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Background This paper is based on the studies of the biogeochemical structure of the water column in the anoxic Fjord Hunnbunn (south-eastern Norway) performed in 2009, 2011 and 2012. This Fjord is an enclosed basin of brackish water separated by a narrow and shallow outlet to the sea with a permanently anoxic layer. We show how an oxygenated intrusion could lead to both positive and negative effects on the ecosystem state in Hunnbunn due to a change in the biogeochemical structure. Results During the stratified periods in 2009 and 2012 the anoxic layer amounted to approximately 10% of the total water volume in the Fjord, while dissolved oxygen (DO) was present in 80- 90% of the water. In the autumn of 2011 the water chemistry structure observed in Fjord Hunnbunn was clearly affected by a recent oxygenated intrusion defined by abnormal salinity patterns. This led to a shift of the DO boundary position to shallower depths, resulting in a thicker anoxic layer comprising approximately 40% of the total water volume, with DO present only in approximately 60% of the water. The oxygenated water intrusions led to a twofold decrease of the concentrations of hydrogen sulphide, ammonia, phosphate and silicate in the deep layers with a simultaneous increase of these nutrients and a decrease of the pH level in the surface layers. The concentrations of manganese, iron, and mercury species changed dramatically and in particular revealed a significant supply of iron and methylmercury to the water column. Conclusions Oxic water intrusions into anoxic fjords could lead not only to the flushing of the bottom anoxia, but to a dispersal of sulphidic and low oxygen conditions to the larger bottom area. The elevation of the hydrogen sulphide to the shallower layers (that can be rapidly oxidized) is accompanied by the appearance in the subsurface water of methylmercury, which is easily accumulated by organisms and can be transported to the surrounding waters, affecting the ecosystem over a larger area. [ABSTRACT FROM AUTHOR]

Abstract: The joint analysis of manganese species distributions (dissolved Mn, particulate Mn, and bound Mn) obtained in the Black Sea, the Baltic Sea (Landsort Deep) and Oslo Fjord (Baerumsbassenget), enabled common features to be revealed that demonstrate the similarity in the mechanisms of pelagic redox-layer formation. Our investigations demonstrate that Mn bound in stable complexes with hypothetical P-containing compounds or organic matter (OM) is observed in the redox zones at significant (average: 0.5–1.0μM) concentrations and is likely present as Mn (III), an intermediate product of Mn (II) oxidation. This bound Mn (III) can explain phosphate distribution in redox interfaces and the formation of the so-called “phosphate dipole” with a minimum level above the sulfidic boundary, a maximum just below and a rapid increase in concentration within this range. This dipole structure serves as a geochemical barrier that decreases the upward flux of phosphate from the anoxic layer. Modeling results show that the manganese cycle (formation of settling Mn (IV) particles and presence of dissolved Mn (III)) is the main cause of lack of oxygen and hydrogen sulfide contact. According to the model, the formation of the “phosphate dipole” may also be connected with manganese cycling and different factors (i.e. mixing, particle sinking rate, presence of Mn in the water column) should affect the peculiarities of the observed distributions at the different geographical locations. [Copyright &y& Elsevier]

Submarine tailing disposal (STD) in fjords from land-based mines is common practice in Norway and takes place in other regions worldwide. We synthesize the results of a multidisciplinary programme on environmental impacts of STDs in Norwegian fjords, providing new knowledge that can be applied to assess and mitigate impact of tailing disposal globally, both for submarine and deep-sea activities. Detailed geological seafloor mapping provided data on natural sedimentation to monitor depositional processes on the seafloor. Modelling and analytical techniques were used to assess the behaviour of tailing particles and process-chemicals in the environment, providing novel tools for monitoring. Toxicity tests showed biological impacts on test species due to particulate and chemical exposure. Hypersedimentation mesocosm and field experiments showed a varying response on the benthos, allowing to determine the transition zone in the STD impact area. Recolonisation studies indicate that full community recovery and normalisation of metal leakage rates may take several decades due to bioturbation and slow burial of sulfidic tailings. The results are synthesised to provide guidelines for the development of best available techniques for STDs. • Need for high-resolution hydrodynamics and seafloor geological maps to predict plume and sediment dynamics and slope failure. • Novel methods for the assessment of process chemicals' behaviour and transformation in the environment. • Ecotoxicological effect from process chemicals and its transformed components is assessed. • Importance of transition zone thresholds assessment and mobile epifauna are essential • Recolonisation assessment needs to include recovery of community composition and function. [ABSTRACT FROM AUTHOR]

