Mediterranean Intermediate Water Research Articles

The Effect of the Source of Deep Water in the Eastern Mediterranean on Western Mediterranean Intermediate and Deep Water

The deep water in the western Mediterranean Sea was found to be significantly affected by a climatic event that took place in the eastern Mediterranean during the 1990s. Numerical simulations of the entire Mediterranean Sea showed that multiple equilibria states in the eastern Mediterranean can exist under present-day-like conditions. The two stable states that were found are associated with intermediate water exchange between the eastern Mediterranean's Aegean and Adriatic Basins. In the first state, the Adriatic acts as a source of deep water that flows into the deep layers of the eastern Mediterranean; in the second state, there is no source of deep water in the Adriatic and the eastern Mediterranean intermediate water is warmer and saltier. We studied the water pathways, in both stable states, into the western Mediterranean and found that the eastern Mediterranean water's properties signature can be seen as far as the Gulf of Lion, which is an important open-ocean deep water convection site. Meaning that, the eastern Mediterranean water characteristics are manifested in deep and intermediate water properties all over the Mediterranean Sea. The water propagating from the eastern to the western Mediterranean also has different flow regimes, in both states, through the Sicily Strait and in the Tyrrhenian Basin, as seen from a Lagrangian analysis.

133,000 Years of Sedimentary Record in a Contourite Drift in the Western Alboran Sea: Sediment Sources and Paleocurrent Reconstruction

The Djibouti Ville Drift is part of a contourite depositional system located on the southern side of the Djibouti Ville Seamount in the Alboran Sea (Western Mediterranean). The sedimentary record of a core located in the drift deposits has been characterized to achieve the possible sediment sources for the Saharan dust supply and the paleocurrent variability related to Mediterranean intermediate waters for the last 133 kyr. Three end-member grain-size distributions characterize the sediment record transported by the bottom current to address the different aeolian populations, i.e., coarse EM1, silty EM2, and fine EM3. For these particles, the most likely source areas are the Saharan sedimentary basins and deserts, as well as the cratonic basins of the Sahara-Sahel Dust Corridor. The prevalence of these main source areas is shown in the core record, where a noticeable change occurs during the MIS 5 to MIS 4 transition. Some punctual sediment inputs from the seamount have been recognized during sea-level lowstand, but there is no evidence of fluvial supply in the drift deposits. The paleocurrent reconstruction allows the characterizing of the stadial and cold periods by large increases in the mean sortable silt fraction and UP10, which point to an enhanced bottom current strength related to intermediate water masses. Conversely, interglacial periods are characterized by weaker bottom current activity, which is associated with denser deep water masses. These proxies also recorded the intensified Saharan wind transport that occurred during interstadial/stadial transitions. All these results point to the importance of combining sediment source areas with major climatic oscillations and paleocurrent variability in palaeoceanographic sedimentary archives, which may help to develop future climate prediction models.

Paleoceanographic and climatic implications of a new Mediterranean Outflow branch in the southern Gulf of Cadiz

