Rhodes Gyre Research Articles

Anomalous 2022 deep-water formation and intense phytoplankton bloom in the Cretan area

Abstract. The Mediterranean Sea is a quasi-permanently stratified and oligotrophic basin with intense late-winter and early-spring phytoplankton blooms typically limited to few regions (i.e. northwestern Mediterranean Sea, the southern Adriatic Sea, and the Rhodes Gyre). In these areas, blooms are sustained by nutrient injection to surface layers by winter vertical mixing and convective processes. A markedly intense bloom was predicted in spring 2022 in an unusual area of the southeastern Mediterranean Sea (i.e. southeast of Crete) by the Mediterranean Sea Copernicus Marine Forecasting Centre (MED MFC) system. Combining Copernicus modelling and observation products, the 2022 event and a number of driving and concurrent features have been investigated in a multidisciplinary way. A noticeable cold spell that occurred in Eastern Europe at the beginning of 2022 has been identified as the main driver of an intense deep-water formation event, with associated high nutrient concentrations in the surface layers. Consequently, an extreme phytoplankton bloom that was 50 % more intense than usual occurred in the area southeast of Crete, starting nearly 1 month later than usual and lasting for 3–4 weeks. Impacts on primary production were also relevant in the 2022 event area and were 35 % higher than the climatological annual primary production. Furthermore, the documented link between primary productivity and fishery catches suggests possible consequences along the whole food chain up to the marine ecosystem in the eastern Mediterranean Sea.

Seasonal and Interannual Variability in Sea Surface Temperature Fronts in the Levantine Basin, Mediterranean Sea

Sea surface temperature (SST) fronts were analyzed in the Levantine Basin of the Mediterranean Sea over a 20-year period (2003–2022) using a high-resolution (~1 km) satellite dataset. Frontal gradients were strongest in regions of freshwater influence and around the Ierapetra eddies and Rhodes Gyre. Seasonally, maximum frontal activity was observed in fall and summer. Empirical orthogonal function (EOF) analysis revealed both monthly-to-seasonal variability and interannual variability in frontal gradients. Seasonal frontal variability is partially explained by atmospheric forcing; that is, wind stress curl (WSC) and net air–sea heat flux. The maximum frontal activity was observed in 2006, coinciding with the strongest WSC magnitude. The minimum frontal activity was observed in 2017, which saw the largest winter heat loss to the atmosphere. The highest frontal activity was typically observed in years with mild winters followed by strong Etesian winds. Over the study period (2003–2022), frontal gradients declined in the Levantine Basin. Our results suggest that years with a strong frontal boundary current (Asia Minor Current; AMC) coincide with reduced cross-shelf transport. Subsequent studies are recommended to concentrate on the variability in the frontal intensity of the AMC and associated cross-shelf transports, which are important for the oligotrophic Levantine Basin.

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (eastern Mediterranean): influence of winter mixing

Abstract. The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic–biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 gCm-2yr-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm-2yr-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm-2yr-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm-2yr-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.

A high-resolution study of planktonic foraminifera during the Holocene at the Tilos-Symi sea basin in the SE Aegean Sea

High-resolution data for planktonic foraminifera and their groups of ecological interest, including Herbivores and Carnivores, combined with benthic foraminifera and a great variety of biogeochemical indices from the SE Aegean sediment core ST5, contribute to a detailed study concerning major and minor climatic episodes during Holocene. The ST5 sediment record, retrieved from the Tilos-Symi marine basin, evidences the impact of the nearby land, the local climate, and the Rhodes Gyre imprint on the eastern Mediterranean water circulation. Pronounced environmental changes are detected during a preconditioning period of ~400 years before the onset of sapropel S1 deposition and during the deposition phases S1a (10.0–8.4 ka BP) and S1b (8.0–6.1 ka BP). Major freshwater influx episodes (10.5 ka BP, 9.2–8.9 ka BP, 7.4 ka BP, 4.5–4.3 ka BP, and few additional during the Late-Holocene) are revealed, prominent oxygen deficiency time intervals (including a distinct brief anoxic period) as well as several drier, cooler and warmer climatic events. Herbivore planktonic foraminifera alternate their dominance with the Carnivores at the preconditioning period before the S1 onset and at the Late-Holocene (2.0–1.5 ka BP) when conditions of good seawater circulation, oxygenation, and productivity alternate with stratified low oxygenation waters and high freshwater influx from the land.

