Description: The ever increasing impact of the marine industry and transport on vulnerable sea areas puts the marine environment under exceptional pressure and calls for inspired methods for mitigating the impact of the related risks. We describe a method for preventive reduction of remote environmental risks caused by the shipping and maritime industry that are transported by surface currents and wind impact to the coasts. This method is based on characterizing systematically the damaging potential of the offshore areas in terms of potential transport to vulnerable regions of an oil spill or other pollution that has occurred in a particular area. The resulting maps of probabilities of pollution to be transported to the nearshore and the time it takes for the pollution to reach the nearshore are used to design environmentally optimized fairways for the Gulf of Finland, Baltic Proper, and south-western Baltic Sea

Description: Hydrodynamic drift modeling was used to investigate the potential dispersion of Mnemiopsis leidyi from the Bornholm Basin in the Baltic Sea where it has been observed since 2007 further to the east and north. In the brackish surface layer dispersion is mainly driven by wind, while within the halocline dispersion is mainly controlled by the baroclinic flow field and bottom topography. Model runs showed that the natural spreading via deep water currents from the Bornholm Basin towards north and east is limited by topographic features and low advection velocities. Based on the information on ranges of salinity and temperature, which limit survival and reproduction of this ctenophore within the Baltic Sea, areas have been identified where the American comb jelly, M. leidyi could potentially survive and reproduce. While, we could show that M. leidyi might survive in vast areas of the northern Baltic Sea its reproduction is prevented by low salinity (〈10 psu) and temperature (〈12°C). Thus, due to the combined effect of low salinity and temperature, it is not probable that M. leidyi could establish permanent populations in the central or northern Baltic Sea. However, it seems that in the southern parts of the Baltic Sea environmental conditions are suitable for a successful reproduction of M. leidyi.

Description: The rapid spread of Mnemiopsis leidyi across the entire Baltic Sea after its first observation in 2006 gave rise to the question of its invasion pathway and the possible vector of its transport. To investigate pathways of M. leidyi invasion, the years 2005–2008 have been simulated by a three-dimensional coupled sea ice-ocean model of the Baltic Sea. In addition, a Lagrangian particle-tracking model has been utilized to test possible transport routes of this invader for 2006/2007. Based on the model, we exclude advection from the Kattegat as the main area of origin of M. leidyi and further spreading through the entire Baltic Sea. To explain the dispersion of M. leidyi in 2007 an earlier invasion already in 2005 is most probable. Alternatively, an invasion originating from main harbors with high ship traffic could also be a potential pathway. Drift simulations with drifter release in the main harbors are in good agreement with the observed distribution pattern of M. leidyi.

Description: The hydrography and dynamics of the Baltic Sea, although ruled by the same principles and forcing factors as any part of the World Ocean, contain several distinguishing features. Apart from the complicated geometry and bathymetry of the basin, two major factors contribute to the complexity of the processes here. The interplay between inflowing saline, dense waters from the North Sea in the bottom layer with the excess of light, and fresh riverine waters coming into the system in the upper layer leads to the formation of a permanent two-layer structure of density separated by a sharp jump layer (halocline). Due to the layered structure, the direct atmospheric forcing is restricted to the upper layer with a typical thickness of 40–80 m, while in the bottom layer advection and mixing processes govern the patterns of the hydrographic fields. On the top of the upper layer, a well-mixed surface layer, with a typical thickness of 15–20 m, is formed due to summer-time heating, whereas at the bottom of this layer a rather sharp jump layer of temperature (thermocline) exists. During autumn the vertical temperature gradient vanishes due to thermal convection and turbulent mixing. There are four mechanisms which induce currents in the Baltic Sea: the wind stress at the sea surface, the surface pressure gradient, the thermohaline horizontal gradient of density and the tidal forces. The currents are steered furthermore by the Coriolis acceleration, topography and friction, forming a general (cyclonic) circulation in this stratified system with positive fresh water budget. Due to the shallowness of the Baltic Sea, bottom friction damps the currents remarkably. Voluminous river runoffs can produce local changes in the sea level height and consequently also in currents. Inflowing waters penetrate at depths where the density of the ambient water matches the inflowing water masses. Due to the small baroclinic Rossby radius (2–10 km), the proper descriptions of mesoscale eddies, fronts and mixing processes need high-resolution modelling.