Lacaze-Duthiers Canyon Research Articles

Reproductive biology of the two main reef-building cold-water coral species (Desmophyllum pertusum and Madrepora oculata) in the Mediterranean Sea

Although cold-water coral (CWC) biology, physiology, and connectivity, have been extensively studied, few studies have so far focused on their reproductive biology. The reproductive biology of colonies of the reef-building Lophelia pertusa (Linnaeus 1758), now synonymised as Desmophyllum pertusum, and Madrepora oculata (Linnaeus 1758) from the Mediterranean Sea, an area particularly exposed to the effects of anthropogenic pressures, was investigated for the first time using histological techniques. Samples were collected from the Lacaze-Duthiers canyon in the Gulf of Lion, northwestern Mediterranean Sea, where the corals are living at 13 °C, close to their upper limit of thermal tolerance. The gametogenic cycle suggests a seasonal spawning of D. pertusum in autumn to early winter, similarly to population from the Gulf of Mexico. This period corresponds to the formation of storm-induced deep-sea water plumes, likely acting as a time giver for coral spawning and facilitates larval dispersal. Madrepora oculata shows continuous reproduction as already identified in eastern and western North Atlantic, with reproductive features of a species less opportunistic than D. pertusum, consistent with its other biological characteristics. This difference between the two species is of paramount importance to define strategies of conservation.

3D-Representations for studying deep-sea coral habitats in the Lacaze-Duthiers Canyon, from geological settings to individual specimens

The Lacaze-Duthiers Canyon is located in the western Mediterranean Sea and is long known for hosting cold-water coral colonies in the canyon head region at depths ranging from 250 to 550 m. In 2019 during the CALADU cruise, three kinds of 3D-reconstructions were applied to better understand the distribution of coral colonies, their habitat and their skeleton morphologies.The canyon's flanks were mapped using a hull-mounted echosounder and an ROV multibeam echosounder. Digital terrain models were built with resolutions of 5 and 1 m and examined in three dimensions. ROV bathymetric data collected on the canyon's flanks made it possible to highlight a series of sub-parallel structures identified as lithified sedimentary strata along which coral colonies grow.Coral assemblages were explored at four locations and photographic images were assembled using structure from motion techniques to build photogrammetric models. Coral assemblages reconstructed in 3D enabled geo-localizing and recreating coral colonies on 16 models over a total area of 4370 m2. Two colonial species, Madrepora oculata and Desmophyllum pertusum were plotted and reported on bathymetric models to interpret their location at the scale of the canyon. The coordinates and depth of the colonies were used to calculate the vertical distribution (limited to our small bathymetric exploration, between 339 and 214 m depth) and density of populations (up to 4.3 colonies per m2). The spatial coverage of the 16 assemblages measured between 100 and 600 m2 each. The sizes of the colonies were measured to analyze the population structures of both species (mean sizes of 28 cm for D. pertusum and 18 cm for M. oculata, maximum sizes 1 m and 0.5 m, respectively, bushes 2.5 m long). In addition, lost fishing gears were quantified, longlines measured and their densities calculated (0.16 m/m2, up to 0.30 m/m2). An area with exuberant orange colonies of D. pertusum was discovered for the first time in the Lacaze-Duthiers Canyon.Five deep-sea scleractinian species were collected and micro-tomographic scans computed to view their intrinsic skeleton organization. Micro-CT scans of M. oculata, D. pertusum, Desmophyllum dianthus, Caryophyllia smithii, and Dendrophyllia cornigera enabled longitudinal and transversal cuts, highlighting morphological criteria for species identification and the multidirectional examination of specimens. We observed a thin canal connecting calices along the axis of D. pertusum colonies, and separate calices along the axis of M. oculata colonies.

