxation conditions. Outer shelf waters have more variable transport patterns during relaxation conditions, with limited equatorward or onshore movements being most typical. The destinations of surface waters starting at the outer shelf are predominantly offshore, with the majority of particles exiting the radar domain west of Pt Reyes along the shelf edge in less than 2 days. Significant proportions of water from the inner/mid-shelf are exported southward and exit the radar domain inshore or within 20 km of the tip of Pt Reyes, creating possibilities for either nearshore retention in the Bodega region or entrainment of water into the Gulf of Farallons. Approximately 15% of all trajectories remained in the radar domain for 6 days, suggesting that a biologically significant percentage of larvae might be retained in the area for time periods approaching typical larval durations. Calculations of surface divergence indicate where vertical flux may be significant. An extensive area of positive divergence is observed off Bodega during upwelling conditions, while weakly convergent flow is observed where upwelling flows approach Pt Reyes. Positive divergence also is observed during relaxation periods when poleward flow separates from the shore just north of Pt Reyes. Estimates of vertical flux in these divergence zones point to a significant contribution of recently upwelled waters to the observed horizontal fluxes at the surface. Determination of the ultimate source and fate of phytoplankton-rich waters requires further analysis of the detailed time dependence of phytoplankton concentration relative to the time dependence of wind-forced currents.  相似文献   

东海沿岸海区垂直环流及其温盐结构动力过程研究I．环流的基本特征     

刘兴泉  侯一筠  尹宝树 《海洋与湖沼》2004,35(5):393-403

利用三维斜压流体动力学模型 ,通过对东海沿岸海区冬、夏季的斜压环流及其温盐结构的数值研究 ,揭示研究海区垂直环流及其温盐结构的动力过程及其成因。垂直环流的模拟结果表明 :冬季 ,沿岸海区的垂直环流以逆时针流动 ,近表层为向岸流 ,沿岸为下降流 ,近表层以下为离岸流 ,其在外海有明显的上升趋势 ,沿岸下降流自表层至底层逐渐由强变弱 ;夏季 ,沿岸海区的垂直环流以顺时针流动 ,近表层以下为向岸流 ,沿岸为上升流 ,近表层为离岸流 ,其在外海有明显的下降趋势 ,沿岸上升流自底层至表层逐渐由弱变强。就整个沿岸海区而论 ,冬季沿岸下降流和夏季沿岸上升流的强度都随着岸界地形坡度、风速及风向与岸线偏角的变化而变化。沿岸下降流形成的主要原因是由于冬季东北风与岸界地形的耦合效应及海区温盐分布不均匀所致 ,而沿岸上升流形成的主要原因则是由于夏季西南风与岸界地形的耦合效应及海区温盐分布不均匀所致。  相似文献   

Coastal Upwelling Activity on the Pacific Shelf of the Baja California Peninsula     

Oleg Zaytsev  Rafael Cervantes-Duarte  Orzo Montante  Artemio Gallegos-Garcia 《Journal of Oceanography》2003,59(4):489-502

High primary productivity on the Pacific coast of the Baja California Peninsula is usually related to coastal upwelling activity that injects nutrients into the euphotic zone in response to prevailing longshore winds (from the northwest to north). The upwelling process has maximum intensity from April to June, with the coastal upwelling index varying from 50 to 300 m³/s per 100 m of coastline. Along the entire coast of the peninsula, the upwelling intensity changes in accordance with local wind conditions and bottom topography. Spatial variability can also be modulated by the influence of mesoscale meanders of the California Current. We have identified the seasonal and synoptic variability of upwelling signatures on the Baja California shelf, using averaged monthly and weekly sea surface temperature (SST) distributions obtained from remote sensing imagery from the Advanced Very High Resolution Radiometer in the period from 1996 to 2001. Analysis of SST distribution and direct experimental data on temperature and nutrient concentration shows that the areas with the coldest SST anomalies were closely related to the bottom slope, shelf width, and coastline orientation relating to wind direction. We also assume that the nutrient transport into the coastal lagoons may be forced by the coupling of coastal upwelling and tidal pumping of surface waters into the lagoon system. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

River flooding, storm resuspension, and event stratigraphy on the northern California shelf: observations compared with simulations     