The presence of contourite drifts in the southern Gulf of Cadiz (GoC) along the Moroccan margin raises questions about the (re)circulation of Mediterranean Outflow Water (MOW) in the GoC and the origin of the currents depositing them. Here, we compare two cores representative of Iberian and Moroccan contourite drifts, covering the last 22 kyr. Although the whole sequence is contouritic in character, it reflects the interaction of distinctive silty-contourite facies (high flow velocity periods) imbedded in muddy-contourite facies (low flow velocity periods). Evidence from benthic foraminifera δ13C, sortable silt grain-size, oceanographic CTD profiles and numerical simulations, indicate the Mediterranean water mass as the source of the southern contourite deposits. Our data, therefore, suggests an additional branch of upper-MOW veering southwards off the Straits of Gibraltar along the Moroccan margin. During MIS-(Marine Isotope Stage) 2, upper-MOW was a sluggish current while in the Holocene upper-MOW dominated as a fast, semi-steady flow. Throughout the deglaciation, silty contourites associated with higher flow speeds were deposited in the northern and southern GoC during cold events such as Heinrich Stadial 1 (HS1) and the Younger Dryas, forced by global millennial-scale climate variability. Millennial variability also appears to drive the deposition of silty-contourites in the Holocene. We estimated an average duration of 1 ka for the process of depositing a fast contourite unit. The case of silty-contourite I6 (within HS1) allows us to illustrate with extremely high resolution a “rapid” sequential change in circulation, with gradual slow-down of dense Mediterranean water while surface was freshening (HS1), provoking injection of high-salinity intermediate waters (via contour-currents) into the GoC, and hence the North Atlantic. The subsequent brief collapse of dense water formation in the Mediterranean Sea triggered a major increase in sea surface temperatures (10 °C/ka) in the GoC, developing into the next interstadial (Bølling/Allerød). The impact of Mediterranean intermediate waters is manifested here by triggering a substantial rearrangement of intermediate and deep circulation in the North Atlantic, which would have further impacted the Atlantic Meridional Overturning Circulation (AMOC).

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

Abstract. We present the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species from a sediment core collected at 622 m water depth in the Balearic Sea, as well as εNd of scleractinian cold-water corals (CWC; Madrepora oculata, Lophelia pertusa) retrieved between 280 and 442 m water depth in the Alboran Sea and at 414 m depth in the southern Sardinian continental margin. The aim is to constrain hydrological variations at intermediate depths in the western Mediterranean Sea during the last 20 kyr. Planktonic (Globigerina bulloides) and benthic (Cibicidoides pachyderma) foraminifera from the Balearic Sea were also analyzed for stable oxygen (δ18O) and carbon (δ13C) isotopes. The foraminiferal and coral εNd values from the Balearic and Alboran seas are comparable over the last ∼ 13 kyr, with mean values of −8.94 ± 0.26 (1σ; n = 24) and −8.91 ± 0.18 (1σ; n = 25), respectively. Before 13 ka BP, the foraminiferal εNd values are slightly lower (−9.28 ± 0.15) and tend to reflect higher mixing between intermediate and deep waters, which are characterized by more unradiogenic εNd values. The slight εNd increase after 13 ka BP is associated with a decoupling in the benthic foraminiferal δ13C composition between intermediate and deeper depths, which started at ∼ 16 ka BP. This suggests an earlier stratification of the water masses and a subsequent reduced contribution of unradiogenic εNd from deep waters. The CWC from the Sardinia Channel show a much larger scatter of εNd values, from −8.66 ± 0.30 to −5.99 ± 0.50, and a lower average (−7.31 ± 0.73; n = 19) compared to the CWC and foraminifera from the Alboran and Balearic seas, indicative of intermediate waters sourced from the Levantine basin. At the time of sapropel S1 deposition (10.2 to 6.4 ka), the εNd values of the Sardinian CWC become more unradiogenic (−8.38 ± 0.47; n = 3 at ∼ 8.7 ka BP), suggesting a significant contribution of intermediate waters originated from the western basin. We propose that western Mediterranean intermediate waters replaced the Levantine Intermediate Water (LIW), and thus there was a strong reduction of the LIW during the mid-sapropel ( ∼ 8.7 ka BP). This observation supports a notable change of Mediterranean circulation pattern centered on sapropel S1 that needs further investigation to be confirmed.