Particulate organic carbon dynamics in the Gulf of Lion shelf (NW Mediterranean) using a coupled hydrodynamic–biogeochemical model

Abstract. The Gulf of Lion shelf (GoL, NW Mediterranean) is one of the most productive areas in the Mediterranean Sea. A 3D coupled hydrodynamic–biogeochemical model is used to study the mechanisms that drive the particulate organic carbon (POC) dynamics over the shelf. A set of observations, including temporal series from a coastal station, remote sensing of surface chlorophyll a, and a glider deployment, is used to validate the distribution of physical and biogeochemical variables from the model. The model reproduces the time and spatial evolution of temperature, chlorophyll a, and nitrate concentrations well and shows a clear annual cycle of gross primary production and respiration. We estimate an annual net primary production of ∼ 200 × 104 t C yr−1 at the scale of the shelf. The primary production is marked by a coast-slope increase with maximal values in the eastern region. Our results show that the primary production is favoured by the inputs of nutrients imported from offshore waters, representing 3 and 15 times the inputs of the Rhône in terms of nitrate and phosphate. In addition, the empirical orthogonal function (EOF) decomposition highlights the role of solar radiation anomalies and continental winds that favour upwellings, and inputs of the Rhône River, in annual changes in the net primary production. Annual POC deposition (27 × 104 t C yr−1) represents 13 % of the net primary production. The delivery of terrestrial POC favours the deposition in front of the Rhône mouth, and the mean cyclonic circulation increases the deposition between 30 and 50 m depth from the Rhône prodelta to the west. Mechanisms responsible for POC export (24 × 104 t C yr−1) to the open sea are discussed. The export off the shelf in the western part, from the Cap de Creus to the Lacaze-Duthiers canyon, represents 37 % of the total POC export. Maximum values are obtained during shelf dense water cascading events and marine winds. Considering surface waters only, the POC is mainly exported in the eastern part of the shelf through shelf waters and Rhône inputs, which spread to the Northern Current during favourable continental wind conditions. The GoL shelf appears as an autotrophic ecosystem with a positive net ecosystem production and as a source of POC for the adjacent NW Mediterranean basin. The undergoing and future increase in temperature and stratification induced by climate change could impact the trophic status of the GoL shelf and the carbon export towards the deep basin. It is crucial to develop models to predict and assess these future evolutions.

Long term monitoring of cold-water coral growth shows response to episodic meteorological events in the NW Mediterranean

Lophelia pertusa and Madrepora oculata, two cosmopolitan coral species of the deep ocean, build reef-like structures that provide ecological niches and nurseries for many species. These cold-water corals (CWCs) are thought to grow slowly making associated communities vulnerable to disturbance. However, in situ growth dynamics remains poorly known. The aim of this study was thus to quantify the growth rates of L. pertusa and M. oculata and their variability in relation to environmental conditions that are expected to influence their physiological functions. Such data can be useful to ground-truth physiological rates and improve the prediction of their sensitivity to global change.As part of a long-term monitoring programme of the Lacaze-Duthiers canyon in the Northwestern Mediterranean Sea, fragments of the two species were repeatedly sampled and redeployed at 530 m depth, from November 2010 to May 2018. Budding rates and linear extensions of L. pertusa and M. oculata were hence quantified and their inter-annual variability documented. The data were analyzed in relation to in situ monitoring of water temperature, current speed and turbidity.This study presents the longest in situ survey of the growth patterns of L. pertusa and M. oculata. The observations revealed a strong inter-annual variability in growth rates, both for L. pertusa and M. oculata, and suggest an influence of hydrological conditions on the growth dynamics. Particularly, as regard to episodic water plumes events such as NW wind-induced dense water shelf cascades and E-SE storm-induced downwellings in autumn-winter. In this study, three types of responses were observed: (i) high budding rates but low colony linear extension for strong water plume events periods (ii) high budding rates and high linear extension associated to organic matter supply during medium intensity water plume events, and (iii) low budding rates, low linear extension and high mortality due to high sedimentation rates in the absence of water plume event. Lophelia pertusa appears to be more resilient to these hydrological changes. Disturbance of the regional circulation patterns, in addition to longer-term change in water mass properties such as warming, deoxygenation and acidification, are thus important parameters to consider for CWC reef stability. This is particularly important since increased storminess, and decreased cascading/downwelling intensities, are predicted to occur within the next century.

Co-occurring nematodes and bacteria in submarine canyon sediments.

