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International audience; The ecological equilibrium of water reservoirs may differ from that of natural lakes. We questioned this difference by analysing the sediments of a small oligotrophic Alpine lake, whose management was modified for hydroelectric production since 1976. Corne Lake is formed by a shallow depression connected to a deep depression. The hydropower management induced water level fluctuations (þ2 m in summer; À8 m in winter) that emptied the shallow depression during the winter months and promoted the erosion of littoral soils and tributary channel sediment and the sedimentation in the deep depression. The sediment of the original lake was a low-density organic mud. The sediment composition varied according to 3 phases, which chronology is debated. During a first phase we measured an increase in the ratio of Diatom/Chrysophycea and bioavailable P, as well as a decrease in the C/N ratio and bulk radiocarbon age of the sediment, suggesting a trophic surge. A second phase was characterised by a high rate of mineral sedimentation, an increase of benthic diatom genera in the deep depression of the lake and acidophilic diatoms in the shallow depression. In the third phase covering the last upper cm of the cores, the sediment tended to return to its initial composition, but the algae community differed from its initial state. We suggest that the management of Alpine lakes as reservoirs induce long-term ecological changes in relation to water level fluctuations and littoral habitats degradation.

International audience

210Pb and 210Po are naturally occurring radionuclides that are commonly used as a proxy for particle and carbon export. In this study, the distribution of the 210Po/210Pb pair was investigated in the water column of the Barents, Kara and Laptev Seas and the Nansen, Amundsen and Makarov Basins in order to understand the particle dynamics in the Arctic Ocean during the 2007 sea-ice minimum (August–September). Minimum activities of total 210Pb and 210Po were found in the upper and lower haloclines (approx. 60–130 m), which are partly attributed to particle scavenging over the shelves, boundary current transport and subsequent advection of the water with low 210Pb and 210Po activities into the central Arctic. Widespread and substantial (> 50%) deficits of 210Po with respect to 210Pb were detected from surface waters to 200 m on the shelves, but also in the basins. This was particularly important in the Makarov Basin where, despite very low chlorophyll-a levels, estimates of annual new primary production were three times higher than in the Eurasian Basin. In the Nansen, Amundsen and Makarov Basins, estimates of annual new primary production correlated with the deficits of 210Po in the upper 200 m of the water column, suggesting that in situ production and subsequent export of biogenic material were the mechanisms that controlled the removal of 210Po in the central Arctic. Unlike 210Po, 234Th deficits measured during the same expedition were found to be very small and not significant below 25 m in the basins (Cai et al., 2010), which indicates, given the shorter half-life of 234Th, that particle export fluxes in the central Arctic would have been higher before July–August in 2007 than later in the season.

International audience; This study presents the results of the accumulation of Po-210 and Pb-210 in fish tissues and organs in a brackish-water marshland that is characterized by high concentrations of Rn-222 and Ra-226 supplied by submarine groundwater discharge (SGD). Tissues and organs from Cyprinus carpio, Chelon labrosus and Carassius auratus in the wetland were significantly enriched by both Pb-210 and Po-210 (up to 55 and 66 times, respectively) compared to blanks. The major input route of Pb-210 and Po-210 into the fish body seems to be through ingestion, due to the high levels of Pb-210 and Po-210 found in the gut content as well as in organs involved in digestion and metabolism (i.e. gut, kidney and hepatopancreas). Results showed that Po-210 was more accumulated in all fish tissues and organs except for the spine, which showed a higher affinity for Pb-210, due to its capacity to replace Ca from apatite in bones. Over all the variables analyzed, fish tissues/organs and, secondarily, fish species were the most important factors explaining the concentration of radionuclides, whereas fish length and the sampling location played a minor role. The relationship of the two radionuclides varied markedly among tissues and their concentration levels were only correlated in gills, gut and, marginally, in spines. In general, the highest values of Pb-210 and Po-210 concentrations in tissues were found on C. labrosus tissues rather C auratus and C carpio. This study demonstrates that inputs of natural radionuclides supplied by SGD to coastal semi-enclosed areas (such as marshlands, lagoons or ponds) may significantly increase the contents of Pb-210 and Po-210 in fish tissues/organs. Thus, this study represents one of the first evidences of direct ecological effects derived from SGD. (C) 2016 Elsevier Ltd. All rights reserved.

