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International audience; High-energy tidal beaches are exposed to strong physical forcings. The submarine groundwater discharge (SGD) that occurs in intertidal sandy sediments includes both terrestrial, fresh groundwater flow and seawater recirculation, and plays a significant role in regulating biogeochemical cycles in some coastal zones. In this transition zone between land and sea, complex biogeochemical reactions alter the chemical composition of pore waters that discharge to the coastal ocean. Recent studies highlight that SGD can be a significant source of carbon to the coastal ocean but very few have investigated SGD in high-energy environments. We have characterized the dissolved carbon dynamics in such a high-energy environment (Truc Vert Beach, SW France) through pore water sampling in key compartments of the SGD system. Dissolved organic carbon (DOC), pH, total alkalinity (TA), and the isotopic composition of dissolved inorganic carbon (d 13 C-DIC) were measured in pore waters sampled at regular intervals between 2011 and 2014 in the intertidal zone of the beach, the mixing zone of the subterranean estuary (STE), and the freshwater aquifer upstream from the beach. Results reveal that SGD exports dissolved carbon mostly as DIC to the Aquitaine coast some of which originates from the aerobic respiration of marine organic matter within the beach aquifer. This is highlighted by the opposite spatial trend of DOC, which is consumed, and DIC, which is produced. Saline pore waters expelled from the beach through tidally-driven recirculation of seawater provide about 4400 tons of carbon per year to the coastal zone of the 240 km-long Aquitaine sandy coast. Terrestrial groundwater, characterized by high pCO2 values, is also a significant contributor to the DIC flux to the coastal ocean (16200 tons per year). This flux is abated by CO2 evasion in the upper beach, at the onset of the salinity gradient in the STE, and within the surficial freshwater aquifer along the forest-beach transect below the coastal foredune. Accordingly, the DIC:TA ratio evolves to below 1, suggesting that this SGD increases the buffer capacity of coastal seawater against acidification. This study demonstrates that high-energy beaches are active vectors of DIC from the land to the coastal ocean as well as significant sources of CO2 to the atmosphere, and must therefore be taken into consideration in SGD carbon budgets.

International audience; On the Bay of Biscay continental shelf, there are several mid-shelf mud patches including La Grande Vasière to the north, the West Gironde Mud Patch (WGMP) off the Gironde estuary and the Basque Mud Patch close to the Spanish border. In general, these deposits are several meters thick and cover coarser substrate. Questions remain about their storage capability for fine particles and carbon. This work investigates the sedimentation of the WGMP in order to develop a first estimate of organic carbon (OC) burial. Interface sediment cores were collected at nine stations along two cross-shelf transects in October-November 2016. X-radiograph imaging and grain-size analyses were used to characterize sedimentary structures. 210 Pb xs depth profiles were established to calculate sediment (SAR) and mass (MAR) accumulation rates. Sedimentary structures indicate episodic sandy inputs overlying older deposits at proximal sites, and relatively continuous sedimentation at seaward locations. On the outer-central portion of the northern transect, a maximum SAR (0.47 cm yr − 1) was observed, suggesting a depocenter. On the southern transect, excluding two stations where sedimentary inputs appear massive but sporadic, the SARs are lower (

International audience; We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25m3d−1m−1 longshore. Terrestrial SGD provides a DIN flux of 9·106mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer.

International audience; We have studied the heterogeneity of vertical profiles of biogeochemical components obtained from sediment core slicing. Sediment cores were collected in the intertidal zone of the Arcachon Bay and on the continental slope of the Bay of Biscay, at 550 and 1000m depth. At each station of the Arcachon Bay, five 1m-side zones were delimited at low tide. In each square, 3 cores were collected. Spatial heterogeneity on the continental slope was obtained from the study of 3 cores of multi-corer while 2 or 3 multi-corer deployments were used during the same oceanographic cruise. Similarity between profiles was determined to estimate the spatial heterogeneity at different scales. In the Arcachon Bay, results showed a spatial heterogeneity at a decimetre scale on apparently homogeneous areas. Despite the spatial variability, all the replicated profiles in Arcachon Bay sediments had similar trends, suggesting homogeneous benthic properties. However, a sampling with replicate cores handling is required for seasonal monitoring or study of biogeochemical mechanisms in coastal environments. We observe that the spatial variability on the continental margin of the Bay of Biscay is lower than in coastal environment of the Arcachon Bay. Heterogeneity is equivalent at a decimetre scale and at a hundred meters scale, which is the precision of the sampling positioning due to the drift of the boat and the tilt of the corer cable. In addition, our 10-year-long database for the Bay of Biscay showed that differences in the shape of profiles obtained in sediment cores collected at the same stations throughout this long period may not be interpreted in terms of evolution with time, because the observed variability is as high as the spatial heterogeneity.