• To study the biogeochemical consequences of a release of CO 2 in an anoxic marine environment a FABM family set of models consisting of a transport model, biogeochemical model (including carbonate system processes block) and bubble fate (transport and dissolution) was applied. • The measurements performed during a controlled 2-h long CO 2 release experiments show elevated levels of pCO 2 and simultaneously decreased values in pH, that was used for the model validation. • The model analysis of consequences of a CO 2 discharge demonstrates that CO 2 bubbles are dissolved shortly after termination of the leak, while changes in pH, pCO 2 and TIC can be detected for several days after the leak, but only at a limited distance from the source (< 10 m in the examples evaluated here). In this paper we investigate the spatial extent and biogeochemical properties of a known CO 2 plume using the pelagic transport-biogeochemical model BROM (Bottom RedOx Model). The model consists of a biogeochemical module, a 2-dimensional vertical transport module and gas bubble fate module, parameterizing bubbles rising and dissolution according to existing approaches. A controlled CO 2 release experiment was carried out in the Horten Inner Harbor, Norway, in September 2018. This isolated bay is characterized by limited water mixing and anoxia in the bottom layer. CO 2 was released at a water depth of 18 m either in a gas phase or dissolved in seawater at leak rates ranging from 0.1 l/min to 15.8 l/min. The chemical response to the release events relative to background variations was measured using chemical sensors mounted on two seabed templates located 4 m and 15 m from the release point, respectively, and compared to the values predicted by the model. The measurements show elevated levels of pCO 2 and simultaneously decreased values in pH corresponding to the controlled release experiments. The model's simulations were in good agreement with the baseline observations and the measured changes forced by the experimental leak. The model predicts that after a continuous leak of this magnitude in stagnant conditions of anoxic bottom water, a 2–3 weeks long restoration period occurs, after which the disturbances disappear. This work confirms that the footprint of a potential CO 2 leak is localized in the vicinity of the source (tens of meters) where it can be detectable with available chemical sensors. [ABSTRACT FROM AUTHOR]

Underwater disposal of mine tailings in lakes and seas has been considered favorable due to the geochemical stability obtained during long-term storage in anoxic sediments. Sulfides are stable in the ore; however, oxidation and transformation of some substances into more soluble forms may impact bioavailability processes and enhance the risk of toxic effects in the aquatic environment. The goal of this work was to construct a model for simulating the nickel (Ni) cycle in the water column and upper sediments and apply it to the mine tailing sea deposit in the Jøssingfjord, SouthWest Norway. A one-dimensional (1D) benthic–pelagic coupled biogeochemical model, BROM, supplemented with a Ni module specifically developed for the study was used. The model was optimized using field data collected from the fjord. The model predicted that the current high Ni concentrations in the sediment can be a potential source of Ni leaching to the water column until about 2040. The top 10 cm of sediments were classified as being of "poor" environmental state according to the Norwegian Quality Standards. A numerical experiment predicted that with complete cessation of the discharges there would be an improvement in the environmental state of sediment to "good" in about 20 years. On the other hand, doubling of discharge would lead to an increase in the Ni content in the sediment, approaching the boundary of the "very poor" environmental state. The model results demonstrated that Ni leaching from the sea deposits may be increased due to sediment reworking by bioturbation at the sediment–water interface. The model can be an instrument for analysis of different scenarios for mine tailing activities from point of view of reduction of environmental impact as a component of the best available technology. [ABSTRACT FROM AUTHOR]

Sustainable development of the salmon farming industry requires knowledge of the biogeochemical impacts of fish farm emissions. To investigate the spatial and temporal scales of farm impacts on the water column and benthic biogeochemistry, we coupled the C-N-P-Si-O-S-Mn-Fe transformation model BROM with a 2-dimensional benthic-pelagic transport model (2DBP), considering vertical and horizontal transport in the water and upper 5 cm of sediments along a 10 km transect centered on a fish farm. The 2DBP model was forced by hydrophysical model data for the Hardangerfjord in western Norway. Model simulations showed reasonable agreement with field data from the Hardangerfjord in August 2016 (correlations between the model and observations were significant for most variables, and model biases were mostly <35%). The model predicted significant impacts on seafloor biogeochemistry up to 1 km from the fish farm (e.g., increased organic matter in sediments, oxygen depletion in bottom water and sediments, denitrification, metal and sulfur reduction), as well as detectable decreases in oxygen and increases in ammonium, phosphate and organic matter in the surface water near to the fish farm. [ABSTRACT FROM AUTHOR]