Northwest Mediterranean Sea Research Articles

Size/Age Models for Monitoring of the Pink Sea Fan Eunicella verrucosa (Cnidaria: Alcyonacea) and a Case Study Application

The pink sea fan Eunicella verrucosa is a habitat-forming octocoral living in the East Atlantic and in the Mediterranean Sea where, under proper circumstances, it can form large populations known as coral forests. Although these coral forests represent vulnerable marine ecosystems of great importance, these habitats are still poorly known, and their monitoring is almost non-existent to date. For this reason, we compared two different models to infer the age of E. verrucosa based on nondestructive measurements of the colonies’ size, in order to highlight strengths and weaknesses of the existing tools for a potential application in long-term monitoring. We also applied the two models on a case-study population recently found in the northwest Mediterranean Sea. Our results showed which model was more reliable from a biological point of view, considering both its structure and the results obtained on the case study. However, this model uses solely the height of the colonies as proxy to infer the age, while the total branch fan surface area could represent a more appropriate biometric parameter to monitor the size and the growth of E. verrucosa.

Conservation of Native Wild Ivory-White Olives from the MEDES Islands Natural Reserve to Maintain Virgin Olive Oil Diversity.

Food diversity, and in particular genetic diversity, is being lost at an alarming rate. Protection of natural areas is crucial to safeguard the world’s threatened species. The Medes Islands (MI), located in the northwest Mediterranean Sea, are a protected natural reserve. Wild olive trees also known as oleasters make up part of the vegetation of the Meda Gran island. Among them, in 2012, a wild albino ivory-white olive tree with fruit was identified. Fruits were collected from this tree and their seeds were first sown in a greenhouse and then planted in an orchard for purposes of ex situ preservation. Seven out of the 78 seedling trees obtained (12%) produced ivory-white fruits. In autumn 2018, fruits from these trees were sampled. Although the fruits had low oil content, virgin olive oil with unique sensory, physicochemical, and stability characteristics was produced. With respect to the polyphenols content, oleacein was the main compound identified (373.29 ± 72.02 mg/kg) and the oleocanthal was the second most abundant phenolic compound (204.84 ± 52.58 mg/kg). Regarding pigments, samples were characterized by an intense yellow color, with 12.5 ± 4.6 mg/kg of chlorophyll and 9.2 ± 3.3 mg/kg of carotenoids. Finally, oleic acid was the main fatty acid identified. This study explored the resources of the natural habitat of the MI as a means of enrichment of olive oil diversity and authenticity of this traditional Mediterranean food

Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3− was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2− and NH4+ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m3 s−1 for the entire investigated coastline, determined from simultaneous 224Ra and 226Ra mass balances. The contribution of groundwater PO43−, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

Occurrence of ascaridoid nematodes in Illex coindetii, a commercially relevant cephalopod species from the Ligurian Sea (Northwest Mediterranean Sea)

In this study we investigated the occurrence of larval ascaridoid nematodes in Illex coindetii, a commercially relevant cephalopod, via a cross-sectional prevalence study (January 2015 to February 2018) in two fishing areas (Savona and Piombino) in the Ligurian Sea (northwest Mediterranean Sea). A total of 745 specimens of I. coindetii were caught by local fisherman through bottom trawling. Parasitological examination revealed ascaridoid nematodes (L3) larvae (n = 9): Hysterothylacium sp. (n = 6) and Anisakis spp. (n = 3). All larvae morphologically referred to the genus Anisakis were identified by PCR-RFLP as A. pegreffii. The prevalence of Hysterothylacium sp. infestation was 0.88% for the Savona and 0.56% for the Piombino sampling site; the prevalence of A. pegreffii was 0.18% for Savona and 1.11% for Piombino and did not differ between the two sampling sites (chi-square test; p = 0.0848). Better knowledge of the distribution of ascaridoid nematodes, especially of Anisakis species is essential for defining their epidemiological role and the potential risk for consumer health.