In submarine canyon sediments, bacteria and nematodes dominate the benthic biomass and play a key role in nutrient cycling and energy transfer. The diversity of these communities remains, however, poorly studied. This work aims at describing the composition of bacteria and nematode communities in the Lacaze-Duthiers submarine canyon in the north-western Mediterranean Sea. We targeted three sediment depths for two consecutive years and investigated the communities using nuclear markers (18S rRNA and 16S rRNA genes). High throughput sequencing combined to maximal information coefficient (MIC) statistical analysis allowed us to identify, for the first time, at the same small scale, the community structures and the co-occurrence of nematodes and bacteria Operational Taxonomic Units across the sediment cores. The associations detected by MIC revealed marked patterns of co-occurrences between the bacteria and nematodes in the sediment of the canyon and could be linked to the ecological requirements of individual bacteria and nematodes. For the bacterial community, Delta- and Gammaproteobacteria sequences were the most abundant, as seen in some canyons earlier, although Acidobacteria, Actinobacteria and Planctomycetes have been prevalent in other canyon sediments. The 20 identified nematode genera included bacteria feeders as Terschellingia, Eubostrichus, Geomonhystera, Desmoscolex and Leptolaimus. The present study provides new data on the diversity of bacterial and nematodes communities in the Lacaze-Duthiers canyon and further highlights the importance of small-scale sampling for an accurate vision of deep-sea communities.

In situ growth experiments of reef-building cold-water corals: The good, the bad and the ugly

The ecological study of corals in their habitat is essential to determine the effects of global change and to develop strategies for reef conservation. Based on mark and recovery experiments, we investigated skeletal growth patterns of two reef-building cold-water coral species, Lophelia pertusa and Madrepora oculata, in the Lacaze-Duthiers canyon in the northwestern Mediterranean Sea. Coral fragments were collected, stained and deployed for short-term (2.5 months) and long-term (15 months) growth experiments at two sites located 4.5km and 6.8km from the canyon head. The analysis of distinct growth parameters (budding, new polyp growth and linear extension of the coral fragments) revealed that growth patterns are consistent among branches of different sizes, but discrepancies arose from the different types of staining used. Calcein appeared more suitable than alizarin red, which strongly limited growth by delaying coral recovery, for short-term experiments at least. Both species grew rapidly when redeployed in their habitat. Effects of long-term experiments could not be observed because corals were exposed to harmful environmental stress, particularly the lethal effect of sedimentation on the fragments. Despite limited in situ deployment, the growth analysis from the short-term experiment highlighted species-specific responses according to the location along a longitudinal gradient in the canyon and were likely related to the local environmental conditions. Consistent with the observed species distributions, M. oculata showed optimal growth at the site closer to the canyon head compared with L. pertusa, which had optimal growth at the deeper site.In situ experiments are difficult to conduct in deep-sea ecosystems, but with the use of remotely operated vehicles (ROVs), such a simple approach may be of interest to managers of cold-water coral ecosystems.

Megafauna of vulnerable marine ecosystems in French mediterranean submarine canyons: Spatial distribution and anthropogenic impacts

Vulnerable Marine Ecosystems (VME) in the deep Mediterranean Sea have been identified by the General Fisheries Commission for the Mediterranean as consisting of communities of Scleractinia (Lophelia pertusa and Madrepora oculata), Pennatulacea (Funiculina quadrangularis) and Alcyonacea (Isidella elongata). This paper deals with video data recorded in the heads of French Mediterranean canyons. Quantitative observations were extracted from 101 video films recorded during the MEDSEACAN cruise in 2009 (Aamp/Comex). Qualitative information was extracted from four other cruises (two Marum/Comex cruises in 2009 and 2011 and two Ifremer cruises in 1995 and 2010) to support the previous observations in the Cassidaigne and Lacaze-Duthiers canyons. All the species, fishing impacts and litter recognized in the video films recorded from 180 to 700m depth were mapped using GIS. The abundances and distributions of benthic fishing resources (marketable fishes, Aristeidae, Octopodidae), Vulnerable Marine Species, trawling scars and litter of 17 canyons were calculated and compared, as was the open slope between the Stoechades and Toulon canyons. Funiculina quadrangularis was rarely observed, being confined for the most part to the Marti canyon and, I. elongata was abundant in three canyons (Bourcart, Marti, Petit-Rhône). These two cnidarians were encountered in relatively low abundances, and it may be that they have been swept away by repeated trawling. The Lacaze-Duthiers and Cassidaigne canyons comprised the highest densities and largest colony sizes of scleractinian cold-water corals, whose distribution was mapped in detail. These colonies were often seen to be entangled in fishing lines. The alcyonacean Callogorgia verticillata was observed to be highly abundant in the Bourcart canyon and less abundant in several other canyons. This alcyonacean was also severely affected by bottom fishing gears and is proposed as a Vulnerable Marine Species. Our studies on anthropogenic impacts show that seafloor disturbance by benthic fishing is mainly attributable to trawling in the Gulf of Lion and to long lines where rocky substrates are present. The bauxite residue (red mud) expelled in the Cassidaigne canyon was seen to prevent fauna from settling at the bottom of the canyon and it covered much of the flanks. Litter was present in all of the canyons and especially in considerable quantities in the Ligurian Sea, where the heads of the canyons are closer to the coast. Three Marine Protected Areas and one fishing area with restricted access have recently been established and should permit the preservation of these deep ecosystems.