Shejun Fan  Donald J. P. Swift  Peter Traykovski  Samuel Bentley  Jeffry C. Borgeld  Christopher W. Reed  Alan W. Niedoroda 《Marine Geology》2004,210(1-4):17-41

Coast-hugging surface flood plumes occur on the inner shelf of northern California during the winter season, generating dense, near-bottom suspensions which may attain fluid mud concentrations as particles settle. The period of storm-heightened waves may continue into the flood period, leading to gravity-driven seaward displacement of the bottom suspension; or the wave regime may ameliorate, leaving the suspension to consolidate as a short-lived, inner-shelf flood bed. Such beds tend to be resuspended within days or weeks by subsequent storm events that may recreate the original high concentrations. The sediment is thus dispersed seaward by gravity flows, to be deposited as a muddy flood bed on the central shelf. The locus of deposition of these “high-concentration regimes” is a function of the relative intensities of river discharge and storm wave height. Greater discharge piles thicker storm beds nearer shore, while intense wave regimes allow deposition of the fluid mud further seaward. During events with high values of both parameters, large amount of fluid mud may bypass over the shelf edge. In contrast, “low-concentration regimes” occur during storm periods when there has been no recent flood deposition on the inner shelf. The shelf floor is better consolidated than in the previous case, and the resulting suspended sediment concentrations are lower. As a consequence, low-concentration regimes are winnowing and bypassing regimes, and the beds deposited are thinner and sandier. Algorithms describing deposition by high and low-concentration regimes have been embedded in a probabilistic model. A simulation of a 400-year sequence of beds deposited by winter storms and floods suggests that on the Eel shelf, the Holocene transgressive systems tract consists of back-stepping, seaward-fining event beds, whose timelines (bedding planes) dip more gently than do their gradational facies boundaries. At these longer time scales, flood beds dominate over storm beds.  相似文献   

Evaluation of shelf–basin interaction in the western Arctic by use of short-lived radium isotopes: The importance of mesoscale processes     

David Kadko  Robin Muench   《Deep Sea Research Part II: Topical Studies in Oceanography》2005,52(24-26):3227

Shelf–basin exchange in the western Arctic was evaluated by use of water-column analyses of ²²⁸Ra/²²⁶Ra ratios and the first measurements of the short-lived ²²⁴Ra (T_1/2=3.64 d) in the Arctic. During the 2002 shelf–basin interaction (SBI) program, excess ²²⁴Ra was detected over the shelf but was not found seaward of the shelf-break. Similarly, the ²²⁸Ra/²²⁶Ra ratio dropped rapidly from the shelf across the shelf-break. Consequently, the model age gradient (elapsed time since shelf residence) northward across the Chukchi Shelf increased from 1–5 years nearshore to approximately 14 years in surface waters sampled off shelf at the southern margin of the Beaufort Gyre. This steep gradient is consistent with very slow exchange between the Chukchi Shelf and the Beaufort Gyre, whereby Bering Strait inflow is constrained by the Earth's rotation to follow local isobaths and does not easily move into deeper water. The strong dynamic control inhibiting water that enters the system through Bering Strait from flowing north across isobaths also would lead to a long recirculation time of river water emptied into the Beaufort Gyre. Possible mechanisms that can generate cross-shelf currents that break the topographic constraint to follow isobaths, and thereby transport water (and associated properties) off the shelves include wind-induced upwelling/downwelling, meandering jets, and eddies. Evidence of such a process was found during the ICEX project in the Beaufort Sea in April 2003 when excess ²²⁴Ra was measured over 200 km from any shelf source. This required an NE offshore flow of 40 cm s⁻¹ assuming that the source water derives from the mouth of Barrow Canyon. A weak northeastward flow was measured using an LADCP within the upper 300 m of the ocean, but was of lower speed than required by the ²²⁴Ra_xs at the time of the ICEX occupation.  相似文献   