Impact of Saharan dust events on radionuclides in the atmosphere, seawater, and sediments of the northwest Mediterranean Sea

In February 2004, anthropogenic radionuclides (137Cs, 236U, 239Pu and 240Pu), transported from the Sahara Desert, were observed in the Monaco air, and later in water and sediment samples collected at the DYFAMED site in the northwest (NW) Mediterranean Sea. While 236U and 137Cs in Saharan dust particles showed a high solubility in seawater, Pu isotopes were particle reactive in the water column and in the sediment. The impact of the Saharan deposition was found at 0–1.0 cm of the sediment core for 236U and 137Cs, and between 1.0 and 1.5 cm for Pu isotopes. The excess of 236U was observed more in the water column than in the sediment, whereas the 239+240Pu total inventories were comparable in the water column and the sediment. This single-day particle event represented 72% of annual atmospheric deposition in Monaco. At the DYFAMED site, it accounted for 10% (137Cs) and 15% (239+240Pu) activities of sinking particles during the period of the highest mass flux collected at the 200 and 1000 m water depths, and for a significant proportion of the total annual atmospheric input to the NW Mediterranean Sea (28–37% for 137Cs and 34–45% for 239+240Pu). Contributions to the total 137Cs and 239+240Pu sediment inventories were estimated to be 14% and 8%, respectively. The Saharan dust deposition phenomenon (atmospheric input, water column and sediment) offered a unique case to study origin and accumulation rates of radionuclides in the NW Mediterranean Sea.

Using opportunistic sightings to assess the suitability of Important Marine Mammal Areas (IMMAs) for cetacean conservation in the Western Mediterranean Sea

The Western Mediterranean Sea connects the Atlantic Ocean with the rest of the Mediterranean Sea through the Strait of Gibraltar. The Western Mediterranean Sea is important area for cetaceans and it contains highly productive feeding areas, such as the Pelagos Sanctuary and the South Balearic eddy. The main aim of this study was to assess the suitability of a group of Important Marine Mammal Areas (IMMAs) for the conservation of cetaceans inhabiting this area. There were 398 (46.44%) opportunistic sightings (OS) within a number of IMMAs and 459 (53.56%) sightings outside of IMMAs in this area. Trend surface analysis was used to select the OSs (GPOSs hereafter) most likely to be observed within IMMAs as a function of their geographical position. Significant differences were found between the observed GPOS rate and the expected GPOS rate weighted by the surface area of each IMMA. Specifically, there were more sightings than expected in the Alboran Sea IMMA than in the North West Mediterranean Sea, Slope, and Canyon System IMMA. In the latter area, there were fewer sightings than expected.

Orientation of Mediterranean fish larvae varies with location

Fish larvae at settlement represent a determining stage for maintaining coastal fish populations. This early life stage is decisive for the dispersal, recruitment, and habitat colonisation of coastal fish species. This study aims at observing the orientation behaviour of eight Mediterranean fish taxa, in four families, at settlement stage in two experimental sites along the north-east Corsican coastline (north-west Mediterranean Sea), a Sandy and a Rocky coastal site with different environmental characteristics. The objective was to detect if there were differences in directionality (i.e. the ability of individuals to maintain a fixed bearing in their environment) and orientation (i.e. the consistency among the bearing of individuals at species level) between the two coastal sites for the tested species. We also tried to identify the environmental factors that may influence directionality and orientation. The results show strong directionality for most fish larvae, with proportions of directional individuals generally exceeding 80%, either at community or species level (4 ≤ n ≤ 46 per species). Only the white seabream, Diplodus sargus, showed significant orientation behaviour, towards a cardinal direction, towards the sun in both experimental sites, as well as towards the coast in the Sandy site and towards the open sea in the Rocky site. The other species did not show significant orientation. This study supports the theory that orientation behaviour is dependent both on species and the environment perceived by the fish larvae. This kind of work is important for developing predictive models of marine population settlement and presents key elements for protection and management of coastal areas.

Lessons from photo analyses of Autonomous Reef Monitoring Structures as tools to detect (bio-)geographical, spatial, and environmental effects

We investigated the validity of Autonomous Reef Monitoring Structures (ARMS) as monitoring tools for hard bottoms across a wide geographic and environmental range. We deployed 36 ARMS in the northeast Atlantic, northwest Mediterranean, Adriatic and Red Sea at 7–17 m depth. After 12–16 months, community composition was inferred from photographs, in six plate-faces for each ARMS. Overall, we found a highly significant effect of sea region, site (within seas), and plate-face on community composition. Plate-faces thus represent distinct micro-habitats and provide pseudo-replicates, increasing statistical power. Within each sea region taken individually, there was also a highly significant effect of site and plate-face. Because strong effects were obtained despite the fusion of taxonomic categories at high taxonomic ranks (to ensure comparability among biogeographic provinces), ARMS photo-analysis appears a promising monitoring tool for each sea region. We recommend keeping three ARMS per site and analyzing more numerous sites within a sea region to investigate environmental effects.

