Description: The use of multiphysics computational fluid dynamics (CFD) approaches to simulate surface–subsurface flow processes is evaluated by comparison with flume experiments on current-exposed permeable bed forms. The unique experimental data include measurements of the time-averaged surface water flow velocities, the pressure distribution at the sediment–water interface, and pore water flow paths. The modeling approach first simulates the time-averaged turbulent flow in the channel with CFD and then uses the predicted pressure distribution at the sediment–water interface to drive a flow and transport model for the sediment. The CFD-modeled velocity and pressure distribution and transient particle tracks within the sediment agree reasonably well with observations. Differences that exist between observations and simulations mainly concern the eddies in the wake zone downstream of the ripple crests that are slightly shorter than those predicted by the model. This deviation propagates from the surface to the subsurface domain, appearing in the pressure distribution along the bed and, consequently, the subsurface flow patterns. The good representation of general patterns and rates makes multiphysics CFD modeling a powerful and sufficiently accurate tool that can replace measurements for many studies of surface–subsurface processes involving current-exposed immobile bed forms. The approach can be used for predicting transport processes where they cannot easily be observed, such as in large rivers and coastal systems where boundary conditions such as mean currents and bed forms can be mapped.

Description: The effect of phototrophic biofilm activity on advective transport of cadmium (Cd), copper (Cu), nickel (Ni), and lead (Pb) in sandy sediments was examined using percolated columns. Cd and Ni in the effluent exhibited clear diel cycles in biofilm-containing columns, with concentrations at the end of dark periods exceeding those during illumination by up to 4.5- and 10-fold for Ni and Cd, respectively. Similar cycles were not observed for Pb or Cu. Breakthrough of the latter metals was greatly retarded and incomplete relative to Cd and Ni, and trends in biofilm treatments did not differ greatly from those in control columns. Inhibition of photosystem II by DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) proved that diel cycles of Cd and Ni were controlled by oxygenic photosynthesis, and microsensor measurements showed that metal cycles closely matched metabolic activity-driven pH variations. The sorption edge pH for the sand/biofilm substrate followed the order Ni 〉 Cd 〉 Cu 〉 Pb, and for Ni and Cd, was within the pH 7–10 range observed in the biofilm-containing column. Adsorption dynamics over the light periods matched pH increases, but desorption during dark periods was incomplete and slower than the rate of change of pH. Over a diel cycle, desorption was less than adsorption, resulting in net binding of dissolved metals due to the biofilm metabolic activity. Extraction with selective reagents indicated that the adsorbed metals were readily exchangeable, and potentially bioavailable. Thus, phototrophic benthic biofilms can control the transport of some metals across the sand–water interface, and processes in this very thin surficial layer should be considered when evaluating chemical fluxes in permeable sediments.

Description: Since about fifteen years the Alfred Wegener Institute | Helmholtz Centre for Polar and Marine Research (AWI) carries out time-series observations in the North Atlantic – Arctic transition. Activities are focused on water mass exchange through Fram Strait and on ecosystem studies at the deep-sea observatory ‘HAUSGARTEN’. HAUSGARTEN is the first and by now only open-ocean long-term observatory in a polar region and a key site of the EMSO-Network. The location in Fram Strait – being the sole deep connection between the North-Atlantic and the Arctic Ocean – is of special interest with respect to the exchange of heat and freshwater between the Arctic and lower latitudes. The Fram Strait ecosystem is expected to be particularly vulnerable to Global Change related variations in environmental conditions, including the ongoing sea-ice retreat. Obtained time series of physical and ecological observations demonstrate the tight connection between abiotic habitat properties and ecosystem characteristics – including the rates and the nature of vertical particle fluxes as well as the composition of pelagic assemblages and benthic communities from microbes to megafauna. These findings clearly advocate for a multidisciplinary and multiscale approach that combines fixed-point and region-wide time-series observations and form the basis of our current proposal for the integrated deep-sea observatory ‘FRAM’ (FRontiers in Arctic marine Monitoring). In addition to the integration of existing physical and ecosystem observation components to fully exploit synergies, strategies for FRAM also include the implementation of novel instruments (e.g., ice-tethered platforms, profiling moorings, benthic crawlers) to extend observation-capacities in space and time – including periods of limited access due to permanent ice-coverage.

Description: Since about fifteen years the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI) conducts time-series observations in the transition zone between the North-Atlantic and the Arctic Ocean. Activities are focused on water mass exchange through Fram Strait and ecosystem studies at the deep-sea observatory HAUSGARTEN. The location in Fram Strait – being the sole deep connection between the North-Atlantic and the Arctic Ocean – is of special interest with respect to the exchange of heat and freshwater between the Arctic and lower latitudes. The Fram Strait ecosystem is expected to be particularly vulnerable to Global Change related variations in environmental conditions, including the progressive sea-ice retreat. Obtained time series of physical and biological observations demonstrate the tight connection between abiotic habitat properties and ecosystem characteristics. These findings clearly advocate for a multidisciplinary and multi-scale approach that combines fixed-point and region-wide time-series observations. There are motivated plans for the extension of the current observations and the transition towards the integrated deep-sea observatory FRAM (FRontiers in Arctic marine Monitoring). In addition to the integration of existing physical and ecosystem observation components to fully exploit synergies, strategies for FRAM also include the implementation of novel instruments (e.g., ice-tethered platforms, profiling moorings, benthic crawlers, biooptical instrumentation) to extend observation-capacities in space and time – including periods of limited access due to permanent ice-coverage. FRAM represents a key site of the European Multidisciplinary Seafloor and Water Column Observatory EMSO, and a member of the Long-Term Ecological Research - Network (LTER). FRAM contributes to the ESFRI projects SIOS (Svalbard Integrated Arctic Earth Observing System) and ICOS (Integrated Carbon Observation System). As part of the Fixed-point Open Ocean Observatory network FixO3 (7th framework EU-program; www.fixo3.eu), the FRAM infrastructure provides coordinated, free-of-charge access to external users under the objective of Transnational Access (TNA).

Description: Integrated Sediment Disturber experiment to assess impact of sediment disturbance on meio- and microfauna communities and biogeochemical processes

Description: In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX (“In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies”, www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of watercolumn oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.