Hydrodynamics over the Gulf of Valencia continental slope and their role in sediment transport

Circulation patterns and sediment dynamics were studied over the Gulf of Valencia (GoV) continental slope during spring and winter 2011–2012. Two moorings were deployed at two locations; at 450m depth from February to May 2011, and at 572m depth from October 2011 to February 2012. At both mooring sites, observations were made of currents, temperature and near-bottom turbidity within the lowermost 80m above the seafloor. The temperature measurements allowed distinction of the different water masses and their temporal evolution. The fluctuations of the boundary between the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW) masses were monitored, and several intrusions of Western Mediterranean Intermediate Water (WIW) were observed, generally coinciding with changes in current direction. At both mooring sites, the currents generally maintained low velocities <10cms−1, with several pulses of magnitude increases >20cms−1, and few reaching up to 35cm s−1, associated with mesoscale eddies and topographic waves. The current direction was mainly towards the SSE on the first deployment and to the ESE on the second deployment. This second location was affected by a strong bottom offshore veering presumably generated by local topographic effects. Increases in suspended sediment concentrations (SSC) were observed repeatedly throughout the records, reaching values >3mgl−1. However, these SSC variations were uncorrelated with changes in velocity magnitude and direction and/or with temperature oscillations. Results presented in this paper highlight the complex relation between the hydrodynamics and sediment transport over the GoV continental slope, and suggest that other potential sediment resuspension mechanism not linked with current fluctuations, might play a key role in the present-day sedimentary dynamics. Resuspension due to bottom trawling appears to be the most plausible mechanism.

Vertical structure and temporal evolution of an anticyclonic eddy in the Balearic Sea (western Mediterranean)

An anticyclonic eddy in the Balearic Sea (western Mediterranean) was described using data from a mooring line deployed at the northern slope of Mallorca Island at about 900 m deep. Its surface signature was investigated using sea surface height and sea surface temperature images. The eddy, which lasted around 1 month, modified the thermohaline characteristics and the currents of the entire water column. Levantine Intermediate Waters, usually resident in the region, were displaced by colder and fresher Western Mediterranean Intermediate Waters associated with the eddy. Along‐slope main currents (toward NE) were completely reversed at 500 m and significantly deviated at 900 m. Interestingly, near‐bottom velocities were found to be systematically larger than those at intermediate depths. Furthermore, during the eddy, velocities reached values up to 26 cm/s at the bottom, 5 times larger than the bottom average speed. The recurrence of the phenomenon was explored with an eddy detection tool applied to satellite observations. Results indicated that anticyclonic eddies are common structures in the Balearic Current.

Turbulent mixing in the eddy transport of Western Mediterranean Intermediate Water to the Alboran Sea

Western Mediterranean Intermediate Water (WIW) is formed in winter in the North‐Western Mediterranean. WIW, identifiable as a distinct temperature minimum layer between Atlantic‐Mediterranean Interface waters and the denser Levantine Intermediate Water, is carried down the east coast of Spain in anticyclonic mode water eddies, or “weddies” eventually reaching the Alboran sea. A previous detailed analysis of a weddy in the vicinity of the Almeria‐Oran front indicated that it could have accounted for 10% of a winter's production of WIW, but this analysis was unable to consider turbulent dissipation. In this study we present microstructure measurements across a similar observation of WIW in the vicinity of the Almeria‐Oran front and show that this figure could be conservative by 15–50% due to the turbulent dissipation associated with a weddy.

Small-scale differences in the distribution and population dynamics of pandalid shrimps in the western Mediterranean in relation to environmental factors

