Mediterranean Deep Water Research Articles

Anthropogenic CO 2 fluxes in the Otranto Strait (E. Mediterranean) in February 1995

This study presents the distribution and fluxes of dissolved inorganic carbon (C T), total alkalinity ( A T) and anthropogenic carbon (C ant) along the Otranto strait, during February 1995. Based on a limited number of properties (temperature, dissolved oxygen, total alkalinity and dissolved inorganic carbon), the composite tracer TrOCA was used to estimate the concentration of anthropogenic CO 2 in the Otranto strait. Total alkalinity exhibits high values and weak variability throughout the water column of the strait, probably associated with the dense water formation processes in the Adriatic basin that induce a rapid transport of the coastal alkalinity to the deep waters. Elevated C ant concentrations and high anthropogenic pH variations are observed in the bottom layer of the strait, associated with the presence of Adriatic Deep Water (ADW). The study shows that large amounts of C ant have penetrated the highly alkaline Eastern Mediterranean waters, thereby causing a significant pH reduction since the pre-industrial era. Estimates of the transports of C T and C ant through the strait indicate that during February 1995, the Adriatic Sea imports through the Otranto strait natural and anthropogenic carbon and acts as a net sink of carbon for the Ionian Sea. The anthropogenic carbon that is imported to the Adriatic Sea represents less than 1% of the net C T inflow. The Levantine Intermediate Water (LIW) contributes to about one-third of the total C T and C ant inflow. Although the amounts of C ant annually transported by LIW and ADW are almost equal, the contribution of C ant to the C T transported by each water mass is slightly higher in ADW (3.1%) than in LIW (2.6%), as a result of its higher mean C ant concentration. The ADW, despite its weak contribution to the total outflow of C ant, has a vital role for the sequestration and storage of the anthropogenic carbon, as this water mass is the main component of the Eastern Mediterranean Deep Waters and, thus, the anthropogenic CO 2 is transferred in the deep horizons of the Eastern Mediterranean, where it remains isolated for many years.

Contourites offshore Pantelleria Island (Sicily Channel, Mediterranean Sea): depositional, erosional and biogenic elements

High-resolution seismic reflection profiles and multibeam bathymetry data collected in 2006 and 2008 around Pantelleria Island show the widespread occurrence of contourite drifts and erosional elements ~30 km from the narrowest part (~145 km) of the Sicily Channel, where water masses from the Eastern Mediterranean flow towards the Western Mediterranean. The contourite drifts are rather small (up to 10 km long and 3.3 km wide), at water depths of ~250–750 m. Most are elongated separated drifts with quite well-developed moats and crests, aligned roughly parallel to the regional bathymetric contours. Erosional elements include abraded surfaces, moats, scours and sub-circular depressions. In addition, a wide sector of the seafloor adjacent to a seamount located SW of Pantelleria Island is characterized by numerous biogenic build-ups colonized by deep-water corals (Madrepora oculata). The spatial distribution of sediment drifts, erosional features and biogenic build-ups suggests an origin from a north-westward-flowing bottom current, in this case the outflow of Levantine Intermediate Water and transitional Eastern Mediterranean Deep Water via the Sicily Channel. These findings for the Pantelleria offshore sector demonstrate that contourite processes are able to concentrate a high variety of closely spaced depositional and erosional features even in small areas (in this case, about 2,000 km2). This Pantelleria focusing can plausibly be related to a particular configuration of the prevailing bottom-current regime in complex interaction with an uneven bathymetry shaped mainly by tectonic and volcanic activity. The distribution of bottom currents seems to be strongly influenced by morphological features ranging from major seabed obstacles, such as the Pantelleria volcanic complex and the so-called southwest seamount, to smaller-scale escarpments and banks. This is consistent with previous findings for Mediterranean and other settings characterized by neotectonics and large topographic features.

