Northwest Mediterranean Sea Research Articles

Impact of Saharan dust events on radionuclide levels in Monaco air and in the water column of the northwest Mediterranean Sea

Characterization of atmospheric aerosols collected in Monaco (2004–2008) and in sediment traps at 200 m and 1000 m water depths at the DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (2004) was carried out to improve our understanding of the impact of Saharan dust on ground-level air and on the water column. Activity concentrations of natural (210Pb, 210Po, uranium and radium isotopes) and anthropogenic (137Cs, 239Pu, 240Pu, and 239+240Pu) radionuclides and their isotopic ratios confirmed a Saharan impact on the investigated samples. In association with a large particulate matter deposition event in Monaco on 20 February 2004, the 137Cs (∼40 Bq kg−1) and 239+240Pu (∼1 Bq kg−1) activities were almost a factor of two higher than other Saharan deposition dust events. This single-day particle flux represented 72% of the annual atmospheric deposition in Monaco. The annual deposition of Saharan dust on the sea was 232–407 mBq m−2 for 137Cs and 6.8–9.8 mBq m−2 for 239+240Pu and contributed significantly (28–37% for 137Cs and 34–45% for 239+240Pu) to the total annual atmospheric input to the northwest Mediterranean Sea. The 137Cs/239+240Pu activity ratios in dust samples collected during different Saharan dust events confirmed their global fallout origin or mixing with local re-suspended soil particles. In the sediment trap samples the 137Cs activity varied by a factor of two, while the 239+240Pu activity was constant, confirming the different behaviors of Cs (dissolved) and Pu (particle reactive) in the water column. The 137Cs and 239+240Pu activities of sinking particles during the period of the highest mass flux collected in 20 February 2004 at the 200 m and 1000 m water depths represented about 10% and 15%, respectively, of annual deposition from Saharan dust events.

A multi-proxy record of MIS 11–12 deglaciation and glacial MIS 12 instability from the Sulmona basin (central Italy)

A multi-proxy record (lithology, XRF, CaCO3 content, carbonate δ18O and δ13C) was acquired from a sediment core drilled in the intermountain Sulmona basin (central Italy). Tephrostratigraphic analyses of three volcanic ash layers ascribe the investigated succession to the MIS 12-MIS 11 period, spanning the interval ca. 500–410 ka. Litho-pedo facies assemblage indicates predominant lacustrine deposition, interrupted by a minor sub-aerial and lake low stand episode. Variations in major and minor elements concentrations are related to changes in the clastic input to the lake. The oxygen isotopic composition of carbonate (δ18Oc) intervals is interpreted mainly as a proxy for the amount of precipitation in the high-altitude catchment of the karst recharge system. The record shows pronounced hydrological variability at orbital and millennial time-scales, which appears closely related to the Northern Hemisphere summer insolation pattern and replicates North Atlantic and west Mediterranean Sea Surface Temperature (SST) fluctuations. The MIS 12 glacial inception is marked by an abrupt reduction of precipitation, lowering of the lake level and enhanced catchment erosion. A well-defined and isotopically prominent interstadial with increased precipitation maybe related to insolation maxima-precession minima at ca. 465 ka. This interstadial ends abruptly at ca. 457 ka and it is followed by a phase of strong short-term instability. Drastic lake-level lowering and enhanced clastic flux characterized the MIS 12 glacial maximum. Lacustrine deposition restarted about 440 ka ago. The MIS 12–MIS 11 transition is characterized by a rapid increase in the precipitation, lake-level rise and reduction in the clastic input, interrupted by a short and abrupt return to drier conditions. Comparison with marine records from the Iberian margin and western Mediterranean suggests that major events of ice rafted debris deposition, related to southward migrations of the polar front, match the harshest periods in central Italy. This indicates strong teleconnections between Northern hemisphere ice sheet dynamics, North Atlantic oceanic conditions and Mediterranean continental hydrology.