Nutrient recycling is highly controlled by benthic macrophytes in shallow coastal zones. In seagrass bed, this recycling mainly occurs in sediment porewater, which is considered to be the main source of nutrients for plant growth. This work investigated the effect of a Zostera noltei meadow on the biogeochemistry of inorganic nutrients and carbon in vegetated sediments of the Arcachon Bay. We characterized the vertical distributions of dissolved inorganic C (DIC), NH 4 + , dissolved inorganic P (DIP) and redox-sensitive particulate inorganic P in bare sediments and in sediments heavily or sparsely colonised by Z . noltei . The obtained concentration profiles were used to calculate benthic production rates or particulate stocks. The porewater nutrient concentrations measured in vegetated sediments were depleted by a factor of 10–100 compared to bare sediments. The concentrations of NH 4 + , DIP and DIC exhibited a strong vertical zonation reflecting successive layers of intense production or consumption within the root zone often considered and sampled as a homogeneous zone. The deeper root zone showed higher consumption rates of dissolved inorganic nutrients compared to the upper root zone as young roots growing deeper in the sediment played a major role in nutrient plant uptake. A consumption of DIC was consistently recorded in the root zone. We hypothesized that this could result from a transport of CO 2 from roots to leaves supporting photosynthetic activity as suggested for freshwater plants. We pointed out to what extent the influence of seagrasses changed between sparse and dense meadows. This work emphasizes the high spatial variability in biogeochemical processes occurring at small scale within the root zone or between dense and sparse seagrass patches. Such a variability should be considered when dealing with the role of seagrass in biogeochemical cycles, especially in the context of global decline of seagrass.

International audience; Several studies highlighted the occurrence of circular pools in intertidal flats of different coastal systems and their transient water chemistry over both tidal and diurnal cycles. However, little is known about (1) the response of benthic biogeochemical reactions and fluxes at the sediment-water interface over such short time scales, and (2) the role of these tidal pools in the biogeochemical functioning of coastal systems. Based on in situ microprofiles and water sampling, we investigated the dynamics of dissolved oxygen (O2), nutrients, sulfides and metals, and the associated fluxes at the sediment-water interface in tidal pools from the Arcachon Bay (Atlantic coast of France). Our integrative approach included several tidal and diurnal cycles over two different seasons in the presence and absence of Zostera noltei. The results show that water temperature and light irradiance were the main factors driving the biogeochemical functioning of the tidal pools, as they controlled the physiological activity of the microphytobenthos. Changes in light radiations induced diurnal fluctuations of O2 concentrations within surficial sediment, thus resulting in fluctuations of the O2 diffusive fluxes at the sediment-water interface and of the O2 penetration depth in sediment. At high tide, the increase in turbulence above the sediment induced the advection of oxygenated water within the first millimeters of sediment, resulting in a significant increase in porewater O2 concentrations and sediment O2 penetration depth. Porewater sulfide concentrations and apparition depth were concomitant with the O2 dynamic over both diurnal and tidal cycles, indicating that intermediate redox diagenetic processes were impacted by O2 dynamic over such short time-scale. The rapid changes in redox processes in the sediment column are confirmed by a significant flux of dissolved manganese toward the water column during nighttime. The consumption of nitrate and the release of ammonium and phosphate, associated to the mineralization of the organic matter in the surface sediment did not appeared related however to such short time cycles. The efflux of dissolved silica from the sediment was most likely associated with the enhanced dissolution of Si-bearing particles in surface sediment at higher temperatures, although silica uptake by Z. noltei was also noted. This study clearly shows that tidal pools function as natural incubators of transient biogeochemical processes. A rough assessment of the nutrient budget at the scale of the bay indicates the tidal pools may contribute significantly to the biogeochemical functioning of Arcachon Bay.

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO2 undersaturation, and DOC reached 82 mg L−1, POC 152 mg L−1, and Chl a 800 μg L−1. Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m−2 year−1, representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.

Wave and tide dominated beaches are composed of medium to coarse sandy sediments depleted in organic matter. In these sediments, the availability of organic matter limits benthic heterotrophic respiration processes. This study shows that oxygen consumption linked with nutrient recycling occurs in sands and that the intensity of this process follows a seasonal pattern. Furthermore, substantial seawater exchange occurs through intertidal sandy sediments during each tide. During floods, seawater penetrates sandy sediments, filling the pore spaces with dissolved oxygen and marine organic matter. Water expelled from the sediment during ebb is depleted in oxygen and enriched in nitrate. In order to quantify the extent of respiration processes, we collected pore waters at low tide on tidal cross-shore transects on the Truc Vert beach (France) every two weeks over 20 months. Temperature, salinity and oxygen saturation were recorded in situ and nutrient concentrations were determined from collected samples. The seasonal monitoring showed that aerobic respiration occurred in sands. Organic matter supplied by the seawater entering the pore spaces during floods was trapped long enough to be mineralized. An efficient recycling of nutrients was thus found to occur in tidal sandy beaches. The intensity of the respiration processes varied seasonally. The strongest oxygen depletions occurred in spring, when planktonic organic matter was abundant, as well as in summer when we observed occasional denitrification. Nitrate concentration of recycled seeping seawater reached values up to 40 μM. According to our calculations, the 240-km-long Aquitanian beach recycles 3500 t of carbon and releases 600 t of N-nitrate to the adjacent coastal zone each year. Since phosphorus is readily adsorbed on particles, the N:P ratio of recycled nutrients released to the seawater was much higher than the Redfield ratio.