Pandalidae is one of the most diverse families of marine shrimps. Most studies on pandalid shrimps focus on their vertical zonation, and there are few studies on their horizontal distribution and their relation to environmental factors. The aim of this paper was to study the small-scale spatial and temporal patterns of pandalid shrimps in two nearby areas off the Balearic Islands that have different environmental regimes, and determine the relationship between these patterns and the environmental conditions. Information was obtained from seasonal and annual experimental surveys. The abundance and biomass of all the pandalid shrimps caught as well as the length, weight, sex and maturity stage of the most abundant species were recorded. Environmental parameters were obtained from seasonal experimental surveys and included data on hydrography, trophic resources and sediment characteristics. Information about the trawl fleet was obtained through onboard sampling, and daily landings data were obtained from sales records. Eight pandalid species were found during the annual and seasonal experimental bottom trawl surveys carried out around the Balearic Islands. These species showed clear spatial differences in their population dynamics at the two study sites, the Algerian sub-basin (AsB) and the Balearic sub-basin (BsB), which could be related to the different environmental conditions of the two sub-basins. The relative importance of pandalid shrimps of the genus Plesionika in the slope crustacean communities was higher in the Algerian sub-basin. The abundances of Plesionika martia, Plesionika acanthonotus, Plesionika giglioli and Plesionika antigai were higher in the Algerian sub-basin, and only Plesionika heterocarpus, the most benthic species among those analysed, was more abundant in the Balearic sub-basin, where trophic webs are supported largely by suprabenthos rather than zooplankton biomass. The influence of water masses depends on the specific bathymetric distribution, and the shallowest species (P. antigai) was related to the Western Mediterranean Intermediate Waters, while the deepest species (P. martia and P. acanthonotus) were related to the warmer and more saline Levantine Intermediate Waters and Western Mediterranean Deep Waters. The two species found in intermediate waters (P. heterocarpus and P. giglioli) had a deeper distribution in the Algerian sub-basin, where the layer of Levantine Intermediate Waters is wider. The mean length of P. heterocarpus was higher in the Balearic sub-basin, and that of P. martia was higher in the Algerian sub-basin. The condition index was higher in the BsB than in the AsB for both P. heterocarpus and P. martia. This could be because the BsB is influenced to a greater extent by slope fronts that increase the primary production in the area, and thus enrich the trophic chain. The condition index also showed seasonal differences, which could be related to the reproductive cycle and to the availability of potential food resources. The strong relations found between the geographical distribution of the pandalid shrimps in the Balearic Islands and the environmental variables imply that there are important differences between populations of nearby areas when the environmental conditions in which they live differ.

Interaction between seabed morphology and water masses around the seamounts on the Motril Marginal Plateau (Alboran Sea, Western Mediterranean)

The seabed morphology in the vicinity of the seamounts on the Motril Marginal Plateau (northern Alboran Sea) was investigated using high-resolution (sparker) and very high-resolution (TOPAS) seismic reflection profiles and multibeam bathymetry. The aim of the study was to determine the recent geological processes, and in particular those that control the contourite depositional system associated with the intermediate and deep Mediterranean water masses. Six groups of morphological features were identified: structural features (seamount tops, tectonic depressions), fluid escape-related features (pockmarks), mass-movement features (gullies, slides), bottom-current features (moats, scour marks, terraces, elongated and separated drifts, plastered drifts, confined drifts, sheeted drifts), mixed features (ridges) and biogenic features (including evidence of (dead) cold water corals such as Lophelia pertusa and Madrepora oculata). The main processes controlling the formation of these features are recent tectonic activity and the interaction of Mediterranean water masses with the seafloor topography. Seamounts act as topographic barriers that affect the pathway and velocity of the deep Mediterranean water masses, which are divided into strands that interact with the surrounding seafloor. The influence of the intermediate Mediterranean water mass, by contrast, is restricted mainly to the tops of the seamounts. Sediment instability and fluid-escape processes play a minor role, their occurrence being probably related to seismicity.

Mg/Ca in the planktonic foraminifera Globorotalia inflata and Globigerinoides bulloides from Western Mediterranean plankton tow and core top samples