Population genetic structure of the European conger (Conger conger) in North East Atlantic and West Mediterranean Sea

Assessment of population genetic structure is crucial in species conservation and stock management. The use of multiple markers is recommended to obtain accurate studies. In this study, a panel of five microsatellite loci described for Conger myriaster was optimized for the European conger eel Conger conger. These are the first hypervariable nuclear loci employed for C. conger population study and have a high discrimination power. A total of 67 alleles were observed for Atlantic and Mediterranean samples with an average number of alleles per locus of 13.4 (range 4–24). Variation at these microsatellite loci was combined with mitochondrial control region sequences (395bp) to explore genetic structure of C. conger across the Cantabrian Sea (North East Atlantic ocean) and the West Mediterranean Sea. Overall, there is not total isolation between the Atlantic and the Mediterranean stocks, as previously suggested in other studies, but a subtle differentiation. Cross-species amplification of microsatellite loci in C. conger is hoped to encourage further population studies of this species and the development of new nuclear markers in order to provide scientific advice for the stock management.

Experimental assessment of cumulative temperature and UV-B radiation effects on Mediterranean plankton metabolism

The Mediterranean Sea is a vulnerable region for climate change, warming at higher rates compare to the global ocean. Warming leads to increased stratification of the water column and enhanced the oligotrophic nature of the Mediterranean Sea. The oligotrophic waters are already highly transparent, however, exposure of Mediterranean plankton to ultraviolet radiation (UV-B and UV-A) may increase further if the waters become more oligotrophic, thereby, allowing a deeper UV radiation penetration and likely enhancing impacts to biota. Here we experimentally elucidate the cumulative effects of warming and natural UV-B radiation on the net community production (NCP) of plankton communities. We conducted five experiments at monthly intervals, from June to October 2013, and evaluated the responses of NCP to ambient UV-B radiation and warming (+3oC), alone and in combination, in a coastal area of the northwest Mediterranean Sea. UV-B radiation and warming lead to reduced net community production and resulted in a heterotrophic (NCP<0) metabolic balance. Both UV-B radiation and temperature, showed a significant individual effect in NCP across treatments and time. However, their joint effect showed to be synergistic as the interaction between them (UV x Temp) was statistically significant in most of the experiments performed. Our results showed that both drivers, would affect the gas exchange of CO2-O2 from and to the atmosphere and the role of plankton communities in the Mediterranean carbon cycle

Communities of Cultivable Root Mycobionts of the Seagrass Posidonia oceanica in the Northwest Mediterranean Sea Are Dominated by a Hitherto Undescribed Pleosporalean Dark Septate Endophyte.

Seagrasses, a small group of submerged marine macrophytes, were reported to lack mycorrhizae, i.e., the root-fungus symbioses most terrestrial plants use for nutrient uptake. On the other hand, several authors detected fungal endophytes in seagrass leaves, shoots, rhizomes, and roots, and an anatomically and morphologically unique dark septate endophytic (DSE) association has been recently described in the roots of the Mediterranean seagrass Posidonia oceanica. Nevertheless, the global diversity of seagrass mycobionts is not well understood, and it remains unclear what fungus forms the DSE association in P. oceanica roots. We isolated and determined P. oceanica root mycobionts from 11 localities in the northwest Mediterranean Sea with documented presence of the DSE association and compared our results with recent literature. The mycobiont communities were low in diversity (only three species), were dominated by a single yet unreported marine fungal species (ca. 90 % of the total 177 isolates), and lacked common terrestrial and freshwater root mycobionts. Our phylogenetic analysis suggests that the dominating species represents a new monotypic lineage within the recently described Aigialaceae family (Pleosporales, Ascomycota), probably representing a new genus. Most of its examined colonies developed from intracellular microsclerotia occupying host hypodermis and resembling microsclerotia of terrestrial DSE fungi. Biological significance of this hitherto overlooked seagrass root mycobiont remains obscure, but its presence across the NW Mediterranean Sea and apparent root intracellular lifestyle indicate an intriguing symbiotic relationship with the dominant Mediterranean seagrass. Our microscopic observations suggest that it may form the DSE association recently described in P. oceanica roots.

Seasonal variations in the source of sea bottom organic matter off Catalonia coasts (western Mediterranean): links with hydrography and biological response