Ocean acidification affects calcareous tube growth in adults and reared offspring of serpulid polychaetes.

The energetically costly transition from free-swimming larvae to a benthic life stage and maintenance of a calcareous structure can make calcifying marine invertebrates vulnerable to ocean acidification. The first goal of this study was to evaluate the impact of ocean acidification on calcified tube growth for two Serpulidae polychaete worms. Spirorbis sp. and Spirobranchus triqueter were collected at 11 m depth from the northwest Mediterranean Sea and maintained for 30 and 90 days at three mean pHT levels (total scale): 8.1 (ambient), 7.7 and 7.4. Moderately decreased tube elongation rates were observed in both species at pHT7.7 while severe reductions occurred at pHT7.4. There was visual evidence of dissolution and tubes were more fragile at lower pH but fragility was not attributed to changes in fracture toughness. Instead, it appeared to be due to the presence of larger alveoli covered in a thinner calcareous layer. The second objective of this study was to test for effects on S. triqueter offspring development. Spawning was induced, and offspring were reared in the same pH conditions that the parents experienced. Trochophore size was reduced at the lowest pH level but settlement success was similar across pH conditions. Post-settlement tube growth was most affected. At 38 days post-settlement, juvenile tubes at pHT7.7 and 7.4 were half the size of those at pHT8.1. The results suggest future carbonate chemistry will negatively affect the initiation and persistence of both biofouling and epiphytic polychaete tube worms.

Trace metal concentrations in the muscle of seven marine species: Comparison between the Gulf of Lions (North-West Mediterranean Sea) and the Bay of Biscay (North-East Atlantic Ocean)

Concentrations of 6 trace metals (Ag, Cd, Cu, Ni, Pb, Zn) in the muscle of 2 sharks (Galeus melastomus and Scyliorhinus canicula), 4 teleosts (Helicolenus dactylopterus, Lepidorhombus boscii, Micromesistius poutassou and Phycis blennoides) and 1 crustacean (Nephrops norvegicus) were compared between the Bay of Biscay (Atlantic Ocean) and the Gulf of Lions (Mediterranean Sea). Although average concentrations and the trace element pollution index were generally higher in the Gulf of Lions, significant differences between the two ecosystems were only found for Zn for Helicolenus dactylopterus, and for Ag and Cu for the crustacean N. norvegicus. Moreover, some relationships between trophic level or size and metal concentrations were found for these two species. The absence of clear pattern may result from the blurring effect of contamination and excretion that may act differentially for all species and all elements.

Analysis of the Precipitation Regime over the Ligurian Sea

The regions surrounding the North West Mediterranean Sea are often sites of intense precipitation events, particularly during the autumn months. The many casualties and the high economic costs due to these events demand a continuous improvement in forecasting models in support of early warning systems. The main weather conditions that determine episodes of heavy rain over these regions are known, but the high number of processes and interactions taking place at different time and space scales makes it extremely difficult to increase the skill pertaining to their predictability. To deepen the knowledge of the phenomena, both numerical simulations and analysis of historical data sets are required. This paper presents the analysis of a five-year-long time series of rain data collected in the open Ligurian Sea from the fixed platform W1M3A and coastal stations. The analysis aims to characterize the main features of the precipitation over this area and its seasonal and annual variability. Furthermore, the work includes a description of the main atmospheric and oceanic surface parameters measured from the platform during some intense events that occurred in the period 2009–2013 and suggests to what extent offshore observations may contribute to improve the forecast of rainfall events.

Modelling deep-water formation in the north-west Mediterranean Sea with a new air–sea coupled model: sensitivity to turbulent flux parameterizations

Abstract. In the north-western Mediterranean, the strong, dry, cold winds, the Tramontane and Mistral, produce intense heat and moisture exchange at the interface between the ocean and the atmosphere leading to the formation of deep dense waters, a process that occurs only in certain regions of the world. The purpose of this study is to demonstrate the ability of a new coupled ocean–atmosphere modelling system based on MESONH-SURFEX-SYMPHONIE to simulate a deep-water formation event in real conditions. The study focuses on summer 2012 to spring 2013, a favourable period that is well documented by previous studies and for which many observations are available. Model results are assessed through detailed comparisons with different observation data sets, including measurements from buoys, moorings and floats. The good overall agreement between observations and model results shows that the new coupled system satisfactorily simulates the formation of deep dense water and can be used with confidence to study ocean–atmosphere coupling in the north-western Mediterranean. In addition, to evaluate the uncertainty associated with the representation of turbulent fluxes in strong wind conditions, several simulations were carried out based on different parameterizations of the flux bulk formulas. The results point out that the choice of turbulent flux parameterization strongly influences the simulation of the deep-water convection and can modify the volume of the newly formed deep water by a factor of 2.