Abstract Due to its strong gradient in salinity and small temperature gradient the Mediterranean provides an ideal setting to study the impact of salinity on the incorporation of Mg into foraminiferal tests. We have investigated tests of Globorotalia inflata and Globigerina bulloides in plankton tow and core top samples from the Western Mediterranean using ICP-OES for bulk analyses and LA-ICP-MS for analyses of individual chambers in single specimens. Mg/Ca observed in G. inflata are consistent with existing calibrations, whereas G. bulloides had significantly higher Mg/Ca than predicted, particularly in core top samples from the easterly stations. Scanning Electron Microscopy and Laser Ablation ICP-MS revealed secondary overgrowths on some tests, which could explain the observed high core top Mg/Ca. We suggest that the Mediterranean intermediate and deep water supersaturated with respect to calcite cause these overgrowths and therefore increased bulk Mg/Ca. However, the different species are influenced by diagenesis to different degrees probably due to different test morphologies. Our results provide new perspectives on reported anomalously high Mg/Ca in sedimentary foraminifera and the applicability of the Mg/Ca paleothermometry in high salinity settings, by showing that (1) part of the signal is generated by precipitation of inorganic calcite on the foraminifer test due to increased calcite saturation state of the water and (2) species with high surface-to-volume shell surfaces are potentially more affected by secondary Mg-rich calcite encrustation.

Chemical signature of intermediate water masses along western Portuguese margin

Although the circulation of intermediate water masses in the eastern North Atlantic remains poorly defined, the presence of fresher intermediate waters, the Sub-Artic (SAIW) and the Antarctic Intermediate Water (AAIW), as well the saline intermediate Mediterranean Water (MW), has been tracked using biogeochemical properties. Here we assess the hydrographic and chemical structures of intermediate waters along the western Portuguese margin by examining the vertical distributions and property-property plots of chemical tracers (oxygen and nutrients). AAIW was traced by low oxygen and high nutrients, while SAIW was recognized by low nutrients. The Mediterranean Water (MW) undercurrent is shown to spread towards the eastern flank of Gorringe bank. Concurrently, the fresher waters gained salt by direct incorporation of MW, while this water was enriched in nutrients on its way northward and westward owing, to a great extent, to the entrainment of an AAIW branch. The distributions of nutrients and apparent oxygen utilization are discussed in terms of regional ocean circulation. Our analysis suggests a circulation pattern of the various intermediate waters along the western Portuguese margin: MW extends all over the area, but its presence is more pronounced around cape St. Vincent; SAIW apparently moves southward, reaching the Gorringe bank region, and AAIW flows northward along the coast and around the bank.

Influence of the Icelandic Low on the Variability of Surface Air Temperature in the Gulf of Lion: Implications for Intermediate Water Formation

Abstract The authors investigate the interannual variability of surface air temperature in the Gulf of Lion (northwestern Mediterranean Sea) during the winter season, which has been proposed as the dominant factor for the Mediterranean Intermediate Water (WIW) formation. Recent studies suggest that the variability of WIW formation in the Gulf of Lion can be partially explained by the North Atlantic Oscillation. To examine this process in more detail, the authors separate the North Atlantic Oscillation into two centers of action, the Azores high and the Icelandic low. This approach reveals that the pressure of the Icelandic low controls the surface air temperature in the Gulf of Lion, and the influence of the Azores high is insignificant. It is found that the winds over Europe are predominately northerly during winters with high pressure values of the Icelandic low. These conditions also correspond to colder air temperatures in the Gulf of Lion, which have been proposed previously to be correlated with ocean convection to intermediate depths.

Eddy transport of Western Mediterranean Intermediate Water to the Alboran Sea

During the second cruise of the EU funded OMEGA project the towed undulating vehicle SeaSoar, was deployed to survey the upper 350 m of the water column in the eastern Alboran Sea and extreme western Algerian basin. With an effective along‐track resolution of 4 km, the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them. The Almeria Oran front forms at the eastern boundary of the Alboran Sea gyre system, in the upper 150–200 m of the water column, and separates waters of predominantly Atlantic origin from those formed in the Western Mediterranean Sea. Below these surface waters, but above the Levantine Intermediate Water, Western Mediterranean Intermediate Waters, believed to be formed to the north of the Balearic Sea, are normally observed in this region. However, to our knowledge, this is the first time a discrete eddy of Western Mediterranean Intermediate Water, a “weddy,” has been described in the extreme western Algerian basin. Repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d. A climatological analysis of historical data in the MEDAR/MEDATLAS database provides evidence for the repeatability of this observation and the significance of the estimated transport.