Along-slope oceanographic processes and sedimentary products around the Iberian margin

This contribution to this special volume represents the first attempt to comprehensively describe regional contourite (along-slope) processes and their sedimentary impacts around the Iberian margin, combining numerically simulated bottom currents with existing knowledge of contourite depositional and erosional features. The circulation of water masses is correlated with major contourite depositional systems (CDSs), and potential areas where new CDSs could be found are identified. Water-mass circulation leads to the development of along-slope currents which, in turn, generate contourite features comprising individual contourite drifts and erosional elements forming extensive, complex CDSs of considerable thickness in various geological settings. The regionally simulated bottom-current velocities reveal the strong impact of these water masses on the seafloor, especially in two principal areas: (1) the continental slopes of the Alboran Sea and the Atlantic Iberian margins, and (2) the abyssal plains in the Western Mediterranean and eastern Atlantic. Contourite processes at this scale are associated mainly with the Western Mediterranean Deep Water and the Levantine Intermediate Water in the Alboran Sea, and with both the Mediterranean Outflow Water and the Lower Deep Water in the Atlantic. Deep gateways are essential in controlling water-mass exchange between the abyssal plains, and thereby bottom-current velocities and pathways. Seamounts represent important obstacles for water-mass circulation, and high bottom-current velocities are predicted around their flanks, too. Based on these findings and those of a selected literature review, including less easily accessible “grey literature” such as theses and internal reports, it is clear that the role of bottom currents in shaping continental margins and abyssal plains has to date been generally underestimated, and that many may harbour contourite systems which still remain unexplored today. CDSs incorporate valuable sedimentary records of Iberian margin geological evolution, and further study seems promising in terms of not only stratigraphic, sedimentological, palaeoceanographic and palaeoclimatological research but also possible deep marine geohabitats and/or mineral and energy resources.

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.

Impact of climate variability in the western Mediterranean during the last 20,000 years: oceanic and atmospheric responses

Eolian input fluctuations and paleoceanographic conditions in the western Mediterranean have been reconstructed by using a multi-proxy approach to infer climate variability for the last 20,000 yr. A sediment core from the East Alboran Sea basin provides high-resolution geochemical, mineralogical and grain size distribution records from the Last Glacial Maximum onward. The accurate chronology obtained, closely related with the North Greenland Ice core Project, has provided a detailed paleoclimate reconstruction at the centennial–millennial time scale. Mineral and chemical compositions as well as sediment grain size distribution have been used to establish both fluvial and eolian input variations, deep-water current intensity, and ventilation and productivity conditions at the time of deposition. During the cold period Greenland Stadial-2a, the analyzed proxies evidence significant climate oscillations. Redox proxies, showing marked Mn/Al and Fe/Al peaks, as well as granulometric parameters, support major changes in thermohaline circulation, with active circulation triggered by the Western Mediterranean Deep Water formation. Ba/Al ratios and the presence of authigenic barite point to high productivity during the Last Heinrich event. At this time, Zr/Al, Si/Al and Ti/Al ratios as well as quartz and palygorskite contents also corroborate dryer conditions in the Alboran borderlands, with a major reorganization of atmospheric circulation at the beginning of the deglaciation and during the Last Heinrich event. In contrast, a decreasing trend in the Zr/Al ratio and increasing Mg/Al, K/Al and illite + chlorite/kaolinite ratios would indicate enhanced river runoff and supporting lower eolian dust input and wetter periods during the Bölling–Alleröd. Short-term and abrupt climate oscillations such as the Intra-Alleröd Cold Period are likewise reflected by minor yet conspicuous changes in eolian proxies. During the Younger Dryas, we distinguish an initial cold and dry phase, with major westerlies intensity, thermohaline reactivation, and high productivity levels, and a second wetter phase characterized by sporadic rainfalls over poorly vegetated surrounding continental areas. Then, during the Holocene, alternating phases of major humidity and aridity were recorded by major fluctuations in detrital input. These are also correlated with recurrent forest cover declines reported from pollen records, with a noteworthy early decline in the African Humid Period. During the Holocene, an initial demise of the Last Organic-Rich Layer is also evidenced, tentatively associated with aspiration forces promoted by intense thermohaline circulation through the Gibraltar Strait.

