Bendtsen, J., Gustafsson, K. E., Lehtoranta, J., Saarijarvi, E., Rasmus, K. & Pitkanen, H. 2013: Modeling and tracer release experiment on forced buoyant plume convection from coastal oxygenation. Boreal Env. Res. 18: 37-52. Mechanical oxygenation has been suggested as means of reducing unwanted effects from hypoxia but the near-field dynamics and thereby the potential impact from large scale applications is poorly understood. We present results from a field experiment and model study where the dynamics close to an oxygenator in a shallow coastal area in the Gulf of Finland were studied. The oxygenator created a buoyant plume by pumping surface water through a pipe into the bottom water at a rate of about 1 m(3) s(-1). A passive tracer (rhodamine) was added to the inlet of the pump and its lateral spreading was subsequently observed in a relatively thin layer below the pycnocline. The dispersion of rhodamine was applied to parameterize the entrainment rate in a plume model and the total outflow was increased by a factor of about 7. A sensitivity study with different pump rates and cross flow velocities were analysed.

The relative importance of nutrient inputs from streams and the open sea for the water quality is a key issue for an effective management of eutrophication problems in estuaries. In order to resolve this question we have developed a coupled 3D hydrodynamic–ecological model with numerical nitrogen tracking for Horsens estuary, which is a typical Danish estuary that is shallow and well ventilated with water from the adjacent open sea. The nitrogen tracking technique revealed that nitrogen discharges from local streams accounted for the major part (40%–90%) of the nitrogen in NO3, NH4 and detritus whereas only 15% to 50% of the nitrogen in phytoplankton originated from streams. Effects of nitrogen loading reductions were most pronounced for nutrient concentrations and Chl a concentrations whereas the light condition was only slightly improved. The impact of stream nitrogen reductions on nitrate concentrations, Chl a concentrations and light attenuation was respectively 7, 2 and 3 times higher than the impact of open sea nitrogen reductions. Hence, even in estuaries that are well ventilated with water from the adjacent sea significant environmental improvements can be expected after reduction in local nutrient discharges.

Danish estuaries are characterised by being shallow, eutrophic and with high benthic biomass dominated by suspension feeders. However, the standing stock of benthic fauna is often higher than would be expected from the relation to residence time of sea water or to annual primary production found in other estuaries. The present study investigated how the biomass of benthic suspension feeders was regulated by food supply produced due to eutrophication and supplied to the benthos by vertical mixing. This was tested using a 1D water column model calibrated and validated against monitoring data from a 5years period in the Limfjorden. The model described vertical hydrodynamics and -pelagic biogeochemistry, sediment dynamics and was coupled to a Dynamic Energy Budget model for blue mussels. The modelled biomass of blue mussels showed a dome-shaped relationship to water column stability that was most pronounced combined with high nutrient loadings. At strong stratification, there was little coupling between the upper productive layer and mussels causing food limitation. In addition, hypoxia induced mass mortality of mussels during summer at high nutrient loadings. Under well-mixed conditions, mussels were capable of depleting phytoplankton in the whole water column and food limitation occurred resulting in a lower annual mussel biomass. The optimal conditions for growth of benthic suspension feeders in the long term would therefore be periods of stratification that are long enough to allow the phytoplankton to bloom at the surface layer and short enough to prevent benthic suspension feeders from starving or suffer from hypoxia.

The transition zone between the North Sea and the Baltic Sea is a highly dynamic region where a general estuarine circulation forms a regional scale frontal system from northern Kattegat to the Arkona Sea. This system is characterized by an upper low saline outflowing Baltic water mass from the inflowing saline Skagerrak bottom water to the Kattegat and Belt Sea area. Large and rapid fluctuations of the frontal system are caused by barotropic transports, forced by changing sea level difference between northern Kattegat and the western Baltic Sea, and this results in high variability of the hydrographic conditions and also in frequent in- and outflow events to the Baltic. The dynamics in the region are here analyzed by a regional model of the transition zone, covering the area from the northern Kattegat to the Arkona Sea. The model is validated against water level, temperature and salinity measurements from the region, and the transports through the Danish straits are related to previous estimates and empirical relations. A sensitivity study quantify the role of bathymetry, the tidally induced mixing and the inflowing Skagerrak bottom water for ventilating the bottom water with Skagerrak water or surface water.Furthermore, the dynamics in the region is analyzed with tracers representing the age of the water. The distribution of age tracers with different boundary conditions are analyzed, and the role of advection and mixing for ventilating the bottom water is quantified in terms of the water age. It is shown that the Great Belt area is a very dynamical area where bottom water is ventilated with surface water. The interannual variation of the ventilation of bottom water in the period 2001-2003 is analyzed by various age tracers and related to observed oxygen conditions, and it is shown that the extreme hypoxic event in the autumn 2002 in the southern Kattegat, the Great Belt and in the western Baltic Sea coincide with an unusual low vertical ventilation rate in the Great Belt area, but normal advection rates of bottom water from the northern Kattegat. This indicates that during this particular event, and probably in general, ventilation of bottom water in the Great Belt has significant influence on oxygen conditions in the southern part of the region and for ventilation of bottom waters in the western Baltic Sea. In contrast, the central Kattegat is primarily ventilated by advection of bottom water from the Skagerrak. An age tracer representing the ventilation rate of bottom water with either Skagerrak water or surface water is shown to be inversely correlated to the observed oxygen distribution in the region. (c) 2008 Elsevier B.V. All rights reserved.

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing. transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at Surface and bottom, we quantify the time scales of downward mixing of Surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord. whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations Outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind. it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased Surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus. Our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events. (C) 2007 Elsevier Ltd. All rights reserved.

The Young Sound/Tyrolerfjord system is a 90 km long and 2–7 km wide sill fjord in northeast Greenland, with a mean depth of 100 m. Observations of the bottom topography are presented from different sections of the fjord system, which has a total volume of 40 km3 and a surface area of 390 km2. Hydrographic observations from the summer period show the large influence from the freshwater discharge on the mixed layer depth in the fjord, which, during summer, is less than 5 m, with surface salinity increasing from values below 10 in the inner part of Young Sound to about 30 above the sill in the outer part of the fjord. The deep and intermediate water in the fjord is characterized by a temperature of -1.7 °C and a salinity of 33.1, corresponding to σt < 26.5. The maximum tidal amplitude is 0.8 m and 0.4 m during flood and neap tide, respectively, and is dominated by the lunar semi-diurnal M2 tidal constituent. New model simulations show the evolution of the mixed layer during the summer season. A sensitivity study based on this model is presented, showing that the mixed layer thickness will decrease by about 20% if the runoff is increased by a factor of two, and the implications for the hydrographic conditions in relation to a global warming scenario are discussed.