The influence of oceanographic scenarios on the population dynamics of demersal resources in the western Mediterranean: Hypothesis for hake and red shrimp off Balearic Islands

The aim of the present paper is to study the relationships between some climatic indices and parental stock, recruitment and accessibility to trawl fishery of hake ( Merluccius merluccius) and red shrimp ( Aristeus antennatus) off Balearic Islands (western Mediterranean). Available annual catch per unit effort, recruitment and spawning stock biomass have been used as biological data. As environmental data, the meso-scale IDEA index and the large-scale North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO) indices have been used. To analyze possible links between these indices with the population dynamics of demersal resources, two non-linear approaches have been applied: (i) stock–recruitment relationships from Ricker and Beverton–Holt models, by sequentially incorporating environment factors; (ii) generalized additive modelling, both classical general and threshold non-additive models were considered. The latter simulate an abrupt change in explicative variables across different phases (time periods or climatic index values). The results have shown that two oceanographic scenarios around the Balearic Islands, associated with macro and meso-scale climate regimes, can influence the population dynamics of hake and red shrimp. This is especially true for recruitment, which seems to be enhanced during low NAO and IDEA indices periods. During these periods, colder-than-normal winters generate high amounts of cold Western Mediterranean Intermediate Waters (WIW) in the Gulf of Lions, which flow southwards and reach the Balearic Islands channels in spring, increasing the productivity in the area. This oceanographic scenario could also be favourable to the distribution of hake on the fishing grounds where the trawl fleet targets this species, increasing its accessibility to the fishery. Both spawning stock and abundance of red shrimp seems to be also enhanced by high MO index periods, which could reflect the increased presence of the saline and warm Levantine Intermediate Waters (LIW) in the study area, extending over the fishing grounds of this species. The proposed interactions can be useful to assess and manage these important demersal resources.

Hydrographic conditions affecting two fishing grounds of Mallorca island (Western Mediterranean): during the IDEA Project (2003–2004)

This paper describes the hydrographic conditions observed during six surveys carried out during 2003 and 2004, in the framework of the “IDEA Project” (acronym for “Influence of oceanographic structure and dynamics on demersal populations in waters of the Balearic Islands”). The surveys were developed on the shelf and slope of Mallorca Island, in particular in two fishing grounds at the north and south of the Mallorca channel. Periodic movements of the fishing fleet between these two areas have been regularly reported, suggesting a seasonal variability of the resources which could be in turn associated with the hydrodynamic variability. With this motivation, water masses affecting these grounds have been identified and their seasonal variability has been studied. Different oceanographic and environmental conditions have been found between the two fishing grounds. These differences are related to the presence of mesoscale structures, associated with the Western Mediterranean Intermediate Water (WIW) at the north of the Ibiza channel and big gyres detached from the Algerian Current. The former has been shown to have influence on the regional oceanic circulation and the latter could affect the progress of fresh Atlantic Water (AW) towards the channels and make possible the presence of high salinity values at intermediate waters at the south of Mallorca Island. Historical data from other oceanographic cruises carried out in the region are finally used to discuss the interannual variability of these mesoscale structures.

A mesoscale index to describe the regional circulation around the Balearic Islands

