Femern A/S has its own environmental survey vessel. On regular cruises, it is the platform for extensive monitoring programmes to observe birds and is used for analysing the water environment. (photo: DHI)

The water of the belt is stratified, as are the majority of inner waters, with brackish water from the Baltic in the upper layers and saltier water originating from the North Sea in the bottom layers. The salty water in the bottom layers flows into the Baltic Sea where these large influxes of water are determining factors for the environmental conditions. Water quality is determined by both influx from neighbouring waters and local conditions. Oxygen depletion is occasionally observed, but primarily occurs in areas adjacent to the belt.

A bridge over the Fehmarnbelt may influence the hydrodynamics of the belt. Bridge pillars can obstruct water flow and impact currents and natural water exchange between the surface water and bottom water as well as the water exchange with other marine  

areas. Man-made islands, such as those constructed for the ventilation system of a tunnel, may also affect these conditions. In the worst case scenario, these impacts may not only be local, but may have consequences throughout the rest of the Baltic Sea.

The dynamic sea
Local changes in current patterns, stratification and water exchange may influence water quality in the local marine environment. Incidences of oxygen depletion and algal blooms in the nearby Kiel and Mecklenburg bays may either improve or deteriorate. The dilution and dispersion of contaminated surface water at discharges may also be changed.

On a regional scale, we are especially focused on how a fixed link may influence water exchange with the Baltic Sea. If this water exchange is altered, it may affect the horizontal and vertical distribution of salt water in the Baltic Sea. This may in turn change the water quality and living conditions for plants and animals.

Measurements and modelling
The dynamics of sea water are continuously monitored in order for us to obtain the best possible knowledge as a basis for the environmental assessment of the fixed link. Three large monitoring buoys transmit data around the clock, on sea temperature, salt content, currents, algal biomass, etc., directly to a central data centre. Data from ten smaller buoys along the coasts of Lolland and Fehmarn are also transmitted to this centre. Data from these buoys is then used to simulate the dynamics of the sea water using computer models.