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Sediment compartment is a long term sink for pollutants and a secondary source of contamination for aquatic species. The abiotic factors controlling the bioavailability and thus the toxicity of complex mixtures of pollutants accumulated in sediments are poorly documented. To highlight the different factors influencing sediment toxicity, we identified and analyzed the physico-chemical properties, micro-pollutant contents, and toxicity level of six contrasted sediments in the Lot-Garonne continuum. Sediment toxicity was evaluated using the recently described Japanese medaka ( Oryzias latipes ) embryo-larval assay with direct exposure to whole sediment (MELAc). Multiple toxicity endpoints including embryotoxicity, developmental defects and DNA damage were analyzed in exposed embryos. Chemical analyses revealed significant variations in the nature and contamination profile of sediments, mainly impacted by metallic trace elements and, unexpectedly, polycyclic aromatic hydrocarbons. Exposure to sediments induced different toxic impacts on medaka early life stages when compared with the reference site. Principal component analysis showed that the toxic responses following exposure to sediments from the Lot River and its tributary were associated with micro-pollutant contamination: biometric measurements, hatching success, genotoxicity, craniofacial deformities and yolk sac malabsorption were specifically correlated to metallic and organic contaminants. Conversely, the main biological responses following exposure to the Garonne River sediments were more likely related to their physico-chemical properties than to their contamination level. Time to hatch, cardiovascular injuries and spinal deformities were correlated to organic matter content, fine particles and dissolved oxygen levels. These results emphasize the necessity of combining physico-chemical analysis of sediment with toxicity assessment to accurately evaluate the environmental risks associated with sediment contamination. [ABSTRACT FROM AUTHOR]

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 1000 m depth. At each station of the Arcachon Bay, five 1 m-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. [ABSTRACT FROM AUTHOR]

We conducted six field campaigns to investigate the spatial and temporal evolution of pore water oxygen content on a high-energy sandy beach aquifer during several tidal cycles at different seasons. We buried autonomous probes in intertidal sands to record dissolved oxygen saturation, salinity, temperature and water head at a 2–10 min frequency. Oxygen concentrations display significant changes both with time (tide and seasons) and space (cross-shore and vertical variations). Seawater circulation in tidal sands forms a saline pore water circulation cell with oxygen-saturated pore water in the zone of seawater recharge. Oxygen-depleted pore water in the lower beach is the result of in situ respiration processes that occur during seawater circulation. Oxygen depletion varies throughout the year and anoxic conditions are reached at the end of spring, as planktonic organic matter becomes abundant in seawater and more organic matter is therefore supplied to pore water. On a shorter time scale (weeks to minutes), oxygen variations are driven directly by physical forcing. Tidal amplitude affects the extent of the saltwater circulation cell and the associated location of the recharging and discharging zones of the beach. The evolution of the water table level during the tidal cycle influences the circulation of pore water in the sand and ultimately, the timing of oxygen variations during flood and ebb. This first in situ study in a high-energy sandy beach shows that the dynamics of pore water oxygen are governed by biogeochemical processes at the seasonal scale and by physical forcing at the time scale of minutes to a few days. [ABSTRACT FROM AUTHOR]

Abstract: 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 δ13C) 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. [Copyright &y& Elsevier]

Abstract: The Arcachon lagoon is a 156 km2 temperate mesotidal lagoon dominated by tidal flats (66% of the surface area). The methane (CH4) 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. CH4 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 (<50 nmol l−1). The major CH4 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 CH4 source for the water column and the atmosphere because of a high degree of anaerobic and aerobic CH4 oxidation in sediments. Atmospheric CH4 fluxes at high and low tide were low compared to freshwater wetlands. Temperate tidal lagoons appear to be very minor contributor of CH4 to global atmosphere and to open ocean. [Copyright &y& Elsevier]

Abstract: 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 37kmol of nitrate and 1.9kmol 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. [Copyright &y& Elsevier]