A multispecies approach reveals hot spots and cold spots of diversity and connectivity in invertebrate species with contrasting dispersal modes.

Genetic diversity is crucial for species' maintenance and persistence, yet is often overlooked in conservation studies. Species diversity is more often reported due to practical constraints, but it is unknown if these measures of diversity are correlated. In marine invertebrates, adults are often sessile or sedentary and populations exchange genes via dispersal of gametes and larvae. Species with a larval period are expected to have more connected populations than those without larval dispersal. We assessed the relationship between measures of species and genetic diversity, and between dispersal ability and connectivity. We compiled data on genetic patterns and life history traits in nine species across five phyla. Sampling sites spanned 600km in the northwest Mediterranean Sea and focused on a 50-km area near Marseilles, France. Comparative population genetic approaches yielded three main results. (i) Species without larvae showed higher levels of genetic structure than species with free-living larvae, but the role of larval type (lecithotrophic or planktotrophic) was negligible. (ii) A narrow area around Marseilles, subject to offshore advection, limited genetic connectivity in most species. (iii) We identified sites with significant positive contributions to overall genetic diversity across all species, corresponding with areas near low human population densities. In contrast, high levels of human activity corresponded with a negative contribution to overall genetic diversity. Genetic diversity within species was positively and significantly linearly related to local species diversity. Our study suggests that local contribution to overall genetic diversity should be taken into account for future conservation strategies.

Oligotrophy as a major driver of mercury bioaccumulation in medium-to high-trophic level consumers: A marine ecosystem-comparative study.

Mercury (Hg) is a global contaminant of environmental concern. Numerous factors influencing its bioaccumulation in marine organisms have already been described at both individual and species levels (e.g., size or age, habitat, trophic level). However, few studies have compared the trophic characteristics of ecosystems to explain underlying mechanisms of differences in Hg bioaccumulation and biomagnification among food webs and systems. The present study aimed at investigating the potential primary role of the trophic status of systems on Hg bioaccumulation and biomagnification in temperate marine food webs, as shown by their medium-to high-trophic level consumers. It used data from samples collected at the shelf-edge (i.e. offshore organisms) in two contrasted ecosystems: the Bay of Biscay in the North-East Atlantic Ocean and the Gulf of Lion in the North-West Mediterranean Sea. Seven species including crustaceans, sharks and teleost fish, previously analysed for their total mercury (T-Hg) concentrations and their stable carbon and nitrogen isotope compositions, were considered for a meta-analysis. In addition, methylated mercury forms (or methyl-mercury, Me-Hg) were analysed. Mediterranean organisms presented systematically lower sizes than Atlantic ones, and lower δ13C and δ15N values, the latter values especially highlighting the more oligotrophic character of Mediterranean waters. Mediterranean individuals also showed significantly higher T-Hg and Me-Hg concentrations. Conversely, Me-Hg/T-Hg ratios were higher than 85% for all species, and quite similar between systems. Finally, the biomagnification power of Hg was different between systems when considering T-Hg, but not when considering Me-Hg, and was not different between the Hg forms within a given system. Overall, the different parameters showed the crucial role of the low primary productivity and its effects rippling through the compared ecosystems in the higher Hg bioaccumulation seen in organisms from oligotrophic Mediterranean waters.