Nd isotopic composition of water masses and dilution of the Mediterranean outflow along the southwest European margin

Nd isotopic compositions (epsilon Nd) of seawater profiles and deep-sea corals collected off the coast of Iberia and from the Bay of Biscay were measured (1) to constrain the Nd isotopic composition of water masses along the southwest European margin, (2) to track the Mediterranean Outflow Water (MOW) during its northward propagation, and (3) to establish hydrological changes during the last 1500 years. The Eastern North Atlantic Central Water (ENACW) is characterized by Nd isotopic composition of around -12.0. Mediterranean Sea Water (MSW) is collected from 800 and 1200 m depth and is characterized by epsilon Nd values ranging from -10.9, off the coast of Iberia, to -11.6 in the Bay of Biscay. These epsilon Nd results suggest a strong dilution of the pure MOW at the Strait of Gibraltar (epsilon Nd -9.4) of approximately 40% and 30% along its northward circulation pathway essentially with a contribution from ENACW. At around 2000 m depth, epsilon Nd water profiles display the occurrence of a nonradiogenic water mass (epsilon Nd -13), originating from the Labrador Sea (Labrador Sea Water). Fossil deep-sea corals, dated between 84 and 1500 years, display Nd isotopic compositions that vary moderately from present-day seawater values, suggesting a weaker influence of MOW in the formation of MSW during the Dark Ages and the Little Ice Age. These recent cold events seem to be associated with a reduction in the northward penetration of MSW, which may result from a greater eastward extension of the middepth subpolar gyre and/or a reduction of MSW formation, likely tied to a variation in deep Mediterranean water production.

Surface and deep water conditions in the Sicily channel (central Mediterranean) at the time of sapropel S5 deposition

New centennial-scale data of benthic foraminifera assemblages and of stable isotopes of foraminifera shells from the Sicily Channel, representative of surface and bottom waters, over the interval between about 140 and 110 kyr BP, are presented. During this period anoxia developed on the eastern Mediterranean basin and sapropel S5 deposited. Although anoxic sediments have not been deposited in the Sicily Channel, this area is strategic to study the character of intermediate waters, whose chemical–physical properties strongly precondition the eastern Mediterranean deep water formation. So far, no data from these water masses have been obtained, apart from the isotopic composition of shells of the planktonic foraminifera species Neogloboquadrina pachyderma which lives at about 200 m depth, thus quite far from the intermediate water core. We conclude that, although with a reduced rate, the flowing of intermediate waters coming from the eastern basin, together with the surface water masses with a clear western affinity, implies the maintenance of an anti-estuarine circulation pattern across the Sicily Channel and in the Mediterranean Sea, also during one of the most severe episodes of anoxia of the Late Pleistocene.

Halogenated persistent organic pollutants in deep water fish from waters to the west of Scotland

Halogenated persistent organic pollutants [polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs)] along with total lipid, were measured in the liver and muscle of three species of deep water fish (black scabbard, black dogfish (liver only) and roundnose grenadier) collected from the Rockall fishing area, to the west of Scotland, between 2006 and 2008. Both contaminant groups were detected in the muscle and liver, with concentrations of PCBs being higher than PBDEs. There were no significant differences in the PCB or PBDE concentrations between the three species, or different sampling locations in the Rockall fishing area. PCB concentrations (ΣICES (International Council for the Exploration of the Sea)7 PCBs) greater than 500 μg kg −1 lipid weight were found in 26 of the 106 liver samples. PCB concentrations were compared to OSPAR assessment criteria, concentrations were above background but below Environmental Assessment Criteria. Estimated Toxic Equivalent (TEQ) concentrations, calculated using published models, in the fish muscle and liver indicated that consumption of deep water fish is unlikely to represent a risk to human health. The high squalene content in some of the black dogfish liver necessitated an additional clean-up step, involving gel permeation chromatography, when analyzing for PBDEs. Concentrations of PBDEs were low with many congeners being below detection limits, particularly in the muscle. There are currently no assessment criteria available for PBDEs. Furthermore, there is only very limited data on PBDEs in deep water fish. However, the concentrations observed in this study were similar to the concentrations recently reported in Mediterranean deep water fish.

