Marine System Analysis Group

The Byfjord is a sill fjord in the Orust and Tjörn fjord system at the Swedish west coast. The Byfjord is approximately 4 kilometres long and 1.5 kilometres wide with a maximum depth of about 50 metres. On the eastern shores of the fjord lies the city of Uddevalla. Bäveån, a river, also discharge here. The river is the largest fresh water source to the fjord. The entrence to the fjord is located in the western end where the sill separates th Byfjord from the Havstensfjord outside. The depth of the sill is approximately 13 metres.

Byfjorden from above. The city of Uddevalla is located in the eastern and innermost part of the fjord. The sill is located in the western part of the fjord and separates the Byfjord from the Havstensfjord. The sill is very shallow and the motorway E6/Sunninge bridge goes across it. Click the image for a larger version.

The Byfjord looking east. Uddevalla is discernable in the innermost part of the fjord. Photo by Lena Viktorsson. Click image for a larger version.

The Byfjord looking to the west. The Sunninge bridge crosses the sill. Photo by Lena Viktorsson. Click the image for a larger version.

The water properties of the Byfjord is largerly determined by the low saline water that flows out from the Baltic Sea. This water flows northwards along the Swedish west coast and is continously mixed with the more saline water of the Skagerak. This creates a strongly stratified system in the fjords along the coast. The halocline of the Byfjord is clearly defined at sill depth, separating the well mixed and low saline surface water from the saline deep water. Due to a shallow sill, stagnant conditions prevail in the Byfjord. Every 2 to 5 years the deep water is renewed. As the deep water do not come in contact with the atmosphere, the oxygen levels fall rapidly. Organic material that originates higher up in the water column, or from land, slowly sink to the deep water where it decay in a oxygen consuming process. The stagnant conditions makes the deep water to become hypoxic, which means that the oxygen concentration is below 2 ml O2 per litre, which is the lower limit for what larger organisms demand to survive. Most of the time, however, the oxygen situation deteriorate even further to such an extent that no oxygen is left in the water. This condition is called anoxic condition, i.e. total lack of oxygen. When this occur the poisonous and smelly substance hydrogen sulfide (H2S) is produced.

Oxygen concentrations in the Byfjord measured at the surface and bottom (30 m). The surface water is always well oxygenated while the deep water is almost chronically in an anoxic state (i.e. total lack of oxygen). Please note that the oxygen concentrations is given as millilitres O2 per litre and that negative values represents the presence of hydrogen sulfide. Click on the image for a larger version.

To be able to continously gather data we have placed instruments at several locations in the fjord. Data is transmitted to us with three hours delay. By doing this we may keep a close eye on what happens in the fjord.

At the small island of Ringburen we have placed a mooring with several instruments at different depths. These instruments measure salinity, temperature and oxygen concentrations at their depth. The measurements are automatically published at this website and you can look at the past three weeks of measurements by clicking here.

Another instrument is located at the sill. This instrument is an acoustic current profiler. It sends out a small sound that is reflected on particles in the water. Due to this the instrument can determine current velocities on different depths. You can find the last measurements here.