Analyses of the total organic carbon (TOC), total nitrogen (TN), C/N ratio and isotopic (δ 13C and δ 15N) composition of organic matter (OM) in surficial sediments, integrated with hydrographic measurements, have been used to evaluate the sources and distribution of sedimentary organic matter on the Catalonian shelf and slope (northwest Mediterranean Sea). We examined the effects of the variations on meiofauna. Settled OM in February, April and October displayed δ 13C relatively closer to marine algae (mean −22.4 ‰ ± 0.1) than that in July (−23.3, −23.7 and −22.7 ‰ at 60, 600 and 800 m, respectively). Lowest δ 13C (−23.7 ‰) and high C/N (~10), indicative of mixed continental and marine organic contributions, were detected principally at the head of the Buscarro Canyon (at 600 m) in July, ca. 2–3 months after the maximum river discharge. We observed significant changes in total meiofaunal density among seasons in the canyon, with the highest values in July and October. On the adjacent slope at 800 m there was no significant increase of total meiofauna in those months. Although export of marine organic matter from surface layers was the main food source in the study area (on average 71.2 % estimating marine and terrestrial OC by binary models; 68–71 % at the most by SIAR models), advection of organic matter from the continental edge and/or resuspension of organic matter along submarine canyons may seasonally alter the composition of deposits on the slope, particularly in July with % OC of terrestrial origin reaching 48.7 % in Buscarro canyon, shifting the basal food-web of deep ecosystems.

Diversity of Viruses Infecting the Green Microalga Ostreococcus lucimarinus.

The functional diversity of eukaryotic viruses infecting a single host strain from seawater samples originating from distant marine locations is unknown. To estimate this diversity, we used lysis plaque assays to detect viruses that infect the widespread species Ostreococcus lucimarinus, which is found in coastal and mesotrophic systems, and O. tauri, which was isolated from coastal and lagoon sites from the northwest Mediterranean Sea. Detection of viral lytic activities against O. tauri was not observed using seawater from most sites, except those close to the area where the host strain was isolated. In contrast, the more cosmopolitan O. lucimarinus species recovered viruses from locations in the Atlantic and Pacific Oceans and the Mediterranean Sea. Six new O. lucimarinus viruses (OlVs) then were characterized and their genomes sequenced. Two subgroups of OlVs were distinguished based on their genetic distances and on the inversion of a central 32-kb-long DNA fragment, but overall their genomes displayed a high level of synteny. The two groups did not correspond to proximity of isolation sites, and the phylogenetic distance between these subgroups was higher than the distances observed among viruses infecting O. tauri. Our study demonstrates that viruses originating from very distant sites are able to infect the same algal host strain and can be more diverse than those infecting different species of the same genus. Finally, distinctive features and evolutionary distances between these different viral subgroups does not appear to be linked to biogeography of the viral isolates. Marine eukaryotic phytoplankton virus diversity has yet to be addressed, and more specifically, it is unclear whether diversity is connected to geographical distance and whether differential infection and lysis patterns exist among such viruses that infect the same host strain. Here, we assessed the genetic distance of geographically segregated viruses that infect the ubiquitous green microalga Ostreococcus. This study provides the first glimpse into the diversity of predicted gene functions in Ostreococcus viruses originating from distant sites and provides new insights into potential host distributions and restrictions in the world oceans.

Proxies of community production derived from the diel variability of particulate attenuation and backscattering coefficients in the northwest Mediterranean Sea

A 6 yr time series of high frequency inherent optical property (IOP) measurements in the Mediterranean was used to derive information on the diel and seasonal variability of particulate production. Empirical relationships between particulate attenuation (cp), particulate backscattering (bbp), and particulate organic carbon allowed calculation of estimates of net community production (NCP), daytime NCP (NCPd), and gross community production (GCP) from the diel variations in either IOP. Similar seasonal variations and good correlation (r = 0.83, p &lt; 0.001) between daily means of cp and bbp were observed, yet differences in the timing and amplitude of their diel cycles led to significant differences in their derived production metrics. Best agreement was obtained during bloom proliferation (44.1–66.7% shared variance) when all three production estimates were highest, while worst was during bloom decline. Best overall correlation was found for NCPd. Accordingly, only cp‐derived estimates reproduced predicted seasonal variations in community production and seasonality of “traditional” chlorophyll‐based primary production models. Analysis of the diel cycles of “real‐time” net community production (NCPh), determined from the first‐derivative of either cp or bbp, revealed ca. fourfold to eightfold lower daytime NCPh, twofold to fivefold lower daily maximum NCPh, and twice as much intraseasonal variability relative to the mean amplitude of diel variations for bbp. Although the timing of maximum of cp‐derived NCPh was consistently prior to solar noon, significant seasonal differences in the timing of maximum bbp‐derived NCPh was observed. Particulate backscattering may be used to infer biogeochemical properties, while greater understanding of the diel cycles of bbp is needed before bbp can be used to investigate daily community production.