Tracers confirm downward mixing of Tyrrhenian Sea upper waters associated with the Eastern Mediterranean Transient

Abstract. Observations of tritium and 3He in the Tyrrhenian Sea, 1987–2009, confirm the enhanced vertical mixing of intermediate waters into the deep waters that has been noted and associated with the Eastern Mediterranean Transient in previous studies. Our evidence for the mixing rests on increasing tracer concentrations in the Tyrrhenian deep waters, accompanied by decreases in the upper waters, which are supplied from the Eastern Mediterranean. The downward transfer is particularly evident between 1987 and 1997. Later on, information partly rests on increasing tritium-3He ages; here we correct the observed 3He for contributions released from the ocean floor. The Tyrrhenian tracer distributions are fully compatible with data upstream of the Sicily Strait and in the Western Mediterranean. The tracer data show that mixing reached to the bottom and confirm a cyclonic nature of the deep water circulation in the Tyrrhenian. They furthermore indicate that horizontal homogenization of the deep waters occurs on a time scale of roughly 5 years. Various features point to a reduced impact of Western Mediterranean Deep Water (WMDW) in the Tyrrhenian during the enhanced-mixing period. This is an important finding because it implies less upward mixing of WMDW, which has been named a major process to enable the WMDW to leave the Mediterranean via the Gibraltar Strait. On the other hand, the TDW outflow for several years represented a major influx of enhanced salinity and density waters into the deep-water range of the Western Mediterranean.

What induced the exceptional 2005 convection event in the northwestern Mediterranean basin? Answers from a modeling study

Open‐sea convection occurring in the northwestern Mediterranean basin (NWMED) is at the origin of the formation of Western Mediterranean Deep Water (WMDW), one of the main Mediterranean water masses. During winter 2004–2005, a spectacular convection event occurred, observed by several experimental oceanographers. It was associated with an exceptionally large convection area and unusually warm and salty WMDW. Explanations were proposed tentatively, relating the unusual characteristics of this event to the Eastern Mediterranean Transient (EMT) or to the atmospheric conditions during winter 2004–2005 in the NWMED. They could, however, not be supported until now. Here we used numerical modeling to understand what drove this convection event. The control simulation performed for the period 1961–2006 reproduces correctly the long‐term evolution of the Mediterranean Sea circulation, the EMT, and the NWMED convection event of 2004–2005. Sensitivity simulations are then performed to assess the respective contributions of atmospheric and oceanic conditions to this event. The weakness of the winter buoyancy loss since 1988 in the NWMED prevented strong convection to occur during the 1990s, enabling heat and salt contents to increase in this region. This resulted in the change of WMDW characteristics observed in 2005. The strong buoyancy loss of winter 2004–2005 was responsible for the intensity of the convection observed this winter in terms of depth and volume of newly formed WMDW. The EMT did not fundamentally modify the convection process but potentially doubled this volume by inducing a deepening of the heat and salt maximum that weakened the preconvection stratification.

Abrupt warming and salting of the Western Mediterranean Deep Water after 2005: Atmospheric forcings and lateral advection

The recent major production of anomalously warm, salty deep water in the northwestern Mediterranean Sea (winters 2004–2005 and 2005–2006) is linked to extreme winter air‐sea heat and freshwater forcing of the basin. Fields of heat and density fluxes are determined both from the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis and a daily high‐resolution downscaling of the European Centre for Medium‐Range Weather Forecasts reanalysis and analysis data set ARPERA. In the deep water formation region, during winter 2004–2005, the net heat loss exceeds 300 W m−2 compared with typical values of 200 W m−2. The relationship between the deep water formation episodes and large‐scale atmospheric patterns is investigated and found to be more closely related to the East Atlantic Pattern than the North Atlantic Oscillation. The contributions of atmospheric forcing and lateral advection of anomalously warm, salty water to the convection region are discussed in order to determine their relative roles in causing massive renewal of Western Mediterranean Deep Water and its anomalous properties. The main result shows that the net evaporation during winter 2004–2005, even if very high compared to the climatology, could have induced only 49% of the actual observed increase in the salt content of the deep layer. Thus, lateral advection played a major role in setting the new deep water properties.