Historical oceanographic surveys carried out around the Balearic Islands (western Mediterranean) suggest two different scenarios for the regional ocean circulation. In one scenario, occurring during cold winters, cool water is formed at intermediate layers (100–300 m) in the Gulf of Lions. This Western Mediterranean Intermediate Water (WIW) usually moves southward reaching the Balearic Channels, deflecting the warmer Levantine Intermediate Water (LIW) coming from the Eastern Mediterranean, and even blocking the Ibiza Channel. On the other hand, during mild winters, less WIW is formed and then LIW flows through the channels, appearing at their characteristic depths. The oceanographic surveys around the Balearic Islands (1985–2004) have provided a qualitative index, indicating the presence or not of WIW in the Ibiza Channel, based on the analyses of θS diagrams. A quantitative index based on mean water temperature between 100 and 300 m depth in the channels may also be defined. Both indexes, the qualitative and the quantitative, give consistent information on WIW presence for the period 1985–2004, however, both are short in time and have gaps in the series. In order to obtain a longer and continuous index for WIW presence and then for regional circulation, air–sea heat fluxes at the Gulf of Lions during winter months were obtained from the meteorological NCEP/NCAR reanalysis dataset and compared with other meteorological data such as surface air temperature. The standardized air temperature anomalies at 1000 hPa in the Gulf of Lions during winter (December–March) has been shown to be the simplest and best indicator of absence/presence of WIW in the Balearic Islands channels in late spring. Values above 1.0 of the standardized temperature anomaly would indicate absence of WIW in the Ibiza Channel. The high correlation obtained with available in-situ oceanographic data allows the use of this index as an indicator of presence of WIW and then of different regional circulation scenarios backwards in time and in those years for which the oceanographic data are missing or scarce.

Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 341:37-44 (2007) - doi:10.3354/meps341037 Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean E. J. V. Gillibrand1,3,*, A. J. Jamieson1, P. M. Bagley1, A. F. Zuur2, I. G. Priede1,** 1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeen AB41 6AA, UK 2Highland Statistics Ltd, 6 Laverock Road, Newburgh, Aberdeen AB41 6FN, UK 3Present address: Robert Gordon University, Schoolhill, Aberdeen AB10 1FR, UK *Née Battle**Corresponding author. Email: i.g.priede@abdn.ac.uk ABSTRACT: Vertical distribution of bioluminescent organisms throughout the water column in the Porcupine Seabight and Porcupine Abyssal plain area of the NE Atlantic Ocean was measured using a free-falling autonomous vehicle equipped with a high-sensitivity video system viewing stimulated flashes of light from impacts on a mesh screen. Data were recorded from 500 m depth down to the sea floor at depths from 740 to 4808 m during spring and autumn of 2001 and 2002. Bioluminescent organisms decreased in abundance from a maximum of 80 m–3 at 600 m depth to a mean of 0.5 to 1.2 m–3 at depths greater than 3000 m. During autumn, a seasonally enhanced peak of abundance (mean 27.19 m–3, compared with 7.52 m–3 in spring) centred at 1420 m within the depth range of occurrence of Mediterranean intermediate water. It is hypothesised that this bioluminescent layer reflects seasonal increase in deep pelagic biomass fed by downward transport of organic matter following the spring peak of primary production in the surface layers. The bioluminescent layer is a large-scale regional phenomenon extending over 100s of kilometres. KEY WORDS: Bioluminescence · Deep pelagic · NE Atlantic Ocean · Mediterranean intermediate water · Pelagic–benthic flux Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 341. Online publication date: July 04, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Three‐dimensional modeling of the Gulf of Lion's hydrodynamics (northwest Mediterranean) during January 1999 (MOOGLI3 Experiment) and late winter 1999: Western Mediterranean Intermediate Water's (WIW's) formation and its cascading over the shelf break

The Gulf of Lion's hydrodynamics are investigated with a numerical simulation of the 1998–1999 winter, performed with a three‐dimensional (3‐D) free‐surface model and real forcings. The model initial state derives from the large‐scale outputs of the MOM (1/8°) using a specific initialization method. The model results are validated with an oceanographic data set collected in the gulf during the MOOGLI3 cruise (11–21 January 1999). The modeled outputs agree well with the main hydrological and circulation patterns observed during MOOGLI3. The study focuses on a strong and peculiar event of dense water (Western Mediterranean Intermediate Water (WIW)) formation on the continental shelf and its cascading over the shelf break. This latter was detected by a steep temperature decrease in the time series of a mooring line in the nearby Lacaze‐Duthiers canyon which is well reproduced in the simulation. Modeled dense plumes appear during the MOOGLI3 period but more especially during late winter. Shapiro's theory of dense plume propagating on a sloping bottom is applied and provides information of the plume driving mechanisms. In both cases of plume studied, friction effects and Ekman drainage by the ambient current explain only a part of the plume motion. The complementary forcing is probably the steep local bathymetry irregularities. This 3‐D simulation permits also to evaluate the total amount of WIW formed during the 1998–1999 winter over the Gulf of Lion's shelf to 500 km3.