Temporal changes in the growth of two Mediterranean cold-water coral species, in situ and in aquaria

In situ growth patterns of two-species of reef-building cold-water corals were investigated for the first time at different temporal scales, based on the redeployment of coral nubbins in their natural environment. Lophelia pertusa and Madrepora oculata were collected in November 2010 and May 2011 from the Lacaze-Duthiers canyon in the northwestern Mediterranean Sea (520m depth). Three in situ growth experiments were performed from November 2010 to May 2011, May to September 2011 and November 2010 to September 2011. For comparison, aquaria experiments over comparable lengths of time were conducted with coral colonies collected in November 2010. In the canyon, new polyps from M. oculata and L. pertusa displayed similar growth rates in summer (5.8±3.8 and 7.3±1.7mmyr−1 respectively), but M. oculata growed significantly slower during winter/spring than L. pertusa (4.1±1.8 and 8.4±2.7mmyr−1 respectively). Budding rates however (the rate of new polyp addition per mother polyp per year) were similar between both species in winter/spring (45±40% for M. oculata and 48±72% for L. pertusa), but were significantly lower in summer for M. oculata (14±19%) compared to L. pertusa (58±94%). This seasonal difference in the growth between L. pertusa and M. oculata might reflect differences in species-specific physiology (such as reproduction) or feeding strategy, or a higher sensitivity of M. oculata to the variability of food supply in the Lacaze-Duthiers canyon resulting from periodic cascading events. The comparison of in situ and aquaria growth experiments showed no significant differences for budding and new polyp growth rates, which supports the validity of aquaria experiments for these kinds of investigations. However the budding rates observed were consistently lower in coral maintained in aquaria than those in in situ conditions, a finding which is to be considered when extrapolating laboratory based investigation results to the naturally occurring coral ecosystem.

Bathymetrical distribution and size structure of cold-water coral populations in the Cap de Creus and Lacaze-Duthiers canyons (northwestern Mediterranean)

Abstract. Submarine canyons are known as one of the seafloor morphological features where living cold-water coral (CWC) communities develop in the Mediterranean Sea. We investigated the CWC community of the two westernmost submarine canyons of the Gulf of Lions canyon system: the Cap de Creus Canyon (CCC) and Lacaze-Duthiers Canyon (LDC). Coral associations have been studied through video material recorded by means of a manned submersible and a remotely operated vehicle. Video transects have been conducted and analyzed in order to obtain information on (1) coral bathymetric distribution and density patterns, (2) size structure of coral populations, and (3) coral colony position with respect to the substrate. Madrepora oculata was the most abundant CWC in both canyons, while Lophelia pertusa and Dendrophyllia cornigera mostly occurred as isolated colonies or in small patches. An important exception was detected in a vertical cliff in LDC where a large L. pertusa framework was documented. This is the first record of such an extended L. pertusa framework in the Mediterranean Sea. In both canyons coral populations were dominated by medium and large colonies, but the frequent presence of small-sized colonies also indicate active recruitment. The predominant coral orientation (90° and 135°) is probably driven by the current regime as well as by the sediment load transported by the current flows. In general, no clear differences were observed in the abundance and in the size structure of the CWC populations between CCC and LDC, despite large differences in particulate matter between canyons.