冲绳海槽中南部及其邻近陆架悬浮体的分布、组成和影响因子分析   总被引：7，自引：3，他引：7  

郭志刚  杨作升  雷坤  高谰  曲艳慧 《海洋学报》2001,23(1):66-72

初夏冲绳海槽中南部海域悬浮体含量的分布特点为:表层水体中部海区高,南北海区低;中底层水体自南西至北东由低至高.从陆架到海槽断面的悬浮体含量分布为陆架最高值区、陆坡最低值区和海槽低值区.悬浮体组成包括矿物岩石碎屑、生物骨屑以及絮凝体.矿物岩石碎屑主要分布在陆架区中、下层水体,生物骨屑主要分布在海槽区(整个水层)和陆架区上层水体,絮凝体在研究区都有分布.海槽区与陆架区的悬浮体含量垂直分布和组分特征具有明显的差异.研究区悬浮体分布和组成主要受控于黑潮对陆架物质向冲绳海槽输送的阻隔、生物生产量以及地形、地理环境.  相似文献   

Cross-shore separation of adult and juvenile euphausiids in a shelf-break alongshore current     

Beiwei Lu  David L. Mackas  Douglas F. Moore 《Progress in Oceanography》2003,57(3-4):381

Euphausiids are an important component of the zooplankton in boundary current upwelling regions, including the Pacific Northwest continental margin. Many aspects of euphausiid distribution and ecology in this region are well known. However, some features of their spatial and temporal distribution are less understood:

• How and why euphausiids aggregate near the shelf-break upwelling center.

• How and why there is (within an alongshore band of high abundance of all stages) spatial segregation of adults and larvae.

• Why, despite spatial association with upwelling, euphausiid abundance off Vancouver Island is weakly or negatively correlated at interannual time scales with upwelling intensity.

To address these, we made km-resolution surveys of adult, juvenile, and larval euphausiid horizontal distributions, water properties, and currents across the Vancouver Island shelf break in mid-to-late spring of two successive years. Survey timing was before (1997) and after (1998) the spring transition to upwelling conditions, and near the annual spring reproductive peak. In both years, early developmental stages occupied an alongshore band that was offset from the late juveniles and adults. The direction of the offset differed between the two surveys. Early life history stages (larvae and early juveniles) were shoreward of adults in April 1997 (downwelling-conditions), but seaward of adults in May 1998 (upwelling-conditions). Separation distance (order 5–10 km) was consistent with expected differences in cumulative wind-driven (and vertically-sheared) cross-shore transport of surface-dwelling larvae and early juveniles vs. transport of diel migratory late juveniles and adults. Separation direction was consistent with recent history of winds and Ekman transport—shoreward during poleward winds, and seaward into blue water (and usually into a strong equatorward current) during equatorward winds.  相似文献   

Spatio‐temporal variability of zooplankton biomass and environmental control in the Northern Benguela Upwelling System: field investigations and model simulation          下载免费PDF全文

Bettina Martin  Anja Eggert  Rolf Koppelmann  Rabea Diekmann  Volker Mohrholz  Martin Schmidt 《Marine Ecology》2015,36(3):637-658

Spatial and temporal distribution patterns of zooplankton are highly variable in the Northern Benguela Upwelling System. We studied the distribution of zooplankton (size class ≥ 0.33 mm) and used field data from four cruises that took place between March 2008 and February 2011, as well as simulation results of a regional ecosystem model. Remotely sensed sea surface temperatures (SST) and surface chlorophyll concentrations were analysed to investigate environmental influences on zooplankton biomass. The Intense Benguela Upwelling Index showed a distinct seasonal signal throughout the years and the highest upwelling peaks in August/September. Even though surface chlorophyll concentrations were very variable throughout the year, the highest concentrations were always detected in September, following the upwelling of nutrient‐rich water. In field catches, zooplankton biomass concentration in the upper 200 m was highest above the outer shelf and shelf‐break in December 2010 and February 2011, i.e. 6 months after the upwelling peaks. In contrast, zooplankton biomass simulated by the model in the surface water was highest in September. In March/April, biomass maxima were typically measured in the field at intermediate water depths, but the vertical distribution was also affected by extensive oxygen minimum zones. The ecosystem model reproduced this vertical pattern. Although general trends were similar, simulation data of zooplankton standing stocks overestimated the field data by a factor of 3. In upwelling systems, food webs are generally considered to be short and dominated by large cells. However, our field data indicate more small‐sized zooplankton organisms above the shelf than offshore.  相似文献   

Physical processes contributing to harmful algal blooms in Saldanha Bay,South Africa     