Climatic forcing of eastern Mediterranean deep-water formation and benthic ecosystems during the past 22 000 years

Lateglacial and Holocene faunal and stable-isotope records from benthic foraminifers in the eastern Mediterranean Sea (EMS) suggest a high spatiotemporal variability of deep-water oxygenation and biogeochemical processes at the sea floor during that time. Changes in the oxygenation and food availability of the deep-sea ecosystems are closely linked to the hydrology of the EMS borderlands; they reflect orbital and suborbital climate variations of the high northern latitudes and the African monsoon system. During the last glacial maximum, cool surface waters and high evaporation resulted in maximum convection and oxic deep-waters in all sub-basins. Strong wind-induced mixing fostered surface-water production with seasonal phytodetritus fluxes. During the glacial termination and the Holocene, oxygenation and food availability of deep-sea benthic ecosystems were characterized by a pronounced regional differentiation. Local deep-water formation and trophic conditions were particularly variable in the northern Aegean Sea as a response to changes in riverine runoff and Black Sea outflow. During the interval of sapropel S1 formation in the early Holocene, average oxygen levels decreased exponentially with increasing water depth, suggesting a basin-wide shallowing of vertical convection superimposed by local signals. In the northernmost Aegean Sea, deep-water ventilation persisted during the early period of S1 formation, owing to temperature-driven local convection and the absence of low-salinity Black Sea outflow. At the same time, severe temporary anoxia occurred in the eastern Levantine basin at water depths as shallow as 900 m. This area was likely influenced by enhanced nutrient input of the Nile river that resulted in high organic matter fluxes and related high oxygen-consumption rates in the water column. In the southern Aegean and Levantine Seas, we observe a gradual increase in deep-water residence times, preceding S1 formation by approximately 1–1.5 kyr. Once oxygen levels fell below a critical threshold, the benthic ecosystems collapsed almost synchronously with the onset of S1 deposition. The recovery of benthic ecosystems during the terminal phase of S1 formation is controlled by subsequently deeper convection and re-ventilation over a period of approximately 1500 years. After the re-ventilation of the various sub-basins had been completed during the middle and late Holocene, deep-water renewal was more or less similar to recent rates. During that time, deep-sea ecosystem variability was driven by short-term changes in food quantity and quality as well as in seasonality, all of which are linked to millennial-scale changes in riverine runoff and associated nutrient input.

Hydrological changes in the Ligurian Sea (NW Mediterranean, DYFAMED site) during 1995–2007 and biogeochemical consequences

Abstract. Data obtained during the monthly cruises of the DYFAMED time-series study (northwestern Mediterranean Sea) in the period 1995–2007 were compiled to examine the hydrological changes and the linked variation of some biogeochemical characteristics (nutrients and pigments). A regular increase of temperature and salinity (0.005 °C y−1, 0.0022 psu y−1) was recorded in deep waters of the NW Mediterranean Sea (2000 m depth) during 1995–2005. In February 2006 an abrupt increase in T (+0.1 °C) and S (+0.03 psu) was measured at 2000 m depth as the result of successive intense winter mixing events during the 3 previous years. The February 2006 event led to the mixing of the whole water column (0 to >2000 m) and increased salt and heat content of the Western Mediterranean Deep Water by mixing with saltier and warmer Levantine Intermediate Water. The deficit in fresh water inputs to the western Mediterranean basin in three successive years (2003–2005) was suspected to be the major cause of this event since an increase of salinity in surface waters was monitored during these years. The measured phytoplankton biomass was specifically high after the periods of intense mixing. Chlorophyll a integrated biomass reached 230 mg m−2 in 1999, 175 mg m−2 in 2003, and 206 mg m−2 in 2006. The high levels of biomass were related to the particularly high increases in nutrients content in surface layers following the intense water column mixing and the subsequent development of a diatom bloom (as seen by fucoxanthin content). The occurrence of extreme events (high mixing, high nutrients, and high biomass) increased in recent drought years (2003 to 2006). Our results indicated that the NW Mediterranean Sea productivity is increasing.