Abstract: We describe the depth distribution of phosphorus in relation to the distribution of major redox species (dissolved O2, NO3 −, NH4 +, Mn, and Fe, and particulate S, organic C, reactive Mn- and Fe-oxides) in modern sediments of two stations in the Thau Lagoon. Sediments close to the oyster bank zone are enriched in organic carbon and are highly bioturbated, while those outside the bank are less bioturbated and organic carbon levels are lower. In all sites, early diagenesis follows the well-accepted depth sequence of redox reactions of organic matter mineralization. The upper sediments of the station enriched in organic carbon contain high amounts of reactive particulate organic phosphorus that arrives at the sediment surface through biodeposition. Only a part of this phosphorus is released to the bottom water after mineralization, since more than 50% of total P is buried as an authigenic phosphate mineral. In the middle of the lagoon, outside the oyster bank zone, organic matter seems to be much more refractory, but the distribution of the major redox compounds indicates that this organic matter is partially mineralized. A portion of the phosphate and ammonium released during mineralization does not escape the sediment, since the concentration gradient is close to zero between 1 and 25cm depth below the sediment–water interface. Pore water N and P are likely fixed by biological uptake. The characterisation and magnitude of this process require further study. [Copyright &y& Elsevier]

Abstract: Cadmium bioaccumulation in tubificid oligochaetes in relation to metal vertical distribution in sediment and bioturbation intensity was studied during a 56-day experiment with a constant contamination source in the overlying water (20μgL-1). The indoor microcosms simulate a two-compartment biotope with three experimental treatments based on metal exposure and faunal composition: contaminated water column with or without worms and uncontaminated water column with worms. Cadmium bioaccumulation in worms was studied after 7, 14, 21, 28, and 56 days. Bioturbation was analyzed as a functional parameter representative of organisms’ activity and using conservative particulate tracers: luminophores ( and 100–315μm) and microspheres (). The results show no significant effects of cadmium exposure on bioturbation, despite high bioaccumulation levels in worms (50μgg-1 dry wt.), suggesting the existence of detoxification/sequestration processes. [Copyright &y& Elsevier]

Inland aquifers are connected to the ocean through permeable coastal sediments. The mixing zone between sea water and fresh-groundwater forms a so-called subterranean estuary. The dynamics and the shape of this interface influence fluxes of fresh water to the coastal zone. On tidal sandy beaches, tide and waves actions lead to the formation of an intertidal saline circulation cell that complements the structure of the subterranean estuary between sea water and fresh water. The upper beach corresponds to the recharge zone, where large volumes sea water penetrate the sediment at high tide, whereas the lower beach corresponds to the discharge zone of older pore water that remains in the sand for several tidal cycles. The objective of our study was to characterize the shape and the evolution of this saline cell in a high-energy macro-tidal beach, the Truc Vert beach (SW France). In order to assess the distribution of saline, brackish and fresh water in this high-energy beach aquifer, we conducted electrical resistivity tomography (ERT) measurements from the lower beach face to the sand dune in winter 2012–2013. We also measured water table elevation and salinity in three piezometers situated at the base of the dune and in pore waters at low tide on tidal cross-shore transects. Results show that the intertidal saline plume can be identified and localized with the ERT method. The fresh-saline water interface is almost vertical in the upper beach. The saline circulation cell occupies the upper part of the tidal beach over a thickness of 5 m deep. Below this saline cell, a brackish water flows to the ocean: about 30% of fresh water was found in pore water in the lower part of the beach. Shifting of the vertical front between saline and fresh waters of about 2–5 m occurred during the spring–neap tidal cycle. This migration is small relative to the beach size, suggesting that the transition zone is relatively stable. The variations in salinity of the circulation cell suggest that the position of the saline plume is controlled by the spring tide level. Our results differ slightly from studies based on modelling and piezometric measurement made in less exposed micro-tidal sandy beaches. Our data would allow to better constrain numerical models that would enable to quantify water residence time and solute fluxes in subterranean estuaries of high-energy beaches. [ABSTRACT FROM AUTHOR]