Combining laser diffraction, flow cytometry and optical microscopy to characterize a nanophytoplankton bloom in the Northwestern Mediterranean

The study of particle size distribution (PSD) gives insights on the dynamics of distinct pools of particles in the ocean, which reflect the functioning of the marine ecosystem and the efficiency of the carbon pump. In this study, we combined continuous particle size estimations and discrete measurements focused on phytoplankton to describe a spring bloom in the North West Mediterranean Sea. During April 2013, about 90 continuous profiles of PSD quantified through in situ laser diffraction and transmissiometry (the Laser in situ Scattering and Transmissiometry Deep (LISST-Deep), Sequoia Sc) were complemented by Niskin bottle samples for flow cytometry analyses, taxonomic identification by optical microscopy and pigments quantification. In the euphotic zone, the PSD shape seen by the LISST was fairly stable with two particle volume peaks covering the 2–11 µm and 15–109 µm size fractions. The first pool strongly co-varied with the chlorophyll fluorescence emitted by phytoplankton cells. In addition, over the 2–11 µm fraction, the LISST derived abundance was highly correlated with the abundance of nanophytoplankton counted by flow cytometry. Microscopy identified a phytoplankton community dominated by nanodiatoms and nanoflagellates. High correlation of LISST derived particle carbon and Particulate Organic Carbon and high nitrogen in the Particulate Organic Matter also supported a dominance of actively growing phytoplankton cells in this pool. The second, broader pool of particles covering sizes 15–109 µm was possibly microflocs coming from rivers and/or sediments. This study demonstrates the complementarity of continuous measurements of PSD combined with discrete measurements to better quantify size, abundance, biomass, and spatial (both vertical and horizontal) distribution of phytoplankton in open ocean environments.

Changes in Microbial (Bacteria and Archaea) Plankton Community Structure after Artificial Dispersal in Grazer-Free Microcosms.

Microbes are considered to have a global distribution due to their high dispersal capabilities. However, our knowledge of the way geographically distant microbial communities assemble after dispersal in a new environment is limited. In this study, we examined whether communities would converge because similar taxa would be selected under the same environmental conditions, or would diverge because of initial community composition, after artificial dispersal. To this aim, a microcosm experiment was performed, in which the temporal changes in the composition and diversity of different prokaryoplankton assemblages from three distant geographic coastal areas (Banyuls-sur-Mer in northwest Mediterranean Sea, Pagasitikos Gulf in northeast Mediterranean and Woods Hole, MA, USA in the northwest Atlantic), were studied. Diversity was investigated using amplicon pyrosequencing of the V1–V3 hypervariable regions of the 16S rRNA. The three assemblages were grown separately in particle free and autoclaved Banyuls-sur-mer seawater at 18 °C in the dark. We found that the variability of prokaryoplankton community diversity (expressed as richness, evenness and dominance) as well as the composition were driven by patterns observed in Bacteria. Regarding community composition, similarities were found between treatments at family level. However, at the OTU level microbial communities from the three different original locations diverge rather than converge during incubation. It is suggested that slight differences in the composition of the initial prokaryoplankton communities, resulted in separate clusters the following days even when growth took place under identical abiotic conditions.

Anthropogenic chemical cues can alter the swimming behaviour of juvenile stages of a temperate fish

Human pressure on coastal areas is affecting essential ecosystems including fish nursery habitats. Among these anthropogenic uses, the seasonal increment in the pressure due to leisure activities such as coastal tourism and yachting is an important environmental stressor in many coastal zones. These pressures may elicit understudied impacts due to, for example, sunscreens or other seasonal pollutants. The island of Majorca, northwest Mediterranean Sea, experiences one of the highest number of tourist visits per capita in the world, thus the surrounding coastal habitat is subject to high anthropogenic seasonal stress. Studies on early stages of fishes have observed responses to coastal chemical cues for the selection or avoidance of habitats. However, the potential interferences of human impacts on these signals are largely unknown. A choice chamber was used to determine water type preference and behaviour in naïve settled juvenile gilt-head sea bream (Sparus aurata), a temperate species of commercial interest. Fish were tested individually for behavioural changes with respect to water types from potential beneficial habitats, such as seawater with extract of the endemic seagrass Posidonia oceanica, anthropogenically influenced habitats such as water extracted from a commercial and recreational harbour and seawater mixed with sunscreen at concentrations observed in coastal waters. Using a Bayesian approach, we investigated a) water type preference; b) mean speed; and c) variance in the movement (as an indicator of burst swimming activity, or “sprint” behaviour) as behavioural descriptors with respect to water type. Fish spent similar percentage of time in treatment and control water types. However, movement descriptors showed that fish in sunscreen water moved slower (98.43% probability of being slower) and performed fewer sprints (90.1% probability of having less burst in speed) compared to control water. Less evident increases in sprints were observed in harbour water (73.56% more sprints), and seagrass (79.03% more) in comparison to control water. When seagrass water was tested against harbour water, the latter elicited a higher number of sprints (91.66% increase). We show that juvenile gilt-head seabream are able to react to a selection of naturally occurring chemically different odourscapes, including the increasingly important presence of sunscreen products, and provide a plausible interpretation of the observed behavioural patterns.