Seasonal dynamics of active SAR11 ecotypes in the oligotrophic Northwest Mediterranean Sea.

A seven-year oceanographic time series in NW Mediterranean surface waters was combined with pyrosequencing of ribosomal RNA (16S rRNA) and ribosomal RNA gene copies (16S rDNA) to examine the environmental controls on SAR11 ecotype dynamics and potential activity. SAR11 diversity exhibited pronounced seasonal cycles remarkably similar to total bacterial diversity. The timing of diversity maxima was similar across narrow and broad phylogenetic clades and strongly associated with deep winter mixing. Diversity minima were associated with periods of stratification that were low in nutrients and phytoplankton biomass and characterised by intense phosphate limitation (turnover time<5 h). We propose a conceptual framework in which physical mixing of the water column periodically resets SAR11 communities to a high diversity state and the seasonal evolution of phosphate limitation competitively excludes deeper-dwelling ecotypes to promote low diversity states dominated (>80%) by SAR11 Ia. A partial least squares (PLS) regression model was developed that could reliably predict sequence abundances of SAR11 ecotypes (Q2=0.70) from measured environmental variables, of which mixed layer depth was quantitatively the most important. Comparison of clade-level SAR11 rRNA:rDNA signals with leucine incorporation enabled us to partially validate the use of these ratios as an in-situ activity measure. However, temporal trends in the activity of SAR11 ecotypes and their relationship to environmental variables were unclear. The strong and predictable temporal patterns observed in SAR11 sequence abundance was not linked to metabolic activity of different ecotypes at the phylogenetic and temporal resolution of our study.

Temperature effects on asexual reproduction of the scyphozoan Aurelia aurita s.l.: differences between exotic (Baltic and Red seas) and native (Mediterranean Sea) populations

AbstractMassive occurrences of jellyfish can cause direct impacts on the economy, especially on tourism and commercial fisheries. Translocation of jellyfish species by humans has caused damaging blooms in new habitats. Aurelia aurita s.l. has been introduced in many locations around the world. To test the potential success of Au. aurita s.l. in various habitats, scyphistomae from different climatic locations (Mediterranean, Red and Baltic Seas) were cultured individually for 201 days at three temperatures (14, 21 and 28 °C) with the same salinity, food and light. We tested the null hypotheses that there were no differences in survival or asexual reproduction (budding and strobilation) amongst populations [native (Mediterranean) and exotic (Red and Baltic)]. Survival of the three scyphistoma populations did not differ significantly across temperatures; however, the Red Sea group had lower survival at all temperatures than did the other populations. Most individuals strobilated at 14 °C. Red Sea scyphistomae strobilated more quickly than Baltic and Mediterranean Sea scyphistomae and produced the fewest ephyrae, whereas Baltic Sea scyphistomae produced the most. Our results indicate that Au. aurita from the Baltic or Red Seas introduced into the Northwest Mediterranean Sea would potentially persist and successfully asexually reproduce there. A new invader could even have greater asexual production than the local Au. aurita s.l. Establishment of the invaders could increase genetic variation of subsequent generations and increase their adaptability to environmental changes. Our results suggest that introduction of exotic Au. aurita s.l. populations could increase jellyfish blooms in the Mediterranean Sea.

The role of food availability in regulating the feeding dynamics and reproductive cycles of bathyal benthopelagic fish in the northwest Mediterranean slope

Trophic dynamics and reproductive cycles of fish from the middle slope of the northwest Mediterranean Sea were investigated seasonally from ~ 500 to 1000 m depth. Gut contents of seven species were analyzed, and weights of gonads and livers and C:N ratios of muscle were measured. Additionally, food availability was estimated through analyses of densities of potential prey and of indicators of organic matter sources. The trophic structure of the community was a function of the size of predators and of the height in the near‐bottom water column where megafauna fed. We established three trophic groups: benthic feeders, non‐migrator macroplankton feeders, and small‐sized fish, the latter feeding on macroplankton and hyperbenthos. Seasonal changes in fullness (F) were coupled to cycles of secondary production and to the availability of different food sources. The F of benthic feeders was related to total organic carbon percentage in the sediments, to surface production 2 months before the sampling, and to the abundance of benthic and hyperbenthic prey. For plankton consumers, lipids in the sediments and the abundance of Pasiphaea spp. explained trends in F. Temporal trends in gonadosomatic index (GSI) correlated to food derived from the winter bloom of surface production. Also, increased feeding intensity preceded increases in GSI, but different energy use strategies were deduced from the almost speciesspecific relationships between the GSI and different biological indices. Species from the same trophic guild reproduced at different times of the year, pointing to the existence of species‐specific life strategies that enhance reproductive success.