Can internal processes sustain reversals of the ocean upper circulation? The Ionian Sea example

In 1997 an inversion in the Ionian upper‐layer circulation was documented and ascribed to a massive inflow of Aegean dense waters associated with the Eastern Mediterranean Transient (EMT) and not to the wind‐stress (Borzelli et al., 2009). Here we generalize the concept hypothesizing that such inversions are possible even in the absence of the Aegean influence. Indeed, salinity and density data collected in the Southern Adriatic, the main source of the Eastern Mediterranean deep water, show decadal variations coherent with changes in the sea level height in the northern Ionian. Scaling considerations suggest that the redistribution of Ionian water masses, resulting from changes in the thermohaline properties of waters entering the basin, can sustain inversions of the upper‐layer circulation. Therefore, we propose a feedback mechanism (named the Adriatic‐Ionian Bimodal Oscillating System – BiOS) between variations in the thermohaline properties of waters formed in the Southern Adriatic and the Ionian circulation.

The nature of transitions from dry to wet condition during sapropel events in the Eastern Mediterranean Sea

Benthic foraminifera were analyzed at high-resolution (millennial scale) along core MD84-641 (33°02′ N; 33°38′ E) recovered in the Eastern Mediterranean Sea and covering the last four climatic cycles. The succession of benthic foraminiferal assemblages shows that the decrease in oxygen content of eastern Mediterranean deep water was progressive for sapropels S8, S7, S6, S5, S4, and S3 and rapid for sapropels S10, S9 and S1. This suggests that climatic conditions conducing to the wet epochs associated with sapropels S8, S7, S6, S5, S4 and S3 were progressive, whereas they were rapid for sapropels S10, S9, and S1. However, hydrological conditions responsible for organic matter preservation events seem to be different from one sapropel to another. Benthic foraminifera did not disappear completely during sapropels S10, S8, S6, and S4, suggesting an episodic activation of deep-water convection and an instability of humid period.

Deep and intermediate layer warming in the western Mediterranean: Water mass changes and heaving

Temperature and salinity have increased in the Western Mediterranean Deep Water during the second half of the 20th century, but surprisingly, the two water masses contributing to its formation (Atlantic Water and Levantine Intermediate Water) show no clear warming trends, at least for the same period of time. Previous studies concerned with long term TS property changes in the Western Mediterranean had dealt with changes on isobaric surfaces. In this study we decompose these changes into their two natural contributions: Changes on isopycnal surfaces and changes induced by heaving (vertical displacements of density surfaces). This decomposition shows that LIW temperature and salinity have increased when changes on density surfaces are analyzed. The upward movement of deep layers has induced cooling on isobaric levels masking LIW temperature trends and underestimating deep layers temperature trends.

Hydrographic characteristics of water masses and circulation in the Northern Ionian Sea

Abstract The hydrography of intermediate and deep-water masses in the Northern Ionian Sea (Apulian Plateau) was studied through four quasi-synoptic multidisciplinary surveys carried out in 2004–2006 as an ancillary oceanographic activity in the frame of the APLABES project. This area plays a crucial role for the entire Mediterranean Basin, being influenced by the water outflow of Adriatic origin, which, under severe winter conditions, is a primary source of dense water for the Eastern Mediterranean. At the end of the 1980s such outflow showed a different behavior, and only in the recent years has a gradual re-establishing of the former condition been detected. As such, the Adriatic Sea has regained its role as a main source of the East Mediterranean Deep Water, which was temporarily inhibited during the well-known Eastern Mediterranean Transient which progressively modified the intermediate and deep layers of the Mediterranean Sea. The general structure of water masses was similar through the investigated period, but interesting differences within the bottom layer have been detected. The interaction of the different water types present in the basin is reviewed by means of property–property plots, vertical sections, isopycnal analyses and using an Optimum Multiparameter Analysis (OMP), which is an objective method to measure the mixing of water masses. Due to the lack of any direct information about the dynamics of the water column in the area of the Apulian Plateau during the whole analyzed period, the classical method to infer the baroclinic velocity from the mass field has been applied to hydrographic data. The well-known indeterminacy of this method due to the barotropic component of the velocity field has been resolved using a short time series of current velocities acquired synoptically by a mooring located in the northern part of the studied area. The wavelet transform was adopted for localizing and quantifying the variability of currents simultaneously in both frequency and time domains. The presence of the Adriatic Deep Water close to the bottom was detected on all four surveys, with different signature as underlined by the objectively analysis (with the Optimum Multiparameter Analysis) of the thermohaline field. A core of cold, less-saline and oxygenated water of Adriatic origin coming from the Otranto Channel was identified. This water mass moved in geostrophic balance along the isobaths at 600–1000 m depth at the isopycnal surface of 29.18 kg m−3, not being dense enough to reach the deeper layers of the Ionian Basin, carrying 0.27–0.36 Sv.

