Eastern Mediterranean Water Research Articles

Hydrogeochemistry and Geothermometry of Thermal Groundwater from the Gulf of Suez Region, Egypt

The combined chemical composition, O and H isotopes, and the basic geologic setting of the geothermal system of the Gulf of Suez, Egypt have been investigated to evaluate the origin of the dissolved constituents and subsurface reservoir temperatures. Hydrochemical characterization of thermal waters discharged from springs and flowing artesian wells in the Gulf of Suez region show that there are two groups. One is Hammam Faroun thermal waters with salinity values exceeding 10,000 mg/l, discharge temperatures reach to 70oC, and Na-Cl hydrochemical facies. The other group is thermal waters discharged at Hammam Mousa, Ain Sukhna, and shallow flowing artesian wells at Ayoun Mousa and Ras Sudr. They are characterized by salinity values less than 10,000 mg/l, discharge temperatures ranging from 32.5 to 72oC and Na-Mg-Ca-Cl (Hammam Mousa), Na-Cl-SO4 (Ain Sukhna), and Na-Ca-Cl (Ayoun Mousa-2 well and Ras Sudr-2 well) water types. Different graphical presentations using major and minor ions indicated that little mixing with sea water is probably a source of dissolved constituents. Water/rock interaction is also a major source for the dissolved constituents as revealed from Ca and HCO3 enrichment of the thermal waters that is attributed to dissolution of carbonate minerals. Thermal waters from Hammam Faroun and Ras Sudr-2 well have the highest discharge temperatures and SiO2 concentrations that indicate that ascending hot water at the Hammam Faroun area is slightly mixed with cold water. The thermal waters from the study area are depleted in 18 O and 2 H and fall on the Global Meteoric Water Line (GMWL) and below the local eastern Mediterranean Meteoric Water Line (MMWL) with d-excess values ranging between 3.42 and 10.6‰, which is similar to the groundwater of the Nubian aquifer in central Sinai and the Western Desert of Egypt and suggesting a common origin. This indicates that these waters are paleo-meteoric water which

Evaluation of water quality of goundwater recharge in Wadi Feifa, Southern Jordan Valley, Jordan

A study was carried out during winter 2007-2008 at Ghor Feifa Jordan Valley weir to estimate the quantity and quality of artificially recharged water originating from wadi Feifa surface runoff due to construction of diversion weir at its downstream. Results showed that there was a meaningful relationship between the quantity and quality of the stream feeding the diversion weir and groundwater quality exist. The amounts of water recharging local aquifers were estimated to be 0.15 m 3 /m/h. The major ionic concentration of the water in the investigated site at different periods indicated that there is an increase in TDS during the first two months of the investigated period due to no rain during November and December 2007; thereafter, during January and February 2008 a decrease in all ionic concentrations in both surface and groundwater was observed. This dilution effect is due to recharge during rainfall periods on the mountainous region of Tafileh area. Stable isotopic composition of water collected from surface and groundwater showed that both sources of water are plotted near the Eastern Mediterranean Water Line (EMWL) indicating same origin of water which is rain water without strong evaporation effect. The tritium concentration of both surface and groundwater were less than 1 TU (tritium unit) indicating that the most probable source of water in the investigation area is from old groundwater in form of base flow in addition to treated wastewater. Results also showed that the recharge rate ranged from 0.05 to� 0.21 m 3 /m. These results were in agreement with the measurements of static water level recorded in the monitoring wells downstream as the water level raised about 5 cm within three months which is located very close to the diversion weir.

Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry

‘Clumped isotope’ thermometry is based on analyzing mass 47 in CO 2 extracted from carbonates and uses the tracer mass 47 anomaly (Δ 47). Δ 47 is defined as the deviation of R 47 from that expected for a random distribution of isotopologues and reflects a temperature dependent preference of 13C and 18O to create a bond with each other in CO 2 or in the carbonate lattice. Being an internal characteristic of the carbonate mineral, it is independent of the isotopic composition of the water in which equilibrium precipitation of the carbonate occurs and can therefore be used to independently determine carbonate growth temperatures. This work provides a first examination of the applicability of ‘clumped isotopes’ thermometry to reconstructing the growth temperatures of speleothems, by examining the glacial/interglacial variations of the Δ 47 values of speleothem carbonates from Soreq cave, Israel. The results indicate that the last glacial maximum temperatures were 6–7 °C colder than modern day temperature and a sample at 56 Ky BP was 3 °C colder than the modern. Early Holocene temperatures were slightly above modern day, and late Holocene temperatures were slightly below modern day. These temperature variations are similar to those previously estimated for Eastern Mediterranean sea surface water. Cave water was 18O depleted in the Holocene compared to modern day (by 0.6–1‰) and 1.1‰ more enriched in the last glacial maximum. Comparison of these cave water δ 18O values with fluid inclusion δ D values indicated a late Holocene d-excess value within the range of modern rainfall, implying ∼45% relative humidity. Last glacial maximum and early Holocene d-excess values were significantly lower, suggesting relative humidity of ∼60% and ∼70%, respectively. The temperatures reported in this study were empirically corrected for a non-equilibrium artifact observed in a modern speleothem. The similarity of the temperature variations obtained here to other, independent, records in the region suggests that the Δ 47–temperature calibration slope observed in inorganic synthetic calcite and marine organisms may also be applied in speleothems. But the offset observed in modern temperature suggests that the intercept is different so that a separate calibration is needed for accurate absolute temperature reconstruction using speleothem ‘clumped isotopes’. Similar examination of additional caves would be necessary to determine whether such empirical correction can be generally applied or is it a unique characteristic of Soreq cave.

Genetic variation and population structure of the caramote prawn Penaeus kerathurus (Forskäl) from the eastern and western Mediterranean coasts in Tunisia

Ten populations of Penaeus kerathurus (Forskäl), a prawn of high commercial value, were sampled from the eastern and western Mediterranean coastal waters of Tunisia and screened electrophoretically for genetic variation at 13 allozyme loci. Four among the six polymorphic loci were out of Hardy–Weinberg equilibrium (H–WE) in at least one population. In the same way, the multilocus test showed deviation from H–WE in six populations. These populations showed heterozygote deficiency whereas the average heterozygosity for the four remaining ones is quite similar to the expected levels. Genetic variability was low. The number of alleles per locus ranged from 1.2 to 1.5 (average=1.3), and the observed heterozygosity varied between 0.010 and 0.048 (average=0.021). Significant population differentiation (FST=0.076, P<0.05) in the total data set reflected the differentiation of the two populations, which were at the margins of the range sampled, from all the others (Pairwise FST values ranged from 0.035 to 0.208). Although there was no significant differentiation among the other populations (pairwise FST values ranged from −0.006 to 0.201, P>0.05). Our data suggest a population structure consistent with separation by Mediterranean Sea basins that might reflect different local biogeographical zones.

Marine GIS: Identification of mesoscale oceanic thermal fronts

A new method (the ‘sink’ method) is proposed for the mapping of productive mesoscale oceanic thermal fronts based on the combined analysis of satellite imagery for sea surface temperature (SST) distribution and chlorophyll (CHL) concentration under a Geographic Information System (GIS). In an SST lattice data array, data value sinks describe heterogeneous drops in SST distribution as this is compared to more homogeneously distributed SST in the surrounding area. Using Arc/Info GIS, these thermal discontinuities are flagged and applied on SST and CHL imagery for the calculation of differences in SST and CHL patterns (DSST and DCHL) inside and outside of these flagged areas. Spatially connected sinks that are characterized by simultaneous negative DSST and positive DCHL pattern are mapped as thermal fronts. Results include a time series of monthly front occurrence GIS maps in Eastern Mediterranean waters along with three calculated front characteristics (DSST, DCHL and bathymetry). Comparisons between the mapped fronts and isobath distributions present clearly visible front‐to‐isobath spatial and shape associations while spatial analysis between front and wind data explains 66% of the number of mapped fronts.

Effect of P and N addition to oligotrophic Eastern Mediterranean waters influenced by near-shore waters: A microcosm experiment