Prasinovirus distribution in the Northwest Mediterranean Sea is affected by the environment and particularly by phosphate availability

Numerous seawater lagoons punctuate the southern coastline of France. Exchanges of seawater between these lagoons and the open sea are limited by narrow channels connecting them. Lagoon salinities vary according to evaporation and to the volume of freshwater arriving from influent streams, whose nutrients also promote the growth of algae. We compared Prasinovirus communities, whose replication is supported by microscopic green algae, in four lagoons and at a coastal sampling site. Using high-throughput sequencing of DNA from a giant virus-specific marker gene, we show that the environmental conditions significantly affect the types of detectable viruses across samples. In spatial comparisons between 5 different sampling sites, higher levels of phosphates, nitrates, nitrites, ammonium and silicates tend to increase viral community richness independently of geographical distances between the sampling sites. Finally, comparisons of Prasinovirus communities at 2 sampling sites over a period of 10 months highlighted seasonal effects and the preponderant nature of phosphate concentrations in constraining viral distribution.

Trophic ecology of Lampanyctus crocodilus on north-west Mediterranean Sea slopes in relation to reproductive cycle and environmental variables.

This study examined the population structure, reproductive cycle and feeding pattern of the lanternfish Lampanyctus crocodilus in the Balearic Basin (north-west Mediterranean Sea) from a depth of 450 to 1800 m and at a seasonal scale. Juveniles were mainly located at shallower depths, but also at deepest stations in autumn, while adults mostly inhabited intermediate depths with their centre of population density (CPD) located at 800-1000 m of depth. The migration of adults to deeper depths was detected in late summer to autumn, probably linked to the occurrence of nepheloid layers at c. 1200 m, which in turn enhances the biomass of the zooplankton prey. The diet was mainly based on euphausiids and mysids, with marked seasonal variations both on the upper (450-800 m) and lower (1000-1800 m), where suprabenthic gammariids and pelagic decapods were also dominant. Stomach fullness increased from winter to autumn on the US, while it had a maximum in spring on the LS, in parallel with high consumption of gelatinous zooplankton, which is probably more available after the phytoplankton bloom in late winter. Reproduction occurred in winter, confirmed by the higher percentage of mature females and high gonadosomatic indices (I(G)) at both depth ranges. Hepatosomatic indices (I(H)) showed an inverse trend to I(G) on the US, except in autumn, and was almost parallel on the LS, probably attributable to the migration of adults, which determined different temporal schemes in energy use and storage for reproduction on the US v. LS. Consistent with the different patterns observed at the two depth ranges, environmental drivers of fullness (i.e. feeding intensity) and I(G) (as a proxy of reproductive cycle) differed on the US and LS. The biomass of mysids and euphausiids was the greatest explanatory variables of fullness on the US and LS, pointing to the increasing feeding intensity when a resource was more available. I(H) also explained fullness, suggesting that greater feeding intensity in pre-reproductive periods enabled energy storage in the liver. I(G) was linked directly (i.e. mysids) or indirectly (i.e. surface primary production recorded 2 months before sampling) to food availability, implying a rapid response to vertical food inputs by deep-sea predators. Also, I(G) in L. crocodilus was related to population density, which suggests aggregations for reproduction. Estimates of L. crocodilus trophic levels, and of other accompanying mesopelagic fishes, indicated that the species feed through a continuum spanning the third trophic level, confirming the key role of mesopelagic fishes in transferring organic carbon between trophic levels. Trophic niche segregation among mesopelagic species was pronounced and non-overlapping groups could be distinguished because of the different vertical distribution and migratory behaviour. The study highlights the important role of the benthic boundary layer in sustaining benthopelagic communities in the deep Mediterranean Sea and the need to study the biology of a species throughout its whole depth range and not just at exploited depths (i.e. fishing grounds).

