BGC ARGO Floats Measurements in the Baltic Sea – Dynamics of the Dissolved Oxygen Concentration.

Argo floats were designed to explore the deep ocean. The use of Argo in shallow shelf waters until recently seemed impossible. However, the experience of Finnish and Polish oceanographers has shown that the Baltic Sea can also be explored by the Argo floats. The Baltic Sea is an environment difficult to investigate with Argo floats, mainly due to the small size and shallow depths. In addition, a very strong pycnocline hinders the penetration of floats to the bottom. This applies especially to the Baltic Proper region. Strong pycnocline divides the upper layer of local, brackish waters and the lower, much more saline waters of North Sea origin.

These and other problems have been overcome and seven ARGO floats currently work in the Baltic Sea. Two BGC floats work in the Baltic Proper. At the moment these are the simple BGC floats equipped only with oxygen sensors, however we plan to launch fully equipped BGC floats soon. But even these simple floats after 2 years of work gave extremely valuable results.

There are several hypoxic zones in the Baltic Sea. Hypoxia and anoxia affect mainly Baltic deeps, below the pycnocline. In these waters’ oxygen is supplied mainly by advection of water from the North Sea. Unfortunately, the frequency of large, barotropic inflows has dropped in recent decades and oxygen supplied with the inflows is absorbed very quickly. Fortunately, oxygen is also supplied with smaller, more difficult to observe baroclinic inflows.

BGC Argo fulfill an important role in monitoring dissolved oxygen content and dynamics in three depths of the Baltic Proper: Bornholm, Gdansk and Gotland Deep. These depths are located successively farther away from the North Sea, main source of oxygen. Therefore, the dynamics and size of the hypoxic zones are different in each of them. Due to the large amount of data provided online by Argo floats, we can now monitor these processes more precisely and improve modeling of changes in the biogeochemistry of the Baltic Sea.