Phosphate (P), nitrate (N) or P+N added in a microcosm experiment to oligotrophic waters of the Eastern Mediterranean influenced by near-shore waters triggered a range of responses in the autotrophic and heterotrophic compartments of the system. Chlorophyll a increased in all treatments, including the no-addition control, implying that nutrients became available also from internal sources (recycling). Larger and faster biomass increase as well as a larger P utilization took place in the P+N treatments. Diatoms bloomed in the P+N treatments whereas coccolithophores bloomed following the addition of P ultimately reaching N-limitation. Bacterial activity responded with a transient peak to both low P-alone and N-alone additions (0.01 and 1 μM, respectively). For reasons not well understood, no such response was observed at higher P-alone additions (0.05 and 0.5 μM), whereas at the two highest P+N additions the positive response was delayed. We therefore were unable to conclude conclusively on bacterial limitation. In most cases, the increase in bacterial activity was not matched by an increase in abundance, suggesting a tight top-down control of the biomass. Instead, heterotrophic nanoflagellate and ciliate abundances increased in all treatments. A slightly elevated orthophosphate turnover-time (Tt) (32 h) in the initial waters did not give a clear indication of P-limitation, although the system could absorb the lowest P-addition (0.01 μM) without increase in Tt. N alone lead to a reduction in Tt as would be expected in an N-limited system consuming existing surplus P after N-addition. The response of the near-shore influenced system used in this study was in accord with the ‘classical’ response to nutrient introduction—increase in chlorophyll a and in large size phytoplankton. In contrast, in the ultraoligotrophic Cyprus Eddy [Krom, Thingstad, Carbo, Drakopoulos, Fileman, Flaten, Groom, Herut, Kitides, Kress, Law, Liddicoact, Mantoura, Pasternak, Pitta, Polychronaki, Psarra, Rassoulzadegan, Skjoldal, Spyres, Tanaka, Tselepides, Wassmann, Wexels-Riser, Woodward, Zodiatis, Zohary, 2005. Overview of the CYCLOPS P addition lagrangian experiment in the Eastern Mediterranean. Deep-Sea Research II, this volume.], the short Tt (<4 h) indicated P-limitation, the combined addition of P and N (as ammonium) induced a bloom of picocyanobacteria [Zohary, Herut, Krom, Mantoura, Pitta, Psarra, Rasssoulzadegan, Stambler, Tanaka, Thingstad, Woodward, 2005. P-limited bacteria but N&P co-limited phytoplankton in the Eastern Mediterranean—a microcosm experiment. Deep-Sea Research II, this volume.] and the in situ P alone addition led to a decrease in chlorophyll.

The biology and ecology of juvenile pilotfish (Naucrates ductor) associated with Fish Aggregating Devices (FADs) in eastern Mediterranean waters

The pilotfish (Naucrates ductor) is an epipelagic oceanic species, whose 0-group specimens tend to aggregate below flotsam, constituting a principal by-catch of the western Mediterranean FAD fishery. During a one-year survey sampling, monthly experimental hauls using surrounding nets were conducted at sites in south Peloponnesian waters, where FADs were moored; during the summer-early autumn months bongo tows were also carried out. 0-group pilotfish appeared beneath FADs from mid-summer till early winter and the total length of the collected specimens ranged between 120 and 330 mm. Pilotfish larvae, measuring 2.0- 3.9 mm, were found at FAD sites in mid-summer and in early autumn. The age of juvenile specimens, determined by counting daily increments on their sagittae, ranged between 50-141 increments (days) for males, and 51-131 increments (days) for females. The predicted asymptotic length was found to be 412.3mm TL for males and 435.2 mm TL for females. The onset of sexual maturity appeared to occur when specimens were a few months old. Stomach content analysis suggested that the diet of 0-group pilotfish associated to FADs in Greek waters depended mainly on decapod larvae, hyperiid amphipods, and alciopid polychaetes.

Age and growth of Lessepsian rabbitfish from the eastern Mediterranean

Summary Rabbitfish are currently an important component of the eastern Mediterranean inshore fishery. This study estimates age and calculates growth of Siganus rivulatus and S. luridus from the Lebanese coastal waters based on otolith analysis and compares results with available data from different geographical locations. Annual periodicity of alternating zones was confirmed for S. rivulatus, in which one opaque and one translucent zone were observed to deposit each year, but not for S. luridus. Both species appear to have a maximum lifespan of 6 years. The von Bertalanffy growth functions were Lt = 318.9(1 − e−0.225(t+1.307)) for S. rivulatus and Lt = 274.9(1 − e−0.330(t+1.039)) for S. luridus and length–weight relationships were estimated as for S. rivulatus and for S. luridus. The rabbitfish catches of the Lebanese fishery consist mainly of individuals which have not reached their first maturity. Rabbitfish from the eastern Mediterranean coastal waters appear to grow rapidly most of the year and are thus able to maintain relatively important population size despite intensive fishing pressure.