Annual cycle of mesozooplankton at the coastal waters of Cyprus (Eastern Levantine basin).

This study is the first to explore monthly and seasonal succession of the zooplankton community in coastal waters of Cyprus using a 12-month period time series. A total of 192 taxa of mesozooplankton (MZ), 145 of which were copepods, were identified at three sites at the southern and one site at the northern coasts of the island. Zooplankton distribution and community structure were influenced mostly by stratification, temperature and Chl-a. The combination of upwelling and advection from the Rhodes Gyre during summer, causing cooler waters in the southern coast of Cyprus, seems to control the food supply and offered favorable feeding conditions to zooplankton, enhancing their numbers. The proximity to a fish farm also positively affected MZ abundance and biomass. This study also revealed the importance of smaller species (e.g. Clausocalanus paululus) and juvenile stages (e.g. Clausocalanus, Oithona and Corycaeus spp.) in composition, structure and functionality of the copepod community. These species seems to be more important in low Chl-a environments, where the relative size of primary consumers is expected to be smaller and the microbial components dominant. This baseline study paves the way for further investigation of the elements of marine food webs in the ultra-oligotrophic environment of the Eastern Mediterranean.

The Mediterranean Rhodes Gyre: modelled impacts of climate change, acidification and fishing

The Mediterranean Rhodes Gyre is a cyclonic gyre with high primary production due to local upwelling of nutrients and occasional deep overturning up to 1 km depth. This nutrient-rich state is in sharp contrast to other parts of the Eastern Mediterranean which are oligotrophic. Here we investigated the upwelling system central to the Rhodes Gyre and the impact of different stressors like meteorological changes, acidification and fishing pressure up to the year 2100. A water column model spanning the physical, chemical and biological system up to top predators (GOTM-ERSEM-BFM-EwE) was used to simulate the pelagic environment under single and combined stressors. Results show that due to increasing winter temperatures, deep overturning events will become more rare in the future until they stop occurring around 2060 under the Paris-agreement climate scenario (RCP4.5) or around 2040 under the business-as-usual climate scenario (RCP8.5). Stratification will become stronger as temperature effects outweigh salinity effects in the surface mixed layer. Together with the lack of deep overturning, this stronger stratification limits the nutrient supply to the euphotic zone, significantly reducing primary production. Phytoplankton species shift towards smaller species as nutrients become more scarce, mimicking the situation found currently on the edge of the gyre. Climatic changes and fishing pressure will affect higher trophic levels in an additive way for some species (sardines, dolphins), while in a synergistic way for others (anchovy, mackerel). Acidification impacts are negligible. Fish stocks will reduce significantly under all scenarios involving climate change effects: ~30% under scenarios imposing RCP4.5 and ~40% under scenarios imposing RCP8.5. The beneficial impact of maximum sustained yield-level fishing is very limited, indicating a need for mitigating measures beyond fleet control.

A new assessment of the circulation of Atlantic and Intermediate Waters in the Eastern Mediterranean