《African Journal of Marine Science》2013,35(1):285-297

The study synthesises current understanding of the predominant physical processes responsible for the seasonality of harmful algal blooms, notably Alexandrium catenella and Dinophysis spp., in the nearshore environment of Saldanha Bay on the west coast of South Africa. Saldanha Bay is one of the few naturally sheltered areas on the South African coastline suitable for in situ shellfish farming and is the major site for the production of black mussel Mytilus galloprovincialis in South Africa. Mussel farming started there in 1985 and the present level of production is some 2 700 tons per annum. Since 1994, disruption of harvesting as a result of the presence of harmful algal species has been a regular late-summer phenomenon. Toxic blooms that are ultimately advected into the bay develop on the continental shelf to the north between 32°S and St Helena Bay, a region characterized by favourable conditions for dinoflagellate growth and circulation patterns that facilitate build-up of intense blooms during late summer. Offshore dinoflagellate populations are advected shorewards and polewards in response to relaxation of upwelling at the Namaqua cell to the north. Dinoflagellate blooms are advected south from the southern Namaqua shelf during upwelling relaxation. Under such conditions, the gyre south of Elands Bay moves offshore and a barotropic flow past Cape Columbine is established. Evidence suggests that the near surface component of the flow occurs as a sudden "flood" event. These dinoflagellate-containing shelf waters are in turn advected into Saldanha Bay when upwelling relaxes, when the density gradient between the bay and the shelf drives surface inflow and bottom water outflow. These flows are reversed with the resumption of upwelling over the shelf, resulting in intrusion and entrainment of bottom water and surface outflow. Entrainment dictates that the bay acts as a net importer of bottom water and net exporter of surface waters over a synoptic cycle. This system of exchange between Saldanha Bay and the shelf curtails the duration and severity of toxic episodes in the bay relative to the shelf.  相似文献   

Bedforms and depositional sedimentary structures of a barred nearshore system, eastern Long Island, New York     

R.Craig Shipp   《Marine Geology》1984,60(1-4):235-259

The depositional sedimentary structures and textures of a single-barred nearshore system on the Atlantic coast of eastern Long Island, New York, were studied along seven shore-normal transects. Data along these transects consisted of textural analysis of 160 sediment samples, temporal bedform observations, and 42 can cores for the analysis of sedimentary structures.

Six sedimentary subenvironments were observed, based on distinct combinations of sediment color and texture, bedforms, physical, and biogenic sedimentary structures, and benthic infaunal communities. The shoreface environment is divided into the upper shoreface, the longshore trough, and the longshore bar. The divisions of the inner shelf environment are the shoreface-inner shelf transition, the offshore, and the coarse-grained deposit. The first five subenvironments are arranged in bands parallel to the shoreline, whereas the coarse-grained deposit occurs in patches across the inner shelf.

The location of fair-weather wave base, coinciding with a reduction in slope (3.0–0.3°) from the shoreface to the inner shelf, is characterized by the cessation of debris surge in the troughs of ripples, the formation of a “rust layer” of microorganisms over the bedform surface, and a sediment color change caused by an increase in organic detritus. The sequence of bedforms and physical sedimentary structures observed in this system fits well with existing wave-generated (oscillatory) flow regime models. These models explain the observed sequences as a response to the degree of asymmetric flow created by shoaling waves. Distribution of biogenic structures and assemblages of infaunal organisms is influenced by the distance landward or seaward of fair-weather wave base.

The overall relationships of this nearshore system can then be summarized as a hypothetical prograding stratigraphic sequence. The entire sequence is underlain by organic-rich, bioturbated, offshore deposits. Overlying the offshore is the planar-laminated sediments of the transition. Grading upward from the transition are the cleaner, planar-laminated, seaward slope deposits of the longshore bar. Above this, is a distinct erosional surface indicating the base of the massive to cross-laminated coarse sediments of the longshore trough. Capping the sequence are the cross- to planar-laminated, clean sands of the upper shoreface and foreshore.  相似文献   