Seasonal dynamics of free-living tintinnid ciliate communities revealed by environmental sequences from the North-West Mediterranean Sea

The species-rich order Tintinnida is a group of planktonic ciliates ubiquitous in coastal marine waters, which can be well described using molecular estimates of diversity. We studied temporal changes of tintinnid diversity over 1year in a coastal Mediterranean Sea location (Villefranche-sur-Mer, France) at five different depths (5, 25, 50, 100, and 160m) and one additional year at 50m depth by combining denaturing gel gradient electrophoresis (DGGE) community fingerprinting with direct PCR amplification, cloning and sequencing of small subunit (SSU), 5.8S, and large-subunit (LSU) rRNA genes together with the corresponding internal transcribed spacers (ITS). We amplified tintinnid sequences in all samples. All identified phylotypes were closely related to described species, showing that there is a good phylogenetic reference sequence data set allowing accurate estimation of tintinnid diversity in these waters. Tintinnid community composition exhibited marked seasonal changes. Surprisingly, the tintinnid SSU rDNA-based species composition did not show any clear relationship to measured environmental parameters (temperature, salinity, light, phytoplankton biomass). Nonetheless, the comparison of tintinnid community composition using UniFrac revealed three significant clusters of sequences grouping, respectively, samples collected in winter, autumn, and summer, leading to the hypothesis that seasonal effects on tintinnid community composition might be related to biotic parameters. In addition, phylogenetic trees based on the concatenated SSU+LSU rDNA and ITS sequences showed a better resolution than SSU rDNA alone to discriminate closely related species.

Seasonal circulation over the Catalan inner-shelf (northwest Mediterranean Sea)

[1] This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011, when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall, and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.

Investigating frontal variability from repeated glider transects in the Ligurian Current (North West Mediterranean Sea)

Mesoscale variability of the Ligurian Current (North Western Mediterranean Sea) is studied using glider data from a repeated cross-frontal transect. The Ligurian Current is characterized by a complex time variability, with propagation velocities that can be compared to those of the glider. As a consequence, time variability can appear as folded into space variability in the glider data through the mechanism of Doppler smearing. In this paper, we provide a general framework to illustrate the effects of spatial and temporal variability and introduce a simple methodology to at least partially unfold the uncertainty. We capitalize on the fact that the glider repeatedly covers the transect, and we build time series of isopycnal depth at fixed points using nonlinear interpolation in time and spatial smoothing. Estimates of the isopycnal space patterns are obtained at various times, and a function fitting is used when appropriate allowing to describe the frontal time variability through a few descriptive parameters. Results show a mesoscale signal with scales consistent with what is known in the literature, and characterized by a cross-shore oscillation of the front between approximately 15 and 50km offshore. The frontal oscillation is quite smooth during the first four transects with propagation speed significantly lower than the glider (≈8km/day), so that the deconvolution method is robust. Local wind also appears to play a role. During the last two transects, instead, the system undergoes dramatic changes with time scales of the order of a day, so that the deconvolution is more problematic.

Taxonomic composition of the particle‐attached and free‐living bacterial assemblages in the Northwest Mediterranean Sea analyzed by pyrosequencing of the 16S rRNA

Free-living (FL) and particle-attached (PA) bacterial assemblages in the Northwest Mediterranean Sea were studied using pyrosequencing data of the 16S rRNA. We have described and compared the richness, the distribution of Operational Taxonomic Units (OTUs) within the two fractions, the spatial distribution, and the taxonomic composition of FL and PA bacterial assemblages. The number of OTUs in the present work was two orders of magnitude higher than in previous studies. Only 25% of the total OTUs were common to both fractions, whereas 49% OTUs were exclusive to the PA fraction and 26% to the FL fraction. The OTUs exclusively present in PA or FL assemblages were very low in abundance (6% of total abundance). Detection of the rare OTUs revealed the larger richness of PA bacteria that was hidden in previous studies. Alpha-Proteobacteria dominated the FL bacterial assemblage and gamma-Proteobacteria dominated the PA fraction. Bacteroidetes were important in the PA fraction mainly at the coast. The high number of sequences in this study detected additional phyla from the PA fraction, such as Actinobacteria, Firmicutes, and Verrucomicrobia.