A simulation of the Mediterranean circulation between 2011 and 2020 at a resolution of 3–4 km in the Eastern basin was compared to vertical profiles and horizontal distributions of temperature and salinity from Argo profilers distributed throughout the basin. The comparison is marked by high temporal (∼0.9) and spatial (0.6–0.8) correlations and low biases. Comparisons of SST with satellite imagery have also shown strong similarities for numerous structures over a wide range of spatial scales. The simulation is used to describe the mean circulation of surface Atlantic Waters and Intermediate Waters in winter and summer.The surface circulation is cyclonic alongslope, stronger and more stable in winter. In summer, the current veins are sometimes interrupted and replaced by trains of eddies like in the South Ionian. In other cases, the current becomes very narrow and stuck to the coast as along the Ionian east coast or the Middle East coast. In winter, surface and Levantine Intermediate Waters exit from the Levantine mainly through the Aegean, while in summer, they exit westward south of Crete. The Aegean tends in summer to be isolated by eddies that develop on both sides of the Cretan Arc. The juxtaposition of Ierapetra, the Rhodes Gyre and the Mersa-Matruh Eddies produces a southward path across the Levantine basin at about 27 − 28°E which delimits a large cyclonic circulation to the east which tends to separate the two parts of the basin (west and east Levantine). Concerning the Levantine Intermediate Waters, the alongslope cyclonic circulation all around the Levantine basin in winter is no longer maintained in summer as a large anticyclonic circulation occupies the southeast of the basin. The intermediate waters entering the Ionian either through southern Crete in summer or through the Aegean in winter, are submitted to a strong northward, southward and even westward dispersion by Pelops and the nearby anticyclonic areas. The presence of recurrent anticyclones between 35 and 37°N along a band extending from west to east of the Ionian also produces a vertical dispersion of intermediate water. Finally, the circulation of intermediate water in the South Ionian is marked by an important seasonality with the presence in summer of a large anticyclonic circulation that seems to be wind induced and finally drives a secondary branch to the Sicily Channel. A climatology for the transport through the different straits is discussed and simplified representations of the circulation are proposed.

BGC‐Argo Floats Observe Nitrate Injection and Spring Phytoplankton Increase in the Surface Layer of Levantine Sea (Eastern Mediterranean)

AbstractIn the eastern Mediterranean Sea, satellites have observed events of spring surface‐chlorophyll increase in the Rhodes Gyre region recurring intermittently. Few in situ biogeochemical data, however, exist to confirm their consistency, elucidate their seasonal characteristics, or discriminate among the possible drivers. During the year 2018, an array of BGC‐Argo floats was deployed in the region, collecting the first‐ever annual time series of in situ profiles of biogeochemical parameters in this area. Their observations demonstrated that nitrates, driven by mixed‐layer dynamics, were available at surface from December 2018 onwards and could have sustained phytoplankton growth. Phytoplankton accumulation at the surface was observed by satellite only in March 2019 when the mixed‐layer depth shoaled. These findings confirm that blooms occurring before the start of seasonal stratification are not easily recorded by satellite observations and reaffirm the need to consolidate the BGC‐Argo network to establish time series of the evolution of biogeochemical processes.

Water Mass Variability and Levantine Intermediate Water Formation in the Eastern Mediterranean Between 2015 and 2017

AbstractThe physical characteristics of water masses in the Cilician Basin were analyzed based on recent, comprehensive in‐situ data. Nine seasonal cruises from June 2015 to November 2017 were carried out in an area located in the north‐eastern Levantine Basin between the coasts of Turkey and Cyprus. In this little studied area, the structure of the water column, its seasonal variability, and water formation events are investigated. The extensive data set reveals that in summer Modified Atlantic Water (MAW) is observed at about 50 m depth. Saltier and warmer Levantine Intermediate Water (LIW) lies below the MAW between ∼100 and 250 m. During winter, the water column becomes mixed due to surface cooling and vertical convection, and displays uniform properties from the surface to about 200–300 m depths. The yearly cycle of a vertically uniform water column during winter and the appearance of LIW in the following period suggests that LIW is being formed in this region, which is confirmed through heat and buoyancy flux calculations. It is the first time LIW formation is reported in this area of the north‐eastern Levantine Basin and adds to the areas of LIW formation outside Rhodes Gyre. In the 2.5 years of the study, LIW salinity increased by 0.11 psu in 2017, while surface water salinity increased by 0.5 psu over the same period. The presence of meso‐scale eddies in the study area is observed throughout the year. The eddies are short‐lived, having a time span of a few months, during which they either change location or disappear.

