There are many reasons to feel sorry for the Baltic Sea. Decades of eutrophication, pollution and overfishing have left the poor sea, which is actually the second largest brackish water system in the world, in a pretty bad condition. One of the biggest problems is long periods of hypoxia and anoxia (little or no oxygen) below 80 m depth in the central parts.
The Baltic Sea is relatively small and surrounded by land except for a narrow passage where it is connected to the North Sea. Light fresh water from rain and rivers end up on top of the heavier saline water and acts like a lid. Oxygen rich water from the surface is prevented from being mixed into the deeper layers, and Denmark* more or less blocks the supply of oxygenated water from the North Sea. At the same time eutrophication makes the Baltic Sea heaven on earth for algae. They grow like crazy, and when they die and fall down to the bottom they are eaten by oxygen consuming animals and bacteria. Well… Good bye deep water oxygen.
After several years of oxygen free deep water something very rare happened last year. A mild winter with weak winds had resulted in low water levels in the Baltic Sea. Suddenly the winds changed and started pressing in saline, oxygenated water from the North Sea, through Kattegatt into the Baltic Sea. The stormy winter created the largest inflow of oxygenated water into the Baltic Sea in over 50 years and previously anoxic bottoms were re-oxygenated.
The presence or absence of oxygen strongly affect what chemical processes you find in the sediment (more about this in later posts). This water inflow thus gives us “a UNIQUE opportunity!” (the words of Per Hall, passionate professor of biogeochemistry and cruise leader) to investigate what happens to the sediments during and after an oxygenation event. By comparing data from the oxygen free years 2008 and 2010 with data collected during a cruise last summer, this cruise and cruises the coming years, we want to find out more about how the Baltic Sea respond to re-oxygenation. Hopefully our results will help us finding out exactly how sorry we actually should feel for the Baltic Sea.
We have just left the harbor in Gothenburg for a two-week cruise to the Eastern Gotland Basin. Your eyes and ears on the boat, Astrid Hylén, is a marine master student who likes to dig around in mud and poison bacteria. In later posts you will get to know the rest of the scientists, find out why you would want to put 400 kg of metal on the bottom of the Baltic Sea and learn why adhesive tape is a scientist’s best friend…
Best wishes from the ship Skagerak!
*No offense, Denmark, you are a lovely country. I want you to stay exactly where you are.