Titrating excess nitrogen content of phosphorous-deficient eastern Mediterranean surface water using alkaline phosphatase activity as a bio-indicator

From both an academic and a management point of view, not only the question of which element limits production, but also the excess amount of the next potentially limiting element, is of importance. Activity of the enzyme alkaline phosphatase (APA) produced during phosphorous (P) starvation has been used as a technically easy and sensitive indicator of P limitation. We explore an extension of this technique to estimate also the excess of nitrogen (N) by titrating samples with phosphate until APA disappears. In ultra-oligotrophic eastern Mediterranean surface water, this technique indicated P limitation with a nitrogen excess of 230 ± 60 nM N. Adding ammonia as internal standard gave a molar ratio of 15 ± 2 for balancing amounts of added N and P. Taking advantage of a Lagrangian experiment with in situ phosphate addition, we also used the mirror technique of titrating with ammonia until APA appeared. Excess P inside the experimental patch was estimated to be 33 ± 2 nM 4 days after the addition of 110 nM PO 4 P. In this case, however, a molar N:P ratio of only 3.0 ± 0.2 was determined for balancing amounts of added ammonia and phosphate. The technique should, in principle, estimate excess nutrients accessible on a time scale of ca. 3 d (the incubation time used here). The method applies regardless of the form the excess nutrients are in as long as they are bioavailable. It does not presume that the organisms exhibiting the limitation belong to any particular taxon or functional group as long as they produce APA when P-limited.

Critical regions: A GIS-based model of marine productivity hotspots

Marine productivity hotspots can be defined as areas of high chlorophyll concentration and low temperature distribution. Here we demonstrate how Geographic Information Systems (GIS) can be used to define areas of high productivity and we use fishery data to verify model results and reveal areas of potential fishery locations. We develop a GIS model based on spatial data integration between monthly satellite imagery of Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll concentration (Chl-a) for Eastern Mediterranean waters. Data are analyzed for the production of climatology and mapping of anomaly distribution in these environmental parameters. The geographic distributions of anomalies are spatially integrated to map areas of combined persistent environmental anomaly of below average SST and above average Chl-a (lowSST/highChl-a) indicating marine productivity hotspots and potential fish feeding aggregation regions. A preliminary comparison of surveyed fisheries production data for small pelagic fish and pelagic cephalopods to lowSST/highChl-a areas is performed for the period from December 1997 to November 2001, revealing potential spatial associations among fishery resources and productivity hotspots in terms of unexploited regions as well as overexploited fishing grounds that could be characterized as candidate marine protected areas.

The dynamics of the Sicily Strait: a comprehensive study from observations and models

Abstract The dynamics of the Sicily Strait are investigated in a comprehensive study using recent in situ observations, Ocean General Circulation Model (OGCM) simulations and process model results. In particular, results of a very high-resolution Mediterranean model (MED16) developed in the context of the Mercator project are described. The circulation in the Sicily Strait can be described as a two-layer exchange of Atlantic Water (AW) and Eastern Mediterranean Outflow Water (EOW). The Algerian Current brings AW (the upper layer) eastwards and splits into two branches at the entrance to the Sicily Strait, one flowing to the Tyrrhenian Sea, the other into the Sicily Strait. The later is composed of two streams, referred to as the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). In winter, the ATC signature is more pronounced. In summer, the AIS is associated with a number of well-known semi-permanent features including the intermittent northward extension of the AIS (called NAIS) at the Ionian shelf break, which seems to be driven by the surface density contrast between waters of the Sicilian and the Ionian basins. New intermittent patterns are shown by the MED16 simulations, for example an anticyclonic eddy (called MCA) south of the Malta Channel and an anticyclonic gyre associated to the NAIS (called NAISA). Upwelling is well developed in August, with temperature lower than 24°C along the western and southern coasts of Sicily. The seasonal transport of the AW through the strait has a maximum value (1.4 Sv ) in winter, and a minimum value at the end of summer (0.8 Sv ) . In the lower layer, at the exit of the Sicily Strait, the EOW, mainly composed of Levantine Intermediate Water (LIW), outflows into the Tyrrhenian Sea and veers to the right. Then, as well known, the LIW flows along the northern coast of Sicily before reaching the Sardinia Channel. A new LIW path from the Sicily Strait towards the Sardinia Channel is shown by the MED16 model, and is due to intermediate eddy shedding at the exit of the Sicily Strait. The topography of the sill in the Sicily Strait plays a major role in the circulation. A combination of barotropic/baroclinic double Kelvin waves on both side of the sill provides a mechanism, for splitting the Algerian Current.