The dynamics of the aragonite saturation state (Ωarag) were investigated in the eutrophic coastal waters of Guanabara Bay (RJ-Brazil). Large phytoplankton blooms stimulated by a high nutrient enrichment promoted the production of organic matter with strong uptake of dissolved inorganic carbon (DIC) in surface waters, lowering the concentrations of dissolved carbon dioxide (CO2aq), and increasing the pH, Ωarag and carbonate ion (CO32 −), especially during summer. The increase of Ωarag related to biological activity was also evident comparing the negative relationship between the Ωarag and the apparent utilization of oxygen (AOU), with a very close behavior between the slopes of the linear regression and the Redfield ratio. The lowest values of Ωarag were found at low-buffered waters in regions that receive direct discharges from domestic effluents and polluted rivers, with episodic evidences of corrosive waters (Ωarag < 1). This study showed that the eutrophication controlled the variations of Ωarag in Guanabara Bay.

This work studied the impact of a small estuary (a 4 km-long estuary, at the interface between the Leyre River and the Arcachon lagoon, SW France) with a short flushing time on continental nutrient loads to a coastal lagoon. While large estuaries are known to modify the nutrient load of rivers to the coast, the impact on continental fluxes of a short salinity gradient from small coastal rivers is rarely studied. A survey of nutrient and dissolved organic matter concentration, particulate phosphorus speciation and particulate organic matter (POM) concentration and characteristics (C:N ratio and particulate organic carbon δ 13 C) showed that the estuarine behaviour varied throughout the year. The autochthonous primary production was a sink of dissolved N, P and Si in spring. During this period, the continental load of dissolved nitrogen was reduced by about 1/3 and phosphorus was totally consumed by estuarine processes. Nutrient recycling occurred in summer with ammonium production although this source was low in comparison with continental fluxes of nitrogen. The phosphorus concentration was dominated by the iron oxides-bound fraction. Contrary to large estuaries, the desorption of particulate phosphorus was not a source of dissolved phosphorus. Our results showed that estuarine processes impacted the net nutrient fluxes during productive periods, although estuarine residence time was very short. Passive transport with conservative mixing of nutrient was observed in autumn and winter. The non-conservative behaviour of small estuaries must be taken into account for nutrient mass balance calculation in lagoon.

Sandy sediments of continental shelves and most beaches are often thought of as geochemical deserts because they are usually poor in organic matter and other reactive substances. The present study focuses on analyses of dissolved biogenic compounds of surface seawater and pore waters of Aquitanian coastal beach sediments. To quantitatively assess the biogeochemical reactions, we collected pore waters at low tide on tidal cross-shore transects unaffected by freshwater inputs. We recorded temperature, salinity, oxygen saturation state, and nutrient concentrations. These parameters were compared to the values recorded in the seawater entering the interstitial environment during floods. Cross-shore topography and position of piezometric level at low tide were obtained from kinematics GPS records. Residence time of pore waters was estimated by a tracer approach, using dissolved silica concentration and kinetics estimate of quartz dissolution with seawater. Kinetics parameters were based on dissolved silica concentration monitoring during 20-day incubations of sediment with seawater. We found that seawater that entered the sediment during flood tides remained up to seven tidal cycles within the interstitial environment. Oxygen saturation of seawater was close to 100%, whereas it was as low as 80% in pore waters. Concentrations of dissolved nutrients were higher in pore waters than in seawater. These results suggest that aerobic respiration occurred in the sands. We propose that mineralised organic matter originated from planktonic material that infiltrated the sediment with water during flood tides. Therefore, the sandy tidal sediment of the Aquitanian coast is a biogeochemical reactor that promotes or accelerates remineralisation of coastal pelagic primary production. Mass balance calculations suggest that this single process supplies about 37 kmol of nitrate and 1.9 kmol of dissolved inorganic phosphorus (DIP) to the 250-km long Aquitanian coast during each semi-diurnal tidal cycle. It represents about 1.5% of nitrate and 5% of DIP supplied by the nearest estuary.

Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the sedimentary system located in the Gulf of Cadiz which is under the influence of a strong current, the Mediterranean Outflow Water (MOW). When it comes out from the Strait of Gibraltar, the MOW is either channelled along major or secondary channels, or spills over a sedimentary levee. Frequent earthquakes and the constant current shearing generate widespread sediment deformation and instability of contourite deposits. Secondary channels can form by retrogression following an initial failure. At their mouth, sediment accumulates in the form of small sandy contourite lobes. These observations suggest that the Gulf of Cadiz system shares many similarities with channel-levee complexes formed by turbidity current activity. The main difference is that, in the Gulf of Cadiz, the main process is a strongly flowing saline current which locally interacts with gravity processes.

We present an ecological investigation of live (stained) foraminiferal faunas (>150-μm fraction) from four deep-sea stations sampled in the eastern South Atlantic off Walvis Bay, Namibia. These stations describe a bathymetric transect from ~900 to ~3000-m depth, below the oxygen minimum zone related to the Benguela upweling system. The mineralization of organic detritus, as suggested by oxygen-penetration depths and ΔpH depth profiles, is relatively intense along the overall bathymetrie transect. Organic carbon and amino-acid contents in the surface sediment are high between ~900-2000-m depth (e.g., C org > 1.7%, THAA > 47 nmol/mg), and decrease along the bathymetrie transect. Carbon-14 ages and grain-size distribution determined for the upper sediment suggest a complex balance between erosion, transportation, and deposition at all stations. Bottom nepheloid layers (BNLs) are well-documented in our study area. They are relevant candidates for the dispersion and accumulation of reworked organic compounds along the Namibian slope. Foraminiferal faunas living at the three shallower sites are characterized by species (e.g., Cibicidoides kullembergi, Bulimina inflatalcostata) related to the enhanced productivity. The occurrence of epibiotic species ( Cibicides refulgens, Gavelinopsis translucens, and C kullenbergi ) argues for bottom currents supplying organic compounds in suspension above the sediment-water interface. In those environmental conditions, Reophax bilocularis, R. pilulifer, and Lagenammina difflugiformis contribute significantly to live communities, especially at the lower-slope site. Live foraminiferal faunas are largely restricted to shalow-infaunal oxygenated microhabitats. Finally, we propose that hydro-sedimentary conditions characterized by more or less intermittent near-bottom currents 1) trigger the dominance of arenaceous faunas and 2) preclude the proliferation of a foraminiferal population in deep-infaunal microhabitats.

We report a dataset of the partial pressure of CO2 (pCO2) and methane concentrations (CH4) in the surface waters of Lake Kivu obtained during four cruises covering the two main seasons (rainy and dry). Spatial gradients of surface pCO2 and CH4 concentrations were modest in the main basin. In Kabuno Bay, pCO2 and CH4 concentrations in surface waters were higher, owing to the stronger influence of subaquatic springs from depth. Seasonal variations of pCO2 and CH4 in the main basin of Lake Kivu were strongly driven by deepening of the epilimnion and the resulting entrainment of water characterized by higher pCO2 and CH4 concentrations. Physical and chemical vertical patterns in Kabuno Bay were seasonally stable, owing to a stronger stratification and smaller surface area inducing fetch limitation of wind driven turbulence. A global and regional cross-system comparison of pCO2 and CH4 concentrations in surface waters of lakes highlights the peculiarity of Kabuno Bay in terms of pCO2 values in surface waters. In terms of surface CH4 concentrations, both Kabuno Bay and the main basin of Lake Kivu are at the lower end of values in lakes globally, despite the huge amounts of CH4 and CO2 in the deeper layers of the lake. ispartof: Lake Kivu: Limnology and biogeochemistry of a tropical great lake pages:47-66 ispartof: Aquatic Ecology Series vol:5 pages:47-66 status: published

The Arcachon lagoon is a 156 km 2 temperate mesotidal lagoon dominated by tidal flats (66% of the surface area). The methane (CH 4 ) sources, sinks and fluxes were estimated from water and pore water concentrations, from chamber flux measurements at the sediment–air (low tide), sediment–water and water–air (high tide) interfaces, and from potential oxidation and production rate measurements in sediments. CH 4 concentrations in waters were maximal (500–1000 nmol l −1 ) in river waters and in tidal creeks at low tide, and minimal in the lagoon at high tide ( −1 ). The major CH 4 sources are continental waters and the tidal pumping of sediment pore waters at low tide. Methanogenesis occurred in the tidal flat sediments, in which pore water concentrations were relatively high (2.5–8.0 μmol l −1 ). Nevertheless, the sediment was a minor CH 4 source for the water column and the atmosphere because of a high degree of anaerobic and aerobic CH 4 oxidation in sediments. Atmospheric CH 4 fluxes at high and low tide were low compared to freshwater wetlands. Temperate tidal lagoons appear to be very minor contributor of CH 4 to global atmosphere and to open ocean.