A new approach for assessing cold-water coral growthin situusing fluorescent calcein staining

Research on the biology and ecology of cold-water corals (CWCs) is still in its infancy. The growth patterns of CWCs in their natural environments are poorly known. Growth rate investigations on these deep-sea reef builder species are needed to predict recovery times following damage to their ecosystems. This study investigates a new approach for analysing CWC growth rate, suitable for in situ application. Lophelia pertusa and Madrepora oculata (Scleractinian) were collected from the Lacaze-Duthiers canyon in the northwestern Mediterranean Sea (520 m depth), marked and then either redeployed in situ for 6 months, or maintained in aquaria for growth rate comparison at a constant temperature of 13 °C, corresponding to their habitat conditions. Two different types of staining (calcein and manganese) and three different exposure times (30, 60 and 240 min) were tested. The results show that calcein offers rapid incorporation and easy detection, making it particularly suitable for skeletal growth rate investigations compared with other chemical staining. In situ linear polyp growth rates of 7.5 ± 1.2 mm y-1 and 3.5 ± 2.1 mm y-1 were measured in new polyps of L. pertusa and M. oculata , respectively. Those values were significantly higher in young polyps than in older ones, where they decreased to 1.3 ± 1.5 mm y-1 and 1.2 ± 1.2 mm y-1 . Beyond the study of coral reef growth processes, this approach offers a methodological basis for habitat quality assessment which could be used in the management of deep-sea marine protected areas (MPA).

Lateral advection of organic matter in cascading-dominated submarine canyons

In the Gulf of Lions (GoL), dense water overflowing off the shelf occurs seasonally and represents the main mechanism affecting the shelf-slope exchange of particulate organic matter (OM). Most of the dense water export takes place in the south-western GoL and in particular through Cap de Creus (CdC) submarine canyon. Here, benthic instruments were deployed to collect down-canyon particulate fluxes whereas surface sediments were taken after the cascading event along the sediment dispersal system on the shelf, in CdC canyon and in the nearby Lacaze-Duthiers (LD) canyon. The chemical composition of the suspended material and surface sediments were investigated using several proxies including organic and inorganic carbon, total nitrogen, biogenic silica, δ 13C, Δ 14C, and alkaline CuO oxidation products. Thermohaline anomalies and high current speed events were measured in CdC canyon since December 2004 until mid-April 2005 indicating a marked off-shelf export of dense water trough the canyon. During the cascading, mud and relatively coarse shelf and upper canyon sediments were the major component of the mass flux. Conversely, advection of fine material via nepheloid layers dominated down-slope fluxes during pre- and post-cascading. The resulting change in grain-size affected the flux of mineral-bound terrigenous OC, indicating that the down-canyon transport of land-derived OM did not occur as bulk but rather its composition is driven by sediment sorting associated with different transport mechanisms. Both surface sediments and sediment trap samples indicated that CdC canyon is well connected to the GoL terrigenous dispersal system. Conversely, our results suggest an overall limited influence of land-derived OM in LD canyon. In spite of the reduced fluvial nutrient supply, a significant pulsed input of modern marine OM was observed in April 2005 at the end of the cascading period. Both intense mixing and lack of strong water column stratification likely played a key role allowing for both diffusion of nutrients-rich waters into the euphotic zone and efficient vertical sink of marine OC. On its way toward the seafloor, this fresh pool of OC interacted with the dense plume overflowing off the shelf, becoming part of the material laterally advected to the slope.

Sediment delivery, resuspension, and transport in two contrasting canyon environments in the southwest Gulf of Lions

Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.

Spatial and temporal variability of downward particle fluxes on a continental slope: Lessons from an 8-yr experiment in the Gulf of Lions (NW Mediterranean)