Quantitative aspects of feeding of chaetognaths in the eastern Mediterranean pelagic waters

The diet of all chaetognath species found in a broad pelagic area of the eastern Mediterranean was investigated through gut content analysis. Eight chaetognath species were recovered from four depth intervals between 0 to 300 m in the Ionian, Cretan and Rhodes Seas and in the Cretan Passage. The mean food containing ratio (FCR) value for all chaetognaths combined was 0·048. Copepods comprised nearly 65% of the total food items consumed. The epipelagic species Sagitta enflata, Sagitta serratodentata atlantica, Sagitta bipunctata and Sagitta minima fed mainly in the 0 to 50 m surface layer, while the mesopelagic species Sagitta lyra, Sagitta decipiens, Sagitta hexaptera and Krohnitta subtilis fed in deeper layers. Sagitta s. atlantica showed the highest mean FCR in the integrated water column (0 to 300 m) of the whole sampling area. The estimated impact of chaetognath predation on copepod communities ranged from 0·3 to 7·8% of the copepod standing stock, and was higher in the Ionian Sea and the Cretan Passage than in the Cretan and Rhodes Seas. Sagitta s. atlantica, S. lyra, and S. decipiens had the most important predation impact among the eight chaetognath species found in the 0 to 50 m, 50 to 100 m and 100 to 300 m depth layers respectively.

Eastern Mediterranean surface water temperatures and δ18O composition during deposition of sapropels in the late Quaternary

Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. High‐resolution data sets on sea‐surface temperatures (SST) (estimated from U37k′ indices) and δ18O of planktonic foraminifer calcite (δ18Ofc) across late Pleistocene sapropel intervals show that δ18Ofc decreased (between 1 and 4.6‰) and SST increased (between 0.7° and 6.7°C). Maximal δ18Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0‰, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in δ18Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon‐derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is “glacial” sapropel S6; its largest δ18Oseawater depletion (3‰) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment.

Atmospheric wet deposition of soluble macro-nutrients in the Cilician Basin, north-eastern Mediterranean sea

In order to estimate wet deposition atmospheric fluxes of macro-nutrients into the eastern Mediterranean coastal waters, soluble inorganic phosphate (PO4(3-)), nitrate (NO3-) and nitrite (NO2-) concentrations in precipitation (from February 1996 to June 1997) have been measured at a coastal sampling site, Erdemli, Turkey. Water-soluble inorganic PO4(3)-P, a reactive, bioavailable, limiting macro-nutrient in the oligotrophic waters of the eastern Mediterranean was studied with respect to its contribution to biological productivity. Reactive PO4(3-)-P and NO2(-) + NO3(-)-N concentrations were found to be highly variable in rainwater samples. One of the aims of the study was to determine the contribution of dust transport to the soluble macro-nutrient budget of the eastern Mediterranean. No differences were found between the mean reactive P and NO(2-) + NO3(-)-N concentrations of "red rain" and normal rain events. Most likely as a result of low solubility of crustal phosphorus, dust episodes were not found to be important sources of reactive P, in terms of wet deposition. The annual wet deposition fluxes of reactive PO4(3-)-P and NO2(-) + NO3(-)-N into the Cilician Basin were respectively estimated to be 0.010 g P m(-2) per year and 0.23 g N m(-2) per year, which are comparable to the fluxes from land-based sources in the north-eastern Mediterranean. The incorporation of water soluble bioavailable PO4(3-)-P and NO2(-) + NO3(-)-N delivered via atmospheric wet deposition could be responsible for approximately 3.3% (0.40 g C m(-2) per year) and 11.0% (1.31 g C m(-2) per year) respectively, of the mean annual new production in the north-eastern Mediterranean.