Influence of large-scale atmospheric circulation patterns on nutrient dynamics in the Mediterranean Sea in the extended winter season (October-March) 1961-1999

We investigated the effects of variations in the 4 primary mid-latitude large-scale atmospheric circulation patterns on nutrients potentially limiting phytoplankton growth in the Mediterranean Sea (nitrate and phosphate), with a focus on the key deep convective areas of the basin (Gulf of Lions, Southern Adriatic Sea, Southern Aegean Sea and Rhodes Gyre). Monthly indices of these 4 modes of variability, together with a high-resolution hindcast of the Mediterranean Sea physics and biogeochemistry covering the period 1961-1999, were used to determine the physical mechanisms explaining the influence of these patterns on nutrient distribution and variability. We found a decrease in the concentration of phosphate and nitrate for each unit of increase in the index values of the East Atlantic and East Atlantic/Western Russian variability modes in the area of the Gulf of Lions, while a signal of the opposite sign was associated with the North Atlantic Oscillation in the Aegean Sea and Rhodes Gyre. In both cases, the variability observed was related to a significant variation in the mixed layer depth driven by heat losses and wind stress over the areas. The East Atlantic pattern played a major role in driving the long-term dynamics of both phosphate and nitrate availability in the Gulf of Lions, with a particularly pronounced effect in December and January. For both the Aegean Sea and Rhodes Gyre, the most prominent correlations were found between the North Atlantic Oscillation and phosphate, with a highly consistent behavior in the 2 areas associated with common physical forcing and exchange of properties among them.

Levantine Intermediate and Levantine Deep Water Formation: An Argo Float Study from 2001 to 2017

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea.

The Mediterranean Sea Overturning Circulation

AbstractThe time-mean zonal and meridional overturning circulations of the entire Mediterranean Sea are studied in both the Eulerian and residual frameworks. The overturning is characterized by cells in the vertical and either zonal or meridional planes with clockwise circulations in the upper water column and counterclockwise circulations in the deep and abyssal regions. The zonal overturning is composed of an upper clockwise cell in the top 600 m of the water column related to the classical Wüst cell and two additional deep clockwise cells, one corresponding to the outflow of the dense Aegean water during the Eastern Mediterranean Transient (EMT) and the other associated with dense water formation in the Rhodes Gyre. The variability of the zonal overturning before, during, and after the EMT is discussed. The meridional basinwide overturning is composed of clockwise, multicentered cells connected with the four northern deep ocean formation areas, located in the Eastern and Western Mediterranean basins. The connection between the Wüst cell and the meridional overturning is visualized through the horizontal velocities vertically integrated across two layers above 600 m. The component of the horizontal velocity associated with the overturning is isolated by computing the divergent components of the vertically integrated velocities forced by the inflow/outflow at the Strait of Gibraltar.

A study on causes and recurrence of the Mid-Mediterranean Jet from 2003 to 2015 using satellite thermal and altimetry data and CTD casts

ABSTRACT The existence of the Mid-Mediterranean Jet (MMJ) as part of the pathway of the Atlantic Water (AW) in Eastern Mediterranean is still debated and the reasons for its formation are unknown. The MMJ was investigated over the 2000–2015 summers using thermal and altimetry satellite data and ‘in situ’ casts. In the Levantine basin, the AW flowing along the Libyan-Egyptian coast splits into an along-shore and an offshore branch, the latter following the northern border of the warm anti-cyclones located near the African coast (Libyan-Egyptian Eddies, LEEs). The MMJ is triggered when a surface cold water mass formed south-east of Crete, in part outflowing the Aegean Sea and in part generated within the Rhodes Gyre, moves southward and intrudes the middle Levantine basin. The surface cold water overlays the AW flowing along the northern periphery of LEEs that sinks dozens of metres below increasing its budget of anti-cyclonic vorticity and causing the formation of the jet. Satellite data show that the MMJ formed every summer from 2003 to 2015, sometimes for limited periods of time and at different latitudes, with the exception of summers 2005–2006, probably due to the weakness of the African anti-cyclones that in those years did not advect AW offshore.

