Introduction 
Prior to the reactor accident at Chernobyl the concentration of ar- 
tificial radio nuclides in the North Sea were mainly influenced by 
the discharges from the nuclear fuel reprocessing plants at La Hague 
(France) and Sellafield Works (Great Britain). In the Baltic Sea, 
these radio nuclides were transported only to a slight extent by 
particular weather conditions and hydrographic conditions, so that 
the radioactive inventory of the Baltic Sea was determined by the 
nuclear weapons fallout of the sixties. 
The Baltic Sea, in comparison to the North Sea, owing to the much 
longer water residence time can be regarded as a sea which is less 
tolerant to environmental pollution. In the further monitoring pro- 
grammes of the DHI, this characteristic is to be taken into account. 
The Chernobyl accident has considerably increased and changed the 
environmental inventory of artificial radio nuclides. For that rea- 
son, it will be necessary - besides the investigation of the possi- 
ble immediate hazardous sources - to provide a survey concerning the 
present situation in the sea, in order to be able to find statements 
about long-term effects. 
In the following Report, the essential results in the first weeks 
and months after the accident are to be presented and interpreted. 
The measurement data determined up to the end of the year 1986 per- 
mit a first evaluation of the effects upon the sea areas of the 
North Sea and the Baltic Sea. 
Survey of the measurements carried out 
The input of the fallout from Chernobyl into the sea took place via 
the atmosphere, whereby a large-scale and time synoptic monitoring 
became necessary. After the accident had become known, the DHI 
therefore took water samples immediately as a precaution at several 
positions in the North Sea and the Baltic Sea, in order to investi- 
gate them as soon as possible for radioactive contamination. Aerosol 
investigations carried out in Hamburg simultaneously provided data 
about the time of the arrival of the radioactive cloud and about its 
nuclide composition. 
The DHI's radiological measurement network, delivered the first in- 
formation about radioactive precipitation into the sea. From this 
moment in time, at different measurement network stations in the 
North Sea and the Baltic Sea, water samples were taken at shorter 
intervals of time, which - in the first instance - were classified 
in order of the severity of the contamination by measuring the total