Tracers of physical and biogeochemical processes, past changes and ongoing anthropogenic impacts: the 43rd International Liege Colloquium on Ocean Dynamics, Liege, Belgium, May 2-6, 2011

The 43rd International Liege Colloquium on Ocean Dynamics (http://modb.oce.ulg.ac.be/colloquium/) gathered a hundred scientists from around the world to discuss new developments and insights related to tracers and proxies (from temperature and salinity to gases and isotopes) with a particular attention on the use of Trace Elements and Isotopes (TEI) as tracers. The colloquium was organized in connection with the Geotraces program (an ongoing international study of the global marine biogeochemical cycles of trace elements and their isotopes, http://www.geotraces.org/) and was the occasion to present the wealth of data collected during large oceanographic expeditions that occurred in connection with the International Polar year. In this framework, particular emphasis was given to the BONUS-GoodHope project, a multi-disciplinary oceanographic cruise that coupled full-depth ocean and atmosphere physical and biogeochemical observations, including trace metals and isotopes (Speich et al. 2013; Speich et al. 2008). The special issue presents a collection of papers dealing with these different thematics

Seasonal hypoxia in the Black Sea north-western shelf. Is there any recovery after eutrophication ?

peer reviewedThe Black Sea North-western shelf (NWS) is a shallow eutrophic area in which seasonal tratification of the water column isolates bottom waters from the atmosphere and prevents entilation to compensate for the large consumption of oxygen, due to respiration in the bottom aters and in the sediments. A 3D coupled physical biogeochemical model is used to investigate he dynamics of bottom hypoxia in the Black Sea NWS at different temporal scales from seasonal o interannual (1981-2009) and to differentiate the driving factors (climatic versus eutrophication) f hypoxic conditions in bottom waters. Model skills are evaluated by comparison with 14500 in- itu oxygen measurements available in the NOAA World Ocean Database and the Black Sea ommission data. The choice of skill metrics and data subselections orientate the validation rocedure towards specific aspects of the oxygen dynamics, and prove the model’s ability to esolve the seasonal cycle and interannual variability of oxygen concentration as well as the patial location of the oxygen depleted waters and the specific threshold of hypoxia. During the eriod 1981-2009, each year exhibits seasonal bottom hypoxia at the end of summer. This henomenon essentially covers the northern part of the NWS, receiving large inputs of nutrients rom the Danube, Dniestr and Dniepr rivers, and extends, during the years of severe hypoxia, owards the Romanian Bay of Constanta. In order to explain the interannual variability of bottom ypoxia and to disentangle its drivers, a statistical model (multiple linear regression) is proposed sing the long time series of model results as input variables. This statis- tical model gives a eneral relationships that links the intensity of hypoxia to eutrophication and climate related variables. The use of four predictors allows to reproduce 78% of hypoxia interannual variability: he annual nitrate discharge (N ), the sea surface temperature in the month preceding tratification (T ), the amount of semi-labile organic matter in the sediments (C) and the duration f the stratification (D). Eutrophication (N ,C) and climate (T ,D) predictors explain a similar mount of variability (∼ 35%) when considered separately. A typical timescale of 9.3 years is found to describe the inertia of sediments in the recovering process after eutrophication. From his analysis, we find that under standard conditions (i.e. average atmospheric conditions, ediments in equi- librium with river discharges), the intensity of hypoxia can be linked to the evel of nitrate discharge through a non-linear equation (power law). Bottom hypoxia does not ffect the whole Black Sea NWS but rather exhibits an important spatial variability. This heterogeneous distribution, in addition to the seasonal fluctuations, complicates the monitoring f ottom hypoxia leading to contradictory conclusions when the interpretation is done from different ets of data. We find that it was the case after 1995 when the recovery process was verestimated due to the use of observations concentrated in areas and months not typically ffected by hypoxia. This stresses out the urging need of a dedicated monitoring effort in the WS f the Black Sea focused on the areas and the period of the year concerned by recurrent hypoxic events.P

Sensitivity of the Oxygen Dynamics in the Black Sea North Western Shelf to physical and biogeochemical processes : 3D model approach

The presentation evidences the role of benthic oxygen consumption in the seasonal hypoxia afecting the Norht western shelf of the Black Sea. Spatial variability of related processes is evidenced and finally interanual variability of hypoxia severity is quantified and related to nitrogen riverine inputs

3.6 Decline of the Black Sea oxygen inventory. In: Copernicus Marine Service Ocean State Report, Issue 2

peer reviewedBENTHO

First year of results from a mooring over a Posidonia Oceanica seagrass meadow (Corsica, France)

peer reviewe

Hydrodynamic variability in the Southern Bight of the North Sea in response to typical atmospheric and tidal regimes. Benefit of using a high resolution model

peer reviewedIn this paper, the hydrodynamics of the Southern Bight of the North Sea (SBNS) and in particular, the Belgian Coastal Zone (BCZ) is investigated on daily to seasonal time scales using a high resolution hydrodynamical model. The Regional Ocean Modeling System (ROMS) is implemented over the SBNS with 5 km resolution and downscaled at 1 km resolution over the BCZ in a two-way nesting configuration run over a three years period (i.e. 2006–2008). The benefit of using a high resolution model over the BCZ is assessed through an extensive comparison of model results with data from satellite and in-situ fixed platforms as well as reference products available for the region. The validation exercise and the results analysis are conducted with a particular focus on hydrodynamic features that are expected to impact the sediment transport. We find that despite the validation procedure does not allow to clearly demonstrate better performance of the high resolution model compared to the coarse resolution model in terms of overtidal circulation, sea surface temperature (SST) and salinity (SSS), the high resolution model resolves additional details in the variability of residual circulation and Scheldt salinity plume dynamics. The analysis of the response of the simulated hydrodynamics to atmospheric regimes for neap and spring tide highlights the major role played by the wind direction on the averaged currents and plume extension. The strongest currents and minimum plume extension are obtained under southwestern winds and neap tide while when northeastern winds prevail, the plume extension is at its maximum and the circulation is the weakest. We show that while neap tides allow the establishment of streamlined circulation, the spring tides induce more turbulent circulation which can favor the retention of transported elements. This latter property could not be resolved with the 5 km resolution model.Face-I