On flow in Northwestern Gulf of Alaska,May 1972     

Felix Favorite  W. James Ingraham Jr. 《Journal of Oceanography》1977,33(2):67-81

Southwestward volume transport (referred to 1,500 db) out of the Gulf of Alaska seaward of the continental shelf in May 1972 was 12.5 Sv, and nearly 3/4 of this flow occurred within 50 km of the shelf edge. Mean geostrophic velocities of about 50 cm s^–1 occurred in a band 20 km wide, which extended 500 km along the shelf edge; a maximum velocity of 98 cm s^–1 (nearly 2 knots) was obtained. Bottom flow along the inshore part of the shelf as determined by seabed drifters was generally onshore at 0.5 cm s^–1. Evidence is presented of a large cyclonic gyre on the shelf encompassing the Portlock and Albatross Banks, perturbations in surface flow along the shelf edge, and relations between coastal tidal heights and fluctuations in geopotential topography at the shelf edge.  相似文献   

Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response     

《African Journal of Marine Science》2013,35(3-4):493-509

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.  相似文献   

An upwelling cell inshore of the East Madagascar Current     

J. R. E. Lutjeharms  E. Machu 《Deep Sea Research Part I: Oceanographic Research Papers》2000,47(12)

A distinct, coastal upwelling cell inshore of the southern limb of the East Madagascar Current is described. The upwelling cell has cold surface water, with high concentrations of chlorophyll-a. It is located where this current moves from a bathymetry characterised by a narrow continental shelf and a steep slope to one characterised by a wider shelf and a less steep slope. There seems to be no correlation between the local winds and the presence of upwelled water, suggesting current-induced upwelling. Advection of upwelling filaments is consistent with the concept of an East Madagascar retroflection.  相似文献   

Sources of Cd, Cu, Ni and Zn in Portuguese coastal waters     

Marie-Hlne Cott-Krief  Ccile Guieu  Alain J. Thomas  Jean-Marie Martin 《Marine Chemistry》2000,71(3-4)

In order to identify the major sources of trace metals (TM) in the Portuguese coastal waters, 58 surface water samples were collected during September 1988. The area sampled extended from the Tagus Estuary (down to a salinity of 25) to cape Ste Marie on the southern coast of Portugal. Dissolved metal concentrations in the fully marine waters ranged from 30 to 250 pM for Cd, 0.7–15 nM for Cu, 0.9–20 nM for Zn and 1.8–4.5 nM for Ni. Within the Tagus Estuary (salinity 25), concentrations increased to 3400 pM for Cd, 26 nM for Cu, 14 nM for Ni and 230 nM for Zn.The large-scale distribution of these metals is dominated by two strong continental sources, both probably linked to the exploitation of pyrite ores. In the Tagus Estuary, TM enrichments can be mostly attributed to a pyrite roasting plant located on the shore in front of Lisbon. Concerning the south Portuguese shelf waters, several hypotheses are proposed to explain their elevated metal concentrations. We particularly discussed the likely influence of the Tinto/Odiel rivers located 100 km eastward, an influence well known in the shelf waters of the Gulf of Cadiz. These rivers are extremely metal-rich because of acid mine tailings originating from their catchment. Between these two regions, upwelling of relatively metal-poor water largely contributes to the dilution of the continental inputs. Indeed, water exchanges on the shelf linked to the upwelling involve water fluxes 500 times higher than the Tagus River flow, and renew the coastal waters that are thus cleaned from terrestrial contamination. Contrary to many other upwelling systems in non-contaminated areas, the Portuguese upwelling does not act as a source of trace-metal enrichment of the continental margin waters.  相似文献   

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf     

George P. Allen  Patrice Castaing 《Marine Geology》1973,14(5):47-53

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude.Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.  相似文献   

Dissolved iodine across the Gulf Stream Front and in the South Atlantic Bight     

George T.F. Wong 《Deep Sea Research Part I: Oceanographic Research Papers》1995,42(11-12)