Benthic foraminiferal record of ecosystem variability in the eastern Mediterranean Sea during times of sapropel S 5 and S 6 deposition

High-resolution benthic foraminiferal and geochemical investigations were carried out across sapropels S 5 and S 6 from two sediment cores in the Levantine Sea to evaluate the impact of climatic and environmental changes on benthic ecosystems during times of sapropel formation. The faunal successions indicate that eutrophication and/or oxygen reduction started several thousand years prior to the onset of sapropel formation, suggesting an early response of the bathyal ecosystems to climatic changes. Severest oxygen depletions appear in the early phases of sapropel formation. The initial reduction of deep-water ventilation is caused by a warming and fresh water-induced stratification of Eastern Mediterranean surface waters. During the late phase of S 5 formation improved oxygenation is restricted to middle bathyal ecosystems, indicating that at least some formation of subsurface water took place. During S 6 formation oxygen depletions and eutrophication were less severe and more variable than during S 5 formation. Estimated oxygen contents were low dysoxic at middle bathyal to anoxic at lower bathyal depths during the early phase of S 6 formation but never dropped to anoxic values in its late phase. The high benthic ecosystem variability during S 6 formation suggests that water column stratification at deep-water formation sites was in a very unstable mode and susceptible to minor temperature fluctuations at a millennial time-scale.

Methane-related authigenic carbonates of eastern Mediterranean Sea mud volcanoes and their possible relation to gas hydrate destabilisation

Nautile submersible investigations of mud volcanoes and brine seep areas of the eastern Mediterranean Sea during the MEDINAUT cruise in November 1998 discovered extensive areas of authigenic carbonate crusts associated with methane emissions. Carbonate crusts form pavements, round slabs and circular mounds on the central, most active parts of mud volcanoes and in a fault-related valley where brines have accumulated to form a submarine brine lake. Authigenic carbonate nodules have been recovered from the same areas during the MEDINETH cruise in July 1999. Large 13C depletions of authigenic calcite, aragonite and dolomite indicate methane as a major carbon source for the carbonate. Crust pavements are formed when methane from a freshly emplaced, methane-charged mud flow is oxidised at the seafloor. In this environment, where bottom waters provide the sulphate and magnesium, aragonite is favoured versus calcite and accounts for the majority of the methane-related authigenic carbonates. Calcite, when present, contains significant amounts of Mg2+ (high-Mg calcite), and possibly other divalent ions in its crystal lattice. In areas of brine seep and accumulation, dolomitic nodules are present at shallow depth in the sediment. The 18O enrichment of the authigenic carbonates (up to 4‰ greater than calculated values for carbonates precipitating from modern eastern Mediterranean bottom waters) is interpreted as due to precipitation from 18O-rich fluids rather than as a temperature effect. The source of the 18O-rich fluids may be multiple and possibly includes the destabilisation of gas hydrates present at shallow subbottom depth, and the seepage of relic Messinian brines.

Dynamics and mixing of the Eastern Mediterranean outflow in the Tyrrhenian basin

Hydrographic measurements in the Sicily Channel and in the southern Tyrrhenian Sea show that the water outflowing from the Eastern Mediterranean Basin enters the Tyrrhenian Sea directly, close to the Sicily coast. Two water types have been distinguished: the upper part constituted by the Levantine Intermediate Water (LIW), and the transitional Eastern Mediterranean Deep Water (EMDW) below. Current measurements taken about 50 km after the entrance into the Tyrrhenian evidence a well-developed mean flow from a depth of 500 m to the bottom. While the LIW flows at the known depth for this type of water (namely between 500 and 800 m), a considerable volume of transitional EMDW was seen to sink from a depth of about 300 m in the Sicily Channel to a maximum depth of 1850 m in the Tyrrhenian Sea, where it floats over the Tyrrhenian Deep Water (TDW). The sinking is a consequence of the higher density of the incoming water ( σ θ >29.10) with respect to that of the intermediate Tyrrhenian waters ( σ θ ∼29.00–29.05). The progressive sinking of the transitional EMDW along the principal route from the Sicily Channel to the Tyrrhenian Sea, can be observed in the evolution of the isotherms, isohalines and isopycnals. The isopycnals, in particular, clearly tend to follow the bottom slope. The mean hydrographic characteristics of this vein of Eastern Mediterranean water progressively change along the route, and two regions of strong mixing are identified. The first occurs immediately after the Sicily Channel, while the second is found following the entrance of the vein into the Tyrrhenian Sea. An estimation of the entrainment stress shows that it largely dominates over the acceleration term. When the bottom slope does not influence the water motion anymore, the mixing is entirely ascribable to molecular diffusion via finger instability. The transport of transitional EMDW through the Sicily Channel is estimated to be about 0.2–0.3 Sv, implying a salinity anomaly export of 0.05–0.075 psu per Sv to the deep layers of the Tyrrhenian Sea.