The impact of climate change on circulation patterns in the Eastern Mediterranean Sea upper layer using Med-ROMS model

This paper deals with three hindcasting simulations of circulation and sea level elevation, using Med-ROMS (Mediterranean- Regional Ocean Model System, with high horizontal -resolution (8.6 km). Each simulation has started at the end of previous 100-years spin-up and control run. The model surface boundary conditions, from the corrected ERA-40 re-analyses (Pettenuzzo et al., 2010) and three momentum flux ensemble members are based on a surface wind Bayesian Hierarchical Models (BHM) - Mediterranean Forecasting System (MFS-Wind-BHM) described in Berliner and Kim (2007) and Milliff et al. (2011), cover the period January 1958 to December 2001. The results have been validated by an Empirical Orthogonal Function (EOF) analysis of surface sea height anomalies for satellite and Med-ROMS Hindcasts. The correlation coefficients between Med-ROMS (Principal Component) PC1 (Sea Surface Height anomaly) SSHa hindcasts 1, 2, 3 and AVISO satellite are 0.985, 0.97 and 0.98 respectively, all values are significant at 95% confidence limit. We have defined a Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation (BiOS) (Gacic et al., 2011, 2014) as a pattern of Eastern Mediterranean climate change. The ROMS model successfully simulated the observed general circulation pattern in the Eastern Mediterranean. Over the period from July 1997 till December 2001, the Med-ROMS has successfully simulated the north Ionian inversion; SSHa same as Satellite SSH with an averaged RMSE between them of 0.0792 m. The spatial correlation maps between wind stress curl and Med-ROMS-SSHa/EOF1-SSHa are made and found to be significantly correlated at a 95% confidence limit. The highest significant correlation coefficient for Med-ROMS-SSHa was approximately 0.8 and located off the western Turkish coast inside the Aegean Sea. This strong significant positive correlation reflects the cyclonic motion of Rhodes Gyre. A high significant correlation coefficient for Med-ROMS-EOF1-SSHa, of around 0.51, was found along the western coast of Greece in the northeast Ionian Sea. For the first time, we could correlate the North Ionian Sea inversion to wind stress curl, which was significantly correlated with the EOF1 Med-ROMS SSHa averaged hindcasts during the inversion period. In this transient period, we noticed a significant correlation coefficient, at a 95% confidence limit, between NAO indices moving on average over 12 months and Med-ROMS-SSHa -PC1 over the Eastern Mediterranean.

The effect of wind-stress over the Eastern Mediterranean on deep-water formation in the Adriatic Sea

We investigate the effect of interannual variability of the wind-stress over the Eastern Mediterranean on Adriatic Deep Water (ADW) formation using an oceanic general circulation model of the Mediterranean Sea driven by monthly ERA-Interim wind-stress of the years 1979–2014. This is done by using climatological annual cycle of the surface temperature and freshwater flux but interannually varying wind-stress.A connection between the wind-stress magnitude over the Rhodes Gyre region and ADW outflow is found in association with the formation of Levantine Intermediate Water (LIW). 720 years artificially generated wind-stress fields were constructed based on the first 50 Empirical Orthogonal Functions (EOFs) of the wind-stress monthly anomalies. This simulation strengthen the connection that was found and also reveal a centennial internal variability of both the LIW and the ADW outflow.It is also found that the intensity of the Rhodes Gyre is almost linearly related to the wind-stress amplitude. However, the ADW outflow is non-monotonically related to the wind-stress amplitude. There is an optimal wind-stress amplitude for which the ADW formation is maximal. When the wind-stress amplitude is more than 50% the climatological mean wind-stress amplitude, the ADW outflow is weakened. This implies a different balance between the saline LIW and the fresh Atlantic Water, entering the Eastern Mediterranean from Sicily Strait, under stronger than present wind-stress amplitude.