The distributions of iodide, iodate and total iodine were determined along a transect from the Sargasso Sea and across the Gulf Stream to the continental shelf of the South Atlantic Bight during November 1990. The western boundary of the Gulf Stream at the outer shelf-upper slope was characterized by steeply sloping isotherms and isopleths of iodide and iodate, resulting from a dome of cold water that was rich in iodate and nearly devoid of iodide at the slope. Both the mid and the inner shelf were relatively well mixed vertically. The concentration of iodate in the surface waters decreased shoreward from >0.3 μM in the Sargasso Sea/Gulf Stream/outer shelf, to 0.29 μM in the midshelf, 0.19 μM in the outer-inner shelf and 0.11 /IM in the inner-inner shelf. Concomitantly, the concentration of iodide increased from <161 nM to 175 nM, 257 nM and 300 nM. The concentration changes were more abrupt in the inner-inner shelf within about 30 km from the shore. There was no evidence of significant concentrations of organic iodine. These distributions of iodide and iodate suggest that the South Atlantic Bight may act as a geochemical processor of dissolved iodine. Iodate is added to the shelf during topographically induced upwelling and frontal exchange with the Gulf Stream. In the shelf waters, iodate is reduced to iodide in situ. Iodide is exported from the shelf to the Gulf Stream which may eventually further transport it to the ocean interior. A ☐ model calculation suggests that 28% and 43% of the iodate added to the Bight and the inner shelf, respectively, are converted to another form in these waters, almost all of which is iodide. About a third of the reduction of iodate to iodide in the Bight occurs in the inner shelf. Thus, the inner shelf may be the most geochemically active zone within the Bight. The residence times of iodide relative to its production and that of iodate relative to its removal are 3.1 and 3.6 months in the Bight and 0.9 and 1.8 months in the inner shelf.  相似文献   

Numerical model simulations of continental shelf flows off northern California     

B.T. Kuebel Cervantes  J.S. Allen 《Deep Sea Research Part II: Topical Studies in Oceanography》2006,53(25-26):2956

The three-dimensional circulation on the continental shelf off northern California in the wind events and shelf transport (WEST) experiment region during summer 2001 is studied using the primitive equation regional ocean modeling system (ROMS). The simulations are performed with realistic topography and initial stratification in a limited-area domain with a high-resolution grid. Forcing consists of measured wind-stress and heat flux values obtained from a WEST surface buoy. The general response shows a southward coastal upwelling jet of up to and a weakening or reversal of currents inshore of the jet when upwelling winds relax. Model results are compared to WEST moored velocity and temperature measurements at five locations, to CODAR surface current observations between Pt. Reyes and Bodega Bay, and to hydrographic measurements along shipboard survey lines. The model performs reasonably well, with the highest depth-averaged velocity correlation (0.81) at the inshore mooring (40 m water depth) and lowest correlation (0.68) at the mid-depth mooring (90 m depth). The model shows generally stronger velocities than those observed, especially at the inshore moorings, and a lack in complete reversal of southward velocities observed when upwelling winds relax. The comparison of surface velocities with CODAR measurements shows good agreement of the mean and the dominant mode of variability. The hydrography compares closely at the southern and northern edges of the survey region (correlation coefficients between 0.90 and 0.97), with weaker correlations at the three interior survey lines (correlation coefficients between 0.44 and 0.76). Mean model fields over the summer upwelling period show slight coastal jet separation off Pt. Arena and significant separation off Pt. Reyes. The cape regions also experience relatively strong bottom velocities and nonlinearity in the surface flow. Across-shelf velocity sections examined along the shelf reveal a double jet structure that appears just north of Bodega Bay and shows the offshore jet strengthening to the south. We examine the dynamics during an upwelling and subsequent relaxation event in May 2001 in which the WEST measurements show evidence of a strong flow response. The alongshelf variability in the upwelling and relaxation response introduced by Pt. Reyes is evident. Analysis of term balances from the depth-averaged momentum equations helps to clarify the event dynamics in different regions over the shelf. A clear pattern in the nonlinear advection term is due to the spatial acceleration of the southward jet around the capes of Pt. Arena and Pt. Reyes during upwelling. Results from a three-dimensional Lagrangian analysis of water parcel displacement show significant southward displacement in the coastal jet region, including a strong signal from the double jet. Alongshelf variability in parcel displacements and upwelling source waters due to the presence of Pt. Arena and Pt. Reyes is also apparent from the Lagrangian fields. A cyclonic eddy-like recirculation feature offshore of Pt. Arena prior to the upwelling event causes large patches of onshore-displaced parcels. Additionally, across-shelf variability in the response of water parcels along the D line includes decreased vertical displacement and increased alongshelf displacement in the offshore direction.  相似文献