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A drift experiment was conducted from 23 to 29 April 1991 in the central western part of the German Blght as part of the German research program PRISMA. Observed hydrograph~c time series and results from a numerical model are presented. The discussion focuses on the problem of whether the drift buoy used was suitable for marking a 'selected water body' in order to study specific processes. Based upon the data and the model outcome, the experiment had to be divided into 2 phases. During the first phase the drift buoy was situated in denser water with relatively low concentrations of suspended particulate matter (SPM) and phytoplankton. On 26-27 April, a storm event initiated the second phase. The buoy was then forced to deviate from its expected position into a water mass of a more coastal type characterized by slightly h~g h e r temperatures, lower salinity and lower transmission. Concentrations of phytoplankton and SPM increased. The passage of a weak salinity front between 2 stations on 29 April was identified in the data as well as in the simulated salinity distribution.

The meteorological and oceanographic situation during a 6 d drift experiment in the German Bight at the end of April 1991 is evaluated by combining field data, observation network data and results of different numerical models. By combining all available data, it was possible to represent the complex state of the physical situation during the course of the drift experiment and to gain essential information for the interpretation of chemical and biological measurements performed during the drift experiment. This examination shows that the drift experiment can be subdivided into 2 parts. During the first part the situation was dominated by a high pressure system over Scandinavia. This high was associated with weak easterly winds which transported polluted air masses into the region of the experiment. The prevailing easterly winds led to a strong vertical shear in the oceanic circulation between the upper water levels. This shear caused water masses of different origin to coincide within the drift experiment region. During the second phase the region was influenced by a low which led to precipitation and thereby to a washout of contaminants from the atmosphere. Under the influence of the prevailing northerly winds during the second phase, the vertical shear of the oceanic circulation diminished and a cyclonic circulation established itself over the whole water column, as model results showed.

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