Disentangling the role of connectivity, environmental filtering, and spatial structure on metacommunity dynamics

Dispersal is a key process in metacommunity dynamics, allowing the maintenance of diversity in complex community networks. Geographic distance is usually used as a surrogate for connectivity implying that communities that are closely located are considered more prone to exchange individuals than distant communities. However, in some natural systems, organisms may be subjected to directional dispersal (air or water flows, particular landscape configuration), possibly leading close communities to be isolated from each other and distant communities to be connected. Using geographic distance as a proxy for realised connectivity may then yield misleading results regarding the role of dispersal in structuring communities in such systems. Here, we quantified the relative importance of flow connectivity, geographic distance, and environmental gradients to explain polychaete metacommunity structure along the coasts of the Gulf of Lions (northwest Mediterranean Sea). Flow connectivity was estimated by Lagrangian particle dispersal simulations. Our results revealed that this metacommunity is strongly structured by the environment at large spatial scales, and that both flow connectivity and geographic distance play an important role within homogeneous environments at smaller spatial scales. We thus strongly advocate for a wider use of connectivity measures, in addition to geographic distance, to study spatial patterns of biological diversity (e.g. distance decay) and to infer the processes behind these patterns at different spatial scales.SynthesisEverything is connected, but connections are seldom accurately quantified. Biological communities are often studied separately, using observations, experiments and models to unravel local dynamics of organisms interacting with each other. However, regional processes such as dispersal through ocean and air circulation, likely to connect distant communities and influence their local dynamics, are not always accounted for, or, at best, used as an homogeneous and distance‐related factor. Ocean models have being extensively developed and validated during the past decades with the increasing availability of accurate meteorological data. Using such model outputs, precise quantifi cation of exchange rates of organisms between communities was performed in a marine Mediterranean coastal area. Jointly with local environmental and biological data, these results were used to quantify the effects of realistic connectivity on local and regional polychaete community structure, and revealed that the environmental gradient, geographic distance, and connectivity were responsible for community structure at different spatial scales.

Proliferation of the toxic dinoflagellate Ostreopsis cf. ovata in relation to depth, biotic substrate and environmental factors in the North West Mediterranean Sea

In recent decades, the North West Mediterranean Sea has been seriously affected by the development of the toxic benthic dinoflagellate Ostreopsis cf. ovata, which is associated with harmful effects on human health and the environment. The present work aims to provide a large overview of the multiple environmental factors assumed to regulate or influence the growth of Ostreopsis. An intensive sampling campaign over a full annual cycle was performed along the French and Italian coasts (in six sites from Cassis to Genoa), to determine patterns of temporal and spatial distributions of both O. cf. ovata epiphytic and planktonic cells. Results highlighted substantial seasonal variations in the abundance of Ostreopsis. These variations correlated to seawater temperature, with an optimum growth temperature ranging from 23°C to 27.5°C. Phosphate concentration, rather than nitrogen or silicate, was also positively associated with Ostreopsis. Decreases in oxygen and increases in chlorophyll a concentrations were recorded during the summer blooming period. The maximal Ostreopsis epiphytic abundance was generally higher on Dictyota spp. than on the other two sampled macroalgae (up to 8.54×106cellsg−1FW), even though statistical analysis did not support a clear substrate preference. Epiphytic abundances were significantly higher at a very shallow depth (0.5m), than at 1 and/or 3m depths. High anthropogenic pressure (related to population density) seems to have promoted the occurrence of blooms in urbanized areas, which could partly explain the strong demarcation in Ostreopsis development between Western and Eastern sampling sites. The ecological niche of Ostreopsis cf. ovata needs precise definition, which will require further in situ and in vitro experimental studies, to determine the relative importance of distinct environmental parameters.

Detecting immigrants in a highly genetically homogeneous spiny lobster population (Palinurus elephas) in the northwest Mediterranean Sea

We investigated the genetic structure of early benthic juveniles of the spiny lobster Palinurus elephas in the northwest Mediterranean Sea by means of ten polymorphic microsatellite markers. Non-metric Multidimensional Scaling coupled with assignment tests were used as a new approach to further delimit a reference population inside a genetically homogeneous pool of individuals and test for the presence of long distance immigrants. From this approach, we found that most early benthic juveniles collected while settling in the northwest Mediterranean Sea originated from a common larval pool. However, 4.2% of the individuals were classified as immigrants from other genetically differentiated populations, with more immigrants in the south than in the north of the sampled basin. Given currents in the northwest Mediterranean Sea and the long pelagic larval phase of P. elephas that lasts several months, this result suggest a restricted homogenized zone in the studied basin with some individuals probably coming from more differentiated populations through the Almeria-Oran Front or the Strait of Sicily.