Water masses and hydrography during April and June 2016 in the Cretan Sea and Cretan Passage (Eastern Mediterranean Sea)

Two successive cruises in the Cretan Sea and Cretan Passage (West Levantine basin) during April and June 2016 surveyed the physical characteristics of the water masses in these large Eastern Mediterranean basins. Data confirm that the hydrographical status is far different from that of the Eastern Mediterranean Transient (EMT) period; however, the basins have not yet returned to the pre-EMT status. In the Cretan Sea, intermediate water masses of both Cretan and Levantine origins are detected. The stagnating bottom waters of this basin still hold high salinity, density, and dissolved oxygen values, remnant of the EMT deep water formation episodes. Characterised by low salinity and oxygen values, transitional waters of Mediterranean origin are present between intermediate and bottom layers throughout the Cretan Sea. Intermittent weak outflow of warm and saline masses of Cretan origin towards the Eastern Mediterranean is observed at the bottom of both east and west Cretan Straits. In the Cretan Passage, there is no sign of the Ierapetra anticyclonic gyre, possibly related to the seasonality of the gyre or linked to larger scale Eastern Mediterranean circulation variability. The observed surface circulation in this area is comprised of a series of smaller gyres between the Cretan Cyclone and the Rhodes Gyre. The bottom waters of the Cretan Passage present a west-to-east gradient of increasing salinity and decreasing oxygen related to the propagation of new Adriatic Deep Water from the Ionian Sea towards the Levantine basin.

Spatio-temporal variability of the size-fractionated primary production and chlorophyll in the Levantine Basin (northeastern Mediterranean)

Summary Spatial and temporal variations in size-fractionated primary production (PP) and chl a, in relation to ambient physicochemical parameters, were studied in the three distinct ecosystems of northeastern Levantine Basin namely eutrophic Mersin Bay, mesotrophic Rhodes Gyre, and oligotrophic offshore waters. These ecosystems were visited in July and September 2012 and March and May 2013. Total primary production (TPP) rates ranged between 0.22 and 17.8 mg C m−3 h−1 within the euphotic zone, whereas depth-integrated TPP rates were in the range 21.5–348.8 mg C m−2 h−1 (mean: 105.5 ± 88 mg C m−2 h−1), with the lowest rates recorded for offshore waters. Similar spatio-temporal variations were observed in chl a concentrations, ranging from 2.3 to 117.9 mg m−2 (mean: 28.9 ± 24.9 mg m−2) in the study area. The Mersin Bay TPP rates have exceeded almost 8–12 times those measured in the offshore waters and the Rhodes Gyre; however, the chl a concentrations measured in coastal waters (0.343 mg m−3) and the Rhodes Gyre (0.308 mg m−3) were only threefold larger than the offshore values. PP and chl a were dominated by picoplankton in the study area whereas small nanoplankton, being the most active, displayed the highest assimilation ratio in offshore waters (6.8) and the Rhodes Gyre (2.8). In the upper-layer waters depleted of P (0.02–0.03 μM) of the northeastern Mediterranean, a positive correlation was observed between NO3 + NO2 and PP (and thus, chl a), which strongly suggests that reactive P and inorganic nitrogen are co-limiting factors in the production and biomass distribution of the phytoplankton community in both shelf and offshore waters.