A long-term experiment of downward particle fluxes and currents has been initiated in 1993 on the continental slope of the Gulf of Lions (NW Mediterranean) and pursued within the frame of several French and European projects (PNEC, Euromarge-NB, MTP II-MATER, EUROSTRATAFORM). Sediment traps and current meters were deployed at several locations on this slope deeply incised by numerous canyons, with an extensive spatial coverage for the first 2 years (canyons at the entrance, middle and exit of the gulf with respect to the general along-slope circulation, head and mid-canyon depths, adjacent open slope). From late 1995 onwards, this design was reduced to the two mid-canyon moorings at the entrance and exit of the gulf. Monthly fluxes and hourly temperatures and currents were recorded at 500 m (30 m above bottom, mab) in the canyon heads, at 500 and 1000 m (respectively 530 and 30 mab) nominal depths at the mid-canyon sites, and at 750 m (30 mab) open slope. This study aims at describing the spatial, seasonal and interannual variability of flux intensity and composition of settling particles, and at analyzing the role of diverse forcings in the control of particle exchange across the margin. Results from the first 8 years (1993–2001) show that total mass fluxes – in the 10 1–10 4 mg m − 2 d − 1 range – increase along slope, particularly for the near-bottom traps, between the NE (Planier Canyon) and the SW (Lacaze-Duthiers Canyon) limits of the Gulf of Lions, indicating an increased shelf export of particulate matter in the western part of the system. Bulk chemical composition (organic matter, carbonate, opal and lithogenic fraction) remained rather stable during the course of the experiment, tending towards values typical of superficial shelf sediments at higher mass fluxes. First-order calculations using a simple two-component mixing model suggest a decreasing contribution of primary particles settling out of the overlying waters to the total flux from the entrance towards the exit of the system. Particulate material transferred to the deeper slope in the southwestern part of the Gulf of Lions appears therefore to predominantly originate in resuspended shelf and/or upper slope sediment. Downward particle fluxes and potential forcing parameters exhibit a high seasonal variability, with higher values from late autumn to early spring. Furthermore, unprecedented winter flux peaks observed in 1999 dominated the interannual differences, which otherwise were quite limited. Correlations between sources of particulate material on the shelf (i.e., river and atmospheric inputs, phytoplankton biomass and sediment resuspension), cross-slope exchange mechanisms (derived from in situ temperature and current records) and flux data indicate a predominant effect of dense cold water cascading on the exchange of particulate matter between the shelf and the slope.

Scavenging rates and particle characteristics in and near the Lacaze-Duthiers submarine canyon, northwest Mediterranean

To better understand the particle exchange processes across the Pyrenean continental margin, sampling was conducted in 1994 and 1995 at stations in and near the Lacaze-Duthiers canyon in the northwestern Mediterranean. Moored current meters provide evidence of water transport along the canyon axis (cross-slope) and along-slope with the prevailing regional circulation. Turbidity measurements show surficial nepheloid layers (SNL) roughly coincident with fluorescence resulting from biological productivity. Recurrent benthic nepheloid layers (BNL) (as observed at a station on the outer-shelf) are advected into the canyon, forming intermediate nepheloid layers (INL). The mineralogical composition of suspended particles collected at stations on the outer-shelf and head of the canyon indicates an illite and chlorite-rich assemblage with minor magnesian calcite, indicative of materials resuspended from the adjacent shelf sediments. The mineralogy of suspended particles at stations farther in the canyon shows the presence of material derived from the Rhone River and likely transported along-slope by the Liguro-Provençal current. Measurements of the short-lived naturally occurring radionuclide 234Th show that it is actively scavenged from the nepheloid layers. Within the canyon, scavenging (as indicated by the mean residence time of dissolved Th with respect to uptake onto particles) is similar in both the SNL (30–58 d) and INL (25–85 d). Residence times of particulate Th with respect to removal from the water column follow the same trends, with values of about 5 d (1994) and 12–19 d (1995) in the SNL and 11–20 d (1994) and 15–23 d (1995) in the INL. This research suggests that while the Lacaze-Duthiers canyon serves as a conduit through which sediment associated with nepheloid layers detached from the shelf are transported, the rates of scavenging of Th in the canyon are not appreciably different from those on the adjacent shelf.

Deep water in the western Mediterranean: peculiar 1999 and 2000 characteristics, shelf formation hypothesis, variability since 1970 and geochemical inferences

Time series of hydrological data acquired in the Ligurian Sea (western Mediterranean) over the 1993–2000 period allowed us to complement the monitoring of the multi-annual increases in temperature and salinity (3.4×10 −3 °C and 1.05×10 −3 per year, mean values at depth greater than 2000 m since 1959). They also revealed peculiarities of dense water characteristics. Following a 7-year period (1993–1998) with potential temperature and salinity continuously decreasing between 1000-m depth and the bottom (∼2700 m), the 1999 and 2000 data evidenced a positive anomaly, i.e., increases of potential temperature and salinity in the last hundred meters above the bottom. Records of currents and temperature on the continental slope of the Gulf of Lions, particularly in the Lacaze-Duthiers canyon, as well as wind and heat fluxes data indicated that cascading of dense water occurred in February 1999. The transport of turbid shelf water and re-suspended sediment might explain the near-bottom turbidity anomaly found concomitantly with the hydrological anomaly. Besides, hydrological data collected during summer 1999 in the Algero-Provençal basin showed a spreading of the anomalous bottom water, radiating from the Gulf of Lions at a mean speed of 3.5 cm s −1 over the whole basin. The analysis of historical data suggested that over the 1971–2000 period, four events of intense cascading of dense water from the Gulf of Lions' shelf (in 1971, 1980, 1988 and 1999) affected the hydrology of the basin and contributed to the formation of western Mediterranean deep water. Though these cascading events contribute little to the overall dense water formation, which mostly occurs off-shelf (MEDOC area), they probably have a rather important role in the transfer of matter.