International audience; In marine systems, 210Po and 210Pb disequilibria are being increasingly used to examine oceanic particle formation and export. Here, an updated assessment of current methods for determining 210Po and 210Pb activity in marine samples is provided and includes a complete description of the vast number of calculations and uncertainties associated with Po and Pb loss, decay, and ingrowth during sample processing. First, we summarize the current methods for the determination of 210Po and 210Pb activities in dissolved and particulate seawater samples and recommend areas for improvement. Next, we detail the calculations and associated uncertainties using principles of error propagation, while also accounting for radionuclide ingrowth, decay, and recovery. A spreadsheet reporting these calculations is included as a downloadable Web Appendix. Our analysis provides insight into the contributions of the relative uncertainty for each parameter considered in the calculation of final 210Po and 210Pb activities and gives recommendations on how to obtain the most precise final values. For typical experimental conditions in open seawater, we show that our method allows calculating 210Pb activity with a relative uncertainty of about 7%. However for 210Po activities, the final relative uncertainty is more variable and depends on the 210Po/210Pb activity ratio in the initial sample and the time elapsed between sampling and sample processing. The lowest relative uncertainties on 210Po that can be obtained by this method is 6% and can only be obtained for samples with high 210Po/210Pb activity ratios (>1) that were rapidly processed.

The impact of the earthquake and tsunami on the east coast of Japan on 11 March 2011 caused a loss of power at the Fukushima Dai-ichi nuclear power plant (NPP) that resulted in one of the most important releases of artificial radioactivity into the environment. Although several works were devoted to evaluating the atmospheric dispersion of radionuclides, the impact of the discharges to the ocean has been less investigated. Here we evaluate the distribution of Fukushima-derived 90Sr (n = 57) and 89Sr (n = 19) throughout waters 30–600 km offshore in June 2011. Concentrations of 90Sr and 89Sr in both surface waters and shallow profiles ranged from 0.8 ± 0.2 to 85 ± 3 Bq m−3 and from 19 ± 6 to 265 ± 74 Bq m−3, respectively. Because of its short half-life, all measured 89Sr was due to the accident, while the 90Sr concentrations can be compared to the background levels in the Pacific Ocean of about 1.2 Bq m−3. Fukushima-derived radiostrontium was mainly detected north of Kuroshio Current, as this was acting as a southern boundary for transport. The highest activities were associated with near-shore eddies, and larger inventories were found in the closest stations to Fukushima NPP. The data evidence a major influence of direct liquid discharges of radiostrontium compared to the atmospheric deposition. Existing 137Cs data reported from the same samples allowed us to establish a 90Sr / 137Cs ratio of 0.0256 ± 0.0006 in seawater off Fukushima, being significantly different than that of the global atmospheric fallout (i.e., 0.63) and may be used in future studies to track waters coming from the east coast of Japan. Liquid discharges of 90Sr to the ocean were estimated, resulting in an inventory of 53 ± 1 TBq of 90Sr in the inshore study area in June 2011 and total releases of 90Sr ranging from 90 to 900 TBq, depending upon the reported estimates of 137Cs releases that are considered.

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Using Monte Carlo models, this study investigated how uncertainties in Ra isotope activities and the derived activity ratios (AR) arising from analytical uncertainty and natural variability affect the uncertainty associated with Ra-derived water ages and eddy diffusion coefficients, both of which can be used to calculate SGD. Analytical uncertainties associated with Ra-224, Ra-226, and Ra-228 activities were reported in most published studies to be less than 10% of sample activity; those reported for Ra-223 ranged from 7% to 40%. Relative uncertainty related to natural variability-estimated from the variability in Ra-223 and Ra-224 activities of replicate field samples-ranged from 15% to 50% and was similar for Ra-223 activity, Ra-224 activity, and the Ra-224/Ra-223 AR. Our analysis revealed that AR-based water ages shorter than 3-5 d often have relative uncertainties greater than 100%, potentially limiting their utility. Uncertainties in eddy diffusion coefficients estimated based on cross-shore gradients in short-lived Ra isotope activity were greater when fewer points were used to determine the linear trend, when the coefficient of determination (R-2) was low, and when Ra-224, rather than Ra-223, was used. By exploring the uncertainties associated with Ra-derived water ages and eddy diffusion coefficients, this study will enable researchers to apply these methods more effectively and to reduce uncertainty.