developed will be documented, possibilities and limits will be pointed out, examples of the 
monitoring results of the pilot projects will be given (charts, trends, anomalies), and a cost 
estimate for operation of the systems will be presented. 
Contribution by the Institut für Ostseeforschung an der Universität Rostock 
(IOW), Warnemünde 
- Baltic Sea Research Institute at Rostock University - 
Objectives 
The purpose of IOW’s contribution to GOOS is to monitor the water exchange between the 
North Sea and the Baltic Sea and to record and assess the current physical, chemical and 
biological states of the western Baltic and Baltic Proper including its temporal variations. 
Besides, also contaminant levels in the water and sediment are monitored. The investigations 
are accompanied by research aimed at improving concepts, methods, and measuring strategies 
for the determination of variables. 
Present scientific knowledge 
The Baltic Sea is one of the world’s largest brackish seas. It is a highly structured, shallow 
adjacent sea of the Atlantic Ocean, almost completely enclosed by the European continent. 
The temperature and salinity distribution in the Baltic Sea, and hence the density distribution, 
is governed by substantial river runoff due to the Baltic’s geographic position in the transition 
zone between oceanic and continental climates in temperate latitudes. 
On the other hand, the distribution is also influenced by the limited water exchange resulting 
from narrow connecting channels to the open sea. 
Throughout the year, a stratification of the water column exists between the low-salinity 
surface layer receiving freshwater from over 200 rivers and the more saline deepwater with its 
higher density. Since both water types generally also have different temperatures, a so-called 
thermocline separates the less saline, lower-density surface layer from the high-salinity, high- 
density deepwater. 
Vertical circulation is hindered by the stable density stratification, exchange processes being 
largely prevented by the permanent thermocline. The convective and mixing processes 
beginning in autumn and winter due to decreasing sea surface temperatures and rougher seas, 
which are slowed down by ice formation in the northern and eastern areas of the Baltic, do 
not extend below the permanent thermocline. The high-salinity, low-oxygen deepwater is 
largely excluded from the exchange processes taking place in the oxygen-rich surface water. 
Therefore, deepwater renewal is only possible via horizontal inflow, which is hindered by the 
natural limitation of water exchange resulting from the cascade structure of the Baltic basins. 
This accounts for temporarily stagnant conditions in the bottom layer. The biochemical 
decomposition of dead organic material sinking down from the surface water at first causes 
oxygen deficiency and eventually, with continued stagnation, leads to entirely anoxic 
conditions and to the formation of hydrogen sulphide, which does not support marine life. 
Compared to other seas, vertical and horizontal mixing of the Baltic is taking place on a 
reduced scale, also due to the absence of tidal streams. 
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