Trophic control of benthic foraminiferal abundance and microhabitat in the bathyal Gulf of Lions, western Mediterranean Sea

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Origin and variability of downward biogeochemical fluxes on the Rhone continental margin (NW mediterranean)

A one year study of downward particle fluxes conducted in the northwestern Mediterranean Sea is presented. Two mooring lines equipped with sediment traps and current meters were deployed at around 1000 m depth on the northeastern continental slope of the Gulf of Lions, one inside the Grand-Rhône canyon and the other outside on the adjacent open slope. Mean total mass fluxes increased slightly with trap depth inside the canyon, a feature quite typical of fluxes in continental margin environments. The near-bottom trap inside the canyon collected more material than its counterpart deployed at equivalent depth on the open slope, indicating a preferential transport of material within the canyon. Major biogeochemical constituents (organic and inorganic carbon, opal, and siliciclastic residue) revealed a marked difference in particle composition between the sub-surface (80 m) and deeper traps, suggesting the existence of at least two sources of material. The two shallower traps showed a clear biological signal: flux peaks were related to periods of surface biological production, especially perceptible in summer and autumn. The particulate matter trapped at deeper levels in the canyon and on the open slope was characterized by a more stable composition with a major lithogenic contribution, originating from sedimentary material most probably resuspended on the upper- or mid-slope. The seasonal variability was dominated by the summer/winter alternation; the latter period was characterized by a weak stratification of the water column and an enhanced current variability favoring vertical exchanges. The present results are compared with those obtained previously in the Lacaze-Duthiers canyon on the southwestern side of the Gulf of Lions. The comparison shows strong differences between the NE entrance and the SW exit of the gulf, with respect to the general along-slope circulation of water masses, both in terms of intensity of particulate fluxes and transport processes.

Behaviour of suspended and sedimentary organic matter in the deltaic areas of the Gulf of Lions (Mediterranean Sea)

The behaviour of organic matter has been approached in two fluvio-marine areas of the Gulf of Lions in the Northwestern Mediterranean Sea: deltas of the Rhone and Tet rivers. Elemental analysis, pyrolysis-gas chromatography-mass spectrometry (main classes of organic compounds) and high performance liquid chromatography (phenolic compounds) enabled to show the fractioning processes and the differentiation of water layers from the river mouth to the open sea. Transitory horizontal and vertical geochemical gradients appeared in the water column. They were more marked in surficial sediments. They underlined the effects of the river inputs which were characterized by high phenol contents. The increase of the marine character towards the open sea was shown by increasing nitrogen-containing compounds. In the Rhone delta, the distribution of the suspended material and organic compounds emphasized the occurrence of a multi-layered water system. In the surficial layers the suspended organic matter was in a little degraded state. On the contrary, the bottom nepheloid contained altered organo-mineral complexes enriched in resistant organic compounds. In the deeper areas of the Lacaze-Duthiers Canyon, in front of the Tet prodelta, the abundance of phytoplanktonic material in the euphotic zone was emphasized by the increase of both nitrogen-containing compounds and aminosugars. In the deeper layers, the suspended material transported by advective currents showed a detrital characteri.e. enriched in clays and in degraded organic matter. The observations carried out in the Rhone delta, the Tet prodelta and the Lacaze-Duthiers Canyon underlined the importance of the general transfer system of suspended material from East to West enhanced by the Liguro-Provencal current.