Nutrient, particulate matter and phytoplankton variability in the photic layer of the Otranto strait

The distribution of nutrients, suspended matter and phytoplankton in the photic layer of the Otranto Strait, as observed in four seasonal situations (February, May, August and November 1994), is analysed in relation to hydrography. In winter, two water masses were found: the Adriatic Surface Water (ASW) and the Ionian Surface Water (ISW), located at the western and eastern side of the Strait, respectively. In the fresher and cooler ASW, nutrients, suspended matter and phytoplankton contents were higher than in the warmer and saltier ISW (on average: N–NO 3=2.3 μM, POC=3.6 μM, chl. a=0.4 μg dm −3 in the ASW, against N–NO 3=1.3 μM, POC=2.5 μM, chl. a=0.3 μg dm −3 in the ISW). In the ASW, the mean N:P ratio (50) revealed an excess of nitrogen with respect to phosphorus; the nitrogen supply, as well as the significant presence of diatoms, made us suppose that new production processes were occurring here. In summer, the main features were: (i) high water column stability, (ii) small horizontal differences in hydrological and biological features, (iii) extremely low concentration of nutrients at the surface and (iv) a deep chlorophyll maximum (DCM) at the nutricline level. Phytoplankton summer communities, mainly consisting of nanoflagellates, coccolithophorids and small naked dinoflagellates, were present in low quantities. The low phytoplankton carbon:chlorophyll a ratio (<20), observed at DCM, indicated a high chlorophyll content per single cell, probably as a physiological response to low irradiance. As to the phytoplankton annual cycle, sediment traps and water column observations were highly correlated, evidencing two abundance peaks in spring and autumn, and a summer minimum. Hydro-chemical and biological data suggest that the winter spread of the ASW was the main factor favouring the enhancement of phytoplankton growth and controlling the species composition in the strait, while the DCM formation represents a more typical characteristic of ISW, with an oligotrophic feature similar to that of other Eastern Mediterranean water masses.

Adsorption of dissolved phosphate onto loess particles in surface and deep Eastern Mediterranean water

Abstract The objective of this study was to examine whether dissolved inorganic phosphate (DIP) is removed from the Eastern Mediterranean Sea by adhering to atmospherically deposited loess particles sinking through the water column. In a series of radiolable experiments, loess from the Negev Desert, treated in various ways, was added to surface (SSW) and deep (DSW) seawater spiked with 32 P O 4 −3 . It was shown that when fresh loess reaches the Mediterranean SSW approximately 1.3 μmol P/g are released (∼11% of the total P concentration). Biological activity and inorganic particles removed similar amounts of the tracer (30–40%) from SSW. It was estimated that about 0.2 μmol P/g of `aged loess' (proxy of particles sinking into DSW), were removed from poisoned SSW and DSW, while there was minor adsorption when either nothing or quartz powder was added. The adsorbed DIP accounts for approximately 15% of the released P and is equivalent to about 2% of the remaining P (11.17 μmol P/g loess). Therefore, the process of DIP removal by atmospherically derived particles exists, but due to the higher release of P, the result is a net addition of dissolved atmospheric phosphorus in seawater. It is postulated that in actual SSW where dust concentrations are much lower, biological uptake out-competes inorganic adsorption, although it was demonstrated that the decrease in loess particle concentrations tends to increase their adsorption capacity. As the loess descends into the DSW it continues to remove DIP and thus transports phosphate from the water column to the sediments. A preliminary quantitative estimate suggests that the process of DIP removal by loess particles cannot explain the phosphate `deficit' in the Eastern Mediterranean DSW characterized by unusually high DIN/DIP ratios (∼27) reported by others.

Age and growth of the four-spotted megrim (Lepidorhombus boscii) in eastern Mediterranean waters

This paper analyses the age and growth pattern of the four-spotted megrim (Lepidorhombus boscii), collected in the north Aegean Sea. The otoliths of the species exhibit bilateral asymmetry. Our data indicates that males have shorter lifespans (0–6 y) than females (0–8 y) and reach lower asymptotic lengths. A significant difference in growth rate exists between the two sexes after the first year of life. Growth in length is greater in females than males, whereas males appeared to be heavier than females of the same size. The latter was also corroborated by the higher accumulation of lipids in the muscle of males in relation to females. The growth rate of the species in the Atlantic was higher than in the Mediterranean. The oligotrophic character of the study area seemed to have a negative effect on the growth of megrims.