Microbial activity measurements and molecular approaches were combined to study temporal variation in nitrogen (N) removal mechanisms from muddy and sandy intertidal sediments (Arcachon Bay, France). Stable isotopes ( 15 NH 4 + / 15 NO 3 - ) were used to measure oxidation of NH 4 + to NO 3 − /NO 2 - and its subsequent reduction to N 2 via denitrification and/or anammox. We found that denitrification mainly fuelled N loss in both sediment types. However in sandy sediments, anammox accounted for relatively higher N 2 production (45%). Nitrification–denitrification (D n ) and nitrification–anammox coupling was observed particularly in muddy sediments. Temporal variations in prokaryotic abundance (Bacteria, Archaea) and of functional groups responsible for nitrification and denitrification were studied by qPCR and for anammoxifiers by cloning. Bacterial density in both sediment types varied between 10 7−8 equivalent cell per g −1 dry weight sediment and were overall more abundant in muddy sediments. Archaeal equivalent cell varied between 0.5 and 6.5% of the total prokaryotes, with extreme values observed in muddy sediment during October and January respectively. Denitrifiers were 1–2 orders higher than ammonium oxidizers and archeal nitrifiers appeared insignificant compared to betaproteobacterial counterpart. Furthermore, nitrifiers exhibited greater fluctuations in sandy sediments compared to muddy ones. Anammoxifiers were mainly restricted to the “ Candidatus Scalindua” group, and were detected in both sediment types and all sampling periods providing an indirect proof on the occurrence of anammox. In muddy sediments, N 2 production presented overall higher rate over sandy sediments and exhibited the same trend as nitrifier abundance. This finding suggests a central role of bacterially-mediated benthic nitrification in supporting dissimilatory processes through generation of NO 3 − /NO 2 and preventing eutrophication in this mesotidal lagoon. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40μmolL−1 h−1, dissolved inorganic carbon (DIC) production of 9–12μmolL−1 h−1, very low NH4 + and Mn2+ concentrations, and constant NO3 − production rates (0.4 - 0.7μmolL−1 h−1) due to coupled ammonification and nitrification. The beginning of anoxic periods (24h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6μmolL−1 h−1 and very fast NO3 − consumption (5.6–6.3μmolL−1 h−1) and NH4 + production (1.4–1.5μmolL−1 h−1). The latter rates were positively correlated to NO3 − concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO3 − concentrations and DIC and NH4 + productions of less than 1.3 and 0.1μmolL−1 h−1, respectively. Mn2+ and CH4 were produced at constant rates (respectively 0.3 and 0.015μmolL−1 h−1) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36h following anoxic to oxic switches) showed rapid NH4 + and Mn2+ decreases to zero (1.6 and 0.8–2μmolL−1 h−1, respectively) and very fast NO3 − production (3μmolL−1 h−1). This NO3 − production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface. [Copyright &y& Elsevier]

Sandy beaches are places of active organic matter mineralization due to water renewal providing organic matter and electron acceptors in the porous and permeable sands. Recycled biogenic compounds are efficiently transferred to the coastal marine environment via wave and tidal-driven advective flows. The biogeochemical processes in beach aquifers were mainly studied in semi enclosed systems with low tidal amplitude, and with a connection to continental aquifers contributing to solute fluxes to the coast from terrestrial groundwater. We present here the study of a pocket beach isolated from terrestrial aquifers with a high tidal amplitude and a medium energy wave regime. In situ measurements, cross-shore profiles and vertical sampling were conducted during several tidal cycles in spring and autumn. Cross-shore transects, obtained at low tide from holes that represent a mixture of the upper 20 cm of the water saturated zone, showed concentration gradients of redox and recycled compounds. Increase in pCO 2 , dissolved phosphate and ammonium concentrations downslope revealed that more products from organic matter mineralization accumulated in the lower beach. The related increase in total alkalinity downslope indicated that the part of anaerobic processes in organic matter oxidation was higher in the lower beach. Concentration and δ13C of dissolved inorganic carbon in pore waters suggested that the carbon mineralized in pore waters came from marine plant debris that were mixed with the sand. Continuous probe records of dissolved oxygen saturation and vertical profiles revealed a tidally-driven dynamics of pore water in the first centimetres of the lower beach aquifer. Ventilation of pore waters corresponded to wave pumping and swash-induced infiltration of seawater in the upper 10–20 cm of sediment. Nutrients and reduced compounds produced through organic matter mineralization remained stored in pore water below the layer disturbed by wave. The flux of these components to seawater is possible when this interface is eroded, for example when wave energy increases after a less energetic period. The low extension of the studied aquifer, typical of pocket beaches, limits the connection with continental groundwater. Both tidally-driven and wave-driven recirculation of seawater allows pocket beaches to be efficient bioreactors for marine organic matter mineralization. As such, they provide the coastal environment with recycled nutrients, and not new nutrients. • SGD and early diagenesis in a high tidal amplitude pocket beach. • We observed a vertical and a horizontal redox gradient in the beach aquifer. • Tidally and wave-driven seawater recirculation promotes organic matter mineralization. • Recycled nutrients and alkalinity fluxes to the coastal ocean. • Labile organic matter originates from marine plant debris. [ABSTRACT FROM AUTHOR]