Flux of transuranium nuclides and chlorinated hydrocarbons in the northwestern Mediterranean

The transuranium nuclides, plutonium and americium, and selected chlorinated hydrocarbon compounds have been measured in sediment trap samples, bottom sediments and waters. These measurements have been used to quantify vertical fluxes and elucidate particulate biogeochemical cycles, of these man-made contaminants in the northwestern Mediterranean. Sediment trap experiments in the Gulf of Lions (Lacaze-Duthiers Canyon) have shown that transuranic fluxes are considerably higher in the Gulf of Lions, than those which have been reported for the northeast Pacific; principally this is due to the relatively high mass fluxes in this region of the Mediterranean. Water, surface sediment and sediment trap data indicate that 241Am is being transported downward (via particle settling) more rapidly than 239+240 Pu. From direct flux measurement, residence times for 239+240 Pu and 241Am in the upper 300 m were computed to be 2.5 and 0.14 years, respectively. Comparison of 239+240 Pu concentrations in the water column, in 1986, with similar measurements made 5–11 years earlier suggests that fallout input to the Mediterranean has decreased by nearly a factor of 2 during the period 1975–1986. Fluxes of PCB, HCB, γ-HCH and DDT residues varied by as much as an order of magnitude, or more, both temporally and with depth. The average fluxes of most of the compounds in the Lacaze-Duthiers Canyon, during 1985–1986, were quite similar to those measured off the coast of Monaco several years earlier. Generally, the increased chlorinated hydrocarbon fluxes observed with depth were a direct result of the much higher mass fluxes which occurred at depth. Demonstrated differences in PCB concentrations in the settling particles, deposited surface sediments and overlying sediment floc indicate that simple bottom sediment resuspension is not a major contributor to the enhanced PCB flux noted at depth. The PCB concentration in sedimenting particulate material collected from deeper waters, is similar to previously analysed benthic flocs; it is likely to have been advected into the canyon with the nepheloid layer, which is a common oceanographic feature in this region.

Seasonality and composition of particulate fluxes during ECOMARGE—I, western Gulf of Lions

As part of the ECOMARGE program (ECOsystèmes de MARGE continentale), sequential sediment traps were deployed on the continental margin of the Gulf of Lions, in the northwestern Mediterranean Sea. Two sites located in the southwestern part of this region were selected for the first phase of the experiment: inner shelf (bottom depth: 27 m) and continental slope (Lacaze-Duthiers Canyon, bottom depth: 650 m). The canyon was selected as representative of the continental slope, because canyons comprise more than 50% of the slope area. Total mass, organic matter, opal, carbonate and siliciclastic residue fluxes were measured biweekly at 50, 100, 300 and 600 m in the upper part of the Lacaze-Duthiers Canyon (from July 1985 to April 1986) and for longer periods (2 weeks to 2 months) at 10 and 25 m on the shelf (from May 1985 to June 1986). Mass fluxes increased generally with depth, reaching values as high as 20,000 mg m −2 d −1 in the 600 m slope (canyon) trap; this indicated lateral transport of biogenic and abiogenic particulate matter, from local (adjacent shelf and upper slope waters) and distant origin (Rhoˆne River). Shelf-slope particulate transfer attested by the paracontemporaneity of high flux events on the shelf and the slope and by the continuity of the shelf benthic nepheloid layer and intermediate nepheloid layers over the canyon: this rapid and took generally less than 16 days (trap sample resolution time). Particle fluxes and compositions defined a layered system, in which the surface layers< 100m) were essentially characterized by biogenic material (organic matter and opal), and deep layers chiefly characterized by the collection of elastic material (carbonate and siliciclastic material). Significant temporal increases in total mass and constituent fluxes showed two frequencies: a low seasonal frequency, which was related to seasonal variations of the Liguro-Provençal Current and to the winter increase of the Rhoˆne River and other coastal river discharges; variations at higher frequencies, which were related to various impulse events such as summer internal waves, autumn and winter storms and spring nutrient enrichment. The role of the Lacaze-Duthiers Canyon in the particulate transfer across the continental margin is discussed within the particular context of this advective system. Although the importance of local, intra-canyon resuspension of bottom sediments could not be estimated, it seems that the high suspended particulate matter concentrations and particle fluxes observed are essentially related to the fact that this canyon acts as a natural trap which collects particles from the entire Gulf of Lions. Comparison of the flux necessary to sustain the unsupported 210Pb inventory in the canyon sediments, with the mean 210Pb flux measured by traps and of 210Pb-determined mass accumulation rates with total mass fluxes indicate that the canyon partly acts as a modern sediment depocenter of the particles swept into it from the shelf and the slope; also partly as a modern conduit of particles to the deep basin. Several biological mechanisms are probably responsible for the rapid settling of particles onto the slope (within the canyon); these may, thereby, reduce lateral transfer from the shelf to the open ocean and constitute a kind of “biological barrier”.