Relationship between environment and the occurrence of the deep-water rose shrimp Aristeus antennatus (Risso, 1816) in the Blanes submarine canyon (NW Mediterranean)

We performed a multidisciplinary study characterizing the relationships between hydrodynamic conditions (currents and water masses) and the presence and abundance of the deep-water rose shrimp Aristeus antennatus in a submarine canyon (Blanes canyon in the NW Mediterranean Sea). This species is heavily commercially exploited and is the main target species of a bottom trawl fishery. Seasonal fluctuations in landings are attributed to spatio-temporal movements by this species associated with submarine canyons in the study area. Despite the economic importance of this species and the decreases in catches in the area in recent years, few studies have provided significant insight into the environmental conditions driving shrimp distribution. We therefore measured daily A. antennatus catches over the course of an entire year and analyzed this time series in terms of daily average temperature, salinity, mean kinetic energy (MKE), and eddy kinetic energy (EKE) values using generalized additive models and decision trees. A. antennatus was captured between 600 and 900 m in the Blanes canyon, depths that include Levantine Intermediate Water (LIW) and the underlying Western Mediterranean Deep Water (WMDW). The greatest catches were associated with relatively salty waters (38.5–38.6), low MKE values (6 and 9 cm 2 s −2) and moderate EKE values (10 and 20 cm 2 s −2). Deep-water rose shrimp occurrence appears to be driven in a non-linear manner by environmental conditions including local temperature. A. antennatus appears to prefer relatively salty (LIW) waters and low currents (MKE) with moderate variability (EKE).

Another description of the Mediterranean Sea outflow

Papers about the outflow in the Strait of Gibraltar assume that (i) it is composed of only two Mediterranean Waters (MWs), the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW) from the eastern and western basins, respectively, (ii) both MWs are mixed near 6°W, hence producing a homogeneous outflow that is then split into veins, due to its cascading along different paths and to different mixing conditions with the Atlantic Water (AW). A re-analysis of 1985–1986 CTD profiles (Gibraltar Experiment) indicates two other MWs, the Winter Intermediate Water (WIW) from the western basin and the Tyrrhenian Dense Water (TDW) basically originated from the eastern basin. In the central Alboran subbasin, these four MWs are clearly differentiated, roughly lying one above the other in proportions varying from north to south. Proportions also vary with time, so that the outflow can be mostly of either eastern or western origin. While progressing westward, the MWs can still be differentiated and associated isopycnals tilt up southward as much as being, in the sill surroundings, roughly parallel to the Moroccan continental slope where the densest MWs are. The MWs at the sill are thus juxtaposed and they all mix with AW, leading to an outflow that is horizontally heterogeneous just after the sill (5°45′W) before progressively becoming vertically heterogeneous as soon as 6°15′W. There can be little LIW and/or no WMDW outflowing for a while. An analysis of new 2003–2008 time series from two CTDs moored (CIESM Hydro-Changes Programme) at the sill (270 m) and on the Moroccan shelf (80 m) confirms the juxtaposition of the MWs, their individual and generally intense mixing with AW, as well as the large temporal variability of the outflow composition. Only LIW and TDW were indicated at the sill while, on the shelf, only LIW, TDW sometimes denser there than ∼200 m below, and WMDW were indicated; but none of the MWs has been permanently outflowing at one or the other place. The available data can be analyzed coherently. Intermediate and deep MWs are formed in both basins in amounts that, although variable from year to year, allow their tracing up to the strait. Four major MWs circulate alongslope counterclockwise as density currents and as long as they are not trapped within a basin, which is necessarily the case for the deep MWs. In the Alboran, the intermediate MWs (WIW, LIW and upper-TDW) circulate in the north while the deep MWs (lower-TDW and WMDW) are uplifted, hence relatively motionless and mainly pushed away in the south. Since both the intermediate and deep MWs outflow at the sill, they are considered as light and dense MWs, the light–dense MWs interface possibly intersecting the AW–MWs interface in the sill surroundings. Considering an outflow east of the sill composed of only two (light–dense) homogeneous layers gives significant results. Across the whole strait, the outflow has spatial and temporal variabilities much larger than previously assumed. The MWs are superposed in the sea and lead at the sill to juxtaposed and vertically stratified suboutflows that will cascade independently before forming superposed veins in the ocean. These veins can have similar densities and hydrographic characteristics even if associated with different MWs, which accounts for the features permanency assumed up to now. The outflow structure downstream of the sill depends on its composition upstream and, more importantly, on that of AW in the sill surroundings where fortnightly and seasonal signals are imposed on the whole outflow.