Uncoupling between dinitrogen fixation and primary productivity in the eastern Mediterranean Sea

In the nitrogen (N)‐impoverished photic zones of many oceanic regions, prokaryotic organisms fixing atmospheric dinitrogen (N2; diazotrophs) supply an essential source of new nitrogen and fuel primary production. We measured dinitrogen fixation and primary productivity (PP) during the thermally stratified summer period in different water regimes of the oligotrophic eastern Mediterranean Sea, including the Cyprus Eddy and the Rhodes Gyre. Low N2 fixation rates were measured (0.8–3.2 µmol N m−2 d−1) excluding 10‐fold higher rates in the Rhodes Gyre and Cyprus Eddy (~20 µmol N m−2 d−1). The corresponding PP increased from east to west (200–2500 µmol C m−2 d−1), with relatively higher productivity recorded in the Rhodes Gyre and Cyprus Eddy (2150 and 2300 µmol C m−2 d−1, respectively). These measurements demonstrate that N2 fixation in the photic zone of the eastern Mediterranean Sea contributes only negligibly by direct inputs to PP (i.e., cyanobacterial diazotrophs) and is in fact uncoupled from PP. By contrast, N2 fixation is significantly coupled to bacterial productivity and to net heterotrophic areas, suggesting that heterotrophic N2 fixation may in fact be significant in this ultraoligotrophic system. This is further substantiated by the high N2 fixation rates we measured from aphotic depths and by the results of phylogenetic analysis in other studies showing an abundance of heterotrophic diazotrophs.

On the surface circulation of the Levantine sub-basin derived from Lagrangian drifters and satellite altimetry data

The surface currents of the Levantine sub-basin (Mediterranean Sea) are described using 18 years (1992–2010) of drifter data and satellite-derived sea level anomalies. The combination of drifter and satellite data allowed to estimate maps of surface geostrophic circulation and to obtain more accurate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of surface currents are investigated with particular focus on the main sub-basin eddies of the eastern Levantine. The mean velocity field depicts the typical patterns of the along-slope and offshore currents and outlines the sub-regions where eddies are generated recurrently (west Egyptian coast, Ierapetra, Mersa-Matruh, south-west of Cyprus, Israel–Lebanon coast, Latakia) or persist steadily (Rhodes Gyre). Highly variable and energetic currents are observed between the Ierapetra and Mersa-Matruh regions, as the result of the interaction of the Mid-Mediterranean Jet meandering in between, and interacting with, the eddies generated by the instability of the coastal current. Seasonal pseudo-Eulerian maps show the current field stronger in summer and weaker in winter, mainly in the western Levantine and in the Cyprus–Syria Passage. The Shikmona Eddy displays a periodic nature with higher intensities during the cold months and an enhanced activity in the period 1998–2005. The Cyprus Eddy has a less periodic nature, characterised by events of high activity and periods in which it dominates as a single enlarged eddy in the southeast Levantine, eventually including the Shikmona Eddy. The Latakia Eddy is mainly cyclonic with higher intensities in summer and fall; occasional weekly or monthly inversions of circulation from cyclonic to anticyclonic are triggered by the interaction between the MMJ and the northward coastal meandering current.

Hydrocarbons generated by hydropyrolysis of suspended marine particulate organic matter: Relationship to the oceanography of the Black Sea and the Rhodes Gyre

The suspended particulate organic matter, SPOM, in the autumnal Black Sea has been characterised using catalytic hydropyrolysis (HyPy) of the total (bound plus free) lipid material. The technique, which generates maximum yields of volatile products from sediments, kerogens and phytoplankton, was followed using gas chromatography and gas chromatography/mass spectrometry. The generated alkanes were dominated by n-C18, hypothesised to arise predominantly from unsaturated C18 fatty acids. Steranes were generated from reductive conversion of free and bound sterols. The generation of branched alkanes and especially of hopanes provided formal evidence for the participation of bacteria in the mineralisation of the SPOM. Whereas similar distributions of n-alkanes were generated from SPOM sampled from different depths of the comparatively well-stirred Rhodes Gyre (eastern Mediterranean), mineralisation of the SPOM at each depth of the central Black Sea produced characteristic changes in the composition and concentration of the HyPy products. Depth profiles of the n-alkanes generated from SPOM in the region of the Rim Current were affected by the local hydrography. Polynuclear aromatic hydrocarbons (PAHs) of anthropogenic origin were present in the surface waters of the central Black Sea. Some methyl benzenes, thiophenes and pyrroles were also generated.