Physical and dynamical characteristics of a 300 m-deep anticyclonic eddy in the Ligurian Sea (Northwest Mediterranean Sea): Evidence from a multi-platform sampling strategy

We describe the physical and dynamical characteristics of a mesoscale anticyclonic eddy observed in August 2013 over the shelf-break region of the northeastern sector of the Ligurian Sea, between the northeastern edge of the Northern Current (NC) and the coast. Results derive from a dense dataset of temperature, salinity and current measurements obtained from a multi-platform sampling strategy as well as from a diagnostic simulation with the Regional Ocean Modeling System (ROMS) at a horizontal resolution of 1.8km. Model results are obtained from a strong nudging to observations and, as they are physically balanced, they allow a three-dimensional diagnosis of the dynamics and physical characteristics of the eddy. The eddy is centered around 9.5°E, 43.94°N, about 20km from the coast, and has a radius of 16km. It is characterized by low-density waters and penetrates the thermocline down to at least 300m reflecting the main features of the NC. Horizontal velocities near the surface are around 0.4ms−1, while at 150m are still significantly high and close to 0.2ms−1. Vertical velocities were estimated from model results; absolute values are below 4mday−1 until depths shallower than 150m and increase with depth to 15mday−1. The eddy's presence inverts the northwestwards flow traditionally portrayed in the region determining a southeastwards coastal circulation that replenishes coastal waters with those originating from the NC. We discuss several possible eddy formation mechanisms and suggest that its formation depends on the directionality of the NC when it enters the Ligurian Sea, as a result of the adjustment of the shear and orbital components of the current's relative vorticity.

Probing the Rare Biosphere of the North-West Mediterranean Sea: An Experiment with High Sequencing Effort.

High-throughput sequencing (HTS) techniques have suggested the existence of a wealth of species with very low relative abundance: the rare biosphere. We attempted to exhaustively map this rare biosphere in two water samples by performing an exceptionally deep pyrosequencing analysis (~500,000 final reads per sample). Species data were derived by a 97% identity criterion and various parametric distributions were fitted to the observed counts. Using the best-fitting Sichel distribution we estimate a total species richness of 1,568–1,669 (95% Credible Interval) and 5,027–5,196 for surface and deep water samples respectively, implying that 84–89% of the total richness in those two samples was sequenced, and we predict that a quadrupling of the present sequencing effort would suffice to observe 90% of the total richness in both samples. Comparing the HTS results with a culturing approach we found that most of the cultured taxa were not obtained by HTS, despite the high sequencing effort. Culturing therefore remains a useful tool for uncovering marine bacterial diversity, in addition to its other uses for studying the ecology of marine bacteria.

Excess of 236U in the northwest Mediterranean Sea

In this work, we present first 236U results in the northwestern Mediterranean. 236U is studied in a seawater column sampled at DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (Ligurian Sea, 43°25′N, 07°52′E). The obtained 236U/238U atom ratios in the dissolved phase, ranging from about 2×10−9 at 100m depth to about 1.5×10−9 at 2350m depth, indicate that anthropogenic 236U dominates the whole seawater column. The corresponding deep-water column inventory (12.6ng/m2 or 32.1×1012 atoms/m2) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5ng/m2 or 13×1012 atoms/m2), evidencing the influence of local or regional 236U sources in the western Mediterranean basin. On the other hand, the input of 236U associated to Saharan dust outbreaks is evaluated. An additional 236U annual deposition of about 0.2pg/m2 based on the study of atmospheric particles collected in Monaco during different Saharan dust intrusions is estimated. The obtained results in the corresponding suspended solids collected at DYFAMED station indicate that about 64% of that 236U stays in solution in seawater. Overall, this source accounts for about 0.1% of the 236U inventory excess observed at DYFAMED station. The influence of the so-called Chernobyl fallout and the radioactive effluents produced by the different nuclear installations allocated to the Mediterranean basin, might explain the inventory gap, however, further studies are necessary to come to a conclusion about its origin.