Total mercury in the water column near the shelf edge of the European continental margin

The purpose of this paper is to investigate the distribution and the relative importance of processes that affect mercury distribution in the water column at the shelf edge of the Celtic Sea, on the western European continental margin. The water column, down to 4500 m, was sampled during two cruises, one in winter 1994 and the other in June 1995, on eight stations. Total mercury concentrations ranged from 0.29 to 9.37 pM with the extreme values in the first 200 m. The highly variable concentrations in the surface layers are attributed to intense atmospheric-sea water exchange processes. The low concentrations encountered in the shelf water prelude any significant influence of continental waters in this area. In the mixed layer, biological uptake and regenerative processes appear to dominate in controlling the vertical mercury distribution. The intermediate and deep-water mercury distribution is mainly governed by the hydrographic features. In summer, a mercury enrichment was observed in the upwelling zone of the shelf break. Mean mercury concentration in the Lower Deep Water (LDW) was typically 1.96±0.23 pM and in the Labrador Sea Water (LSW) 1.97±0.63 pM. The Mediterranean Intermediate Water (MIW) revealed similar levels, with a mean mercury concentration of 2.04±0.84 pM. Between LDW and LSW, the Iceland–Scotland Overflow Water (SOW) exhibited higher concentrations (2.76±0.35 pM). The upper part of the North East Atlantic Central Water (NEADW) and lower part of the North Atlantic Deep Water (NEACW) were characterized by marked peaks of concentration, which are interpreted in term of atmospheric influence and particle regeneration.

Trace metal (Cd, Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)

Abstract This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and export to deep waters). The concentrations of dissolved Cd, Cu, Ni and Pb were measured with this aim in January 1994 and June 1995 at vertical stations across slope, including stations with upwelling, and in the surface waters along the Celtic Sea shelf. Additionally, deep sea (from sediment trap data) and atmospheric fluxes were estimated. The metal profiles over the slope off the Celtic Sea are quite similar to open ocean profiles already described in the northeast Atlantic, and the concentrations in surface waters are only slightly enriched compared to the nearby open ocean (1.2–1.3× for Cd and Ni). The external sources to the system appear to be of weak influence: the fluvial input is locally strong at the coast and then “diluted” along the large continental shelf; the atmospheric deposition is not significant at the annual scale in comparison to the metal content in the upper waters of the shelf edge (at least for Cd, Ni and Cu). In the upwelling zone, a significant increase in concentrations was observed in the summer surface mixed layer (×2 for nitrate and Cd and ×1.5 for Ni) in comparison to the non-upwelling zone. In winter, concentrations of bioreactive metals increased significantly in the surface waters in comparison to the low summer levels (×5 for nitrate and Cd). Our results suggest that upwelling and winter mixing act as regenerated sources that lead to the resupply of the bioreactive elements above the permanent thermocline with a low export to deeper waters. The tracing of the Mediterranean intermediate waters (MIW) from Gibraltar to the studied area shows indeed that its elemental content at the Celtic shelf edge is mainly due to the conservative mixing of the three “end-member” component waters which are thought to make up the MIW. The remineralization of organic matter within this water mass during its transport to the north would contribute only 20% of the nutrients and Cd concentrations recorded at the Celtic Sea shelf edge. According to the correlation found with nutrients in the 10–200-m layer, dissolved Pb would also be subjected to biological uptake and regeneration within the seasonal thermocline. Particulate scavenging removal of Pb would take place below the permanent thermocline throughout the water column.