Environmental significance of dinoflagellate cysts from the proximal part of the Po-river discharge plume (off southern Italy, Eastern Mediterranean)

To determine the relationship between region specific dinoflagellate cyst distribution in the Po-river discharge area and oceanic environmental conditions, surface sediments of 48 sites in the middle and distal part of the discharge plume area have been studied. Establishing such a relationship is a prerequisite to create reconstructions of the eutrophication history as well as the palaeoclimatic and palaeoceanography history of the area. Literature based information about the sedimentation rates based on 210Pb dating methods are available for 18 sites. This enables the calculation of cyst accumulation rates of individual cyst species which reflect their cyst production. Correlation of the accumulation rates of individual species with environmental parameters of the upper waters allows us to adapt and refine the ecological characteristics of a selection of cyst species. This latter is trivial since the current concepts on the ecological significance of dinoflagellate cyst have to be revised as a result of the current developments in the dinoflagellate research field. These developments have elucidated that a considerable part of the relative abundance datasets that form the basis for the present day ecological concepts of dinoflagellate cysts might have suffered from so called “closed sum effects” and have been overprinted by early diagenetic processes. The dinoflagellate cyst association reflects both upper and bottom water circulation. Based on the relative abundance data four associations can be distinguished that are characteristic for the major oceanographic settings in the region. (1) River discharge association. This association consists of Echinidinium spp., Lejeunecysta sabrina, Lingulodinium machaerophorum, Polykrikos kofoidii, Polykrikos schwarzii, cysts of Protoperidinium stellatum, Selenopemphix quanta and reworked cysts. These species have high relative abundances in sites where bottom waters are low in oxygen and upper waters are influenced by river discharge waters that are characterized by high productivity and relative low salinity. (2) Warm water association. This association consists of Spiniferites mirabilis, Spiniferites pachydermus, Spiniferites ramosus and Spiniferites spp. which have their highest relative abundances in the sites at the plume margin that are characterized by relative warm upper waters and intermediate chlorophyll- a concentrations. (3) Oxygenated bottom water group. Species of this group; Impagidinium aculeatum, Impagidinium patulum, Impagidinium sphaericum, Operculodinium centrocarpum and Operculodinium israelianum have their highest relative abundances at sites where bottom waters are formed by well ventilated Adriatic Deep Water or Eastern Mediterranean Deep Water. (4) Golfo di Taranto group consisting of round brown cysts produced by Protoperidinium species (grouped as Brigantedinium spp .). The variation in accumulation rates can be related to gradiental changes in the trophic state of the surface waters linked to river outflow. Most species show a positive relationship between cyst production and nutrient/trace element availability in upper waters. No negative correlation between cyst production and nutrient/trace element availability could be documented. Production of Brigantidinium spp., Echinidinium spp., L. sabrina, L. machaerophorum, P. kofoidii/schwarzii, Spiniferites spp., S. mirabilis and S. quanta shows the most pronounced increase with increasing nutrient/trace element availability. These species can be considered as valuable indicators for reconstructing changes in the trophic state of the upper waters within the Po-river area in palaeo-environmental studies.