Description: Scientists of the Institut für Meereskunde at the University of Kiel have for several years been involved in investigations into the role of carbon heterotrophic microorganisms in the food web of the western Baltic. The aim of this work is to obtain information on the transformation of organic material from the primary producers to bacteria and from these to zooplankton and zoobenthos during the annual cycle. The release of phytoplankton exudates was investigated by use of tracer methods and by the uptake of this material by bacteria. lt could be shown that in the Kiel Bight area approximately 15- 30 % of the yearly primary production was transformed to bacterial biomass. In relation to the phytoplankton development the bacteria population exhibits seasonal changes. The growth of aufwuchs was studied and also the sedimentation of algal detritus. During sedimentation a rather high amount of the easily degradable material is remineralized. The remineralization processes are strongly affected by temperature. Laboratory experiments showed that 35 % of the phytoplankton material was remineralized at 20 °C and 3 % at 5 °C per day. The bacterial aufwuchs is a valuable subtrate for grazing organisms like ciliates and rotifers. These processes continue after sedimentation of the detritus and stimulate bacterial activity in the uppermost zone of the ground. The amount of bacterial biomass production influences the development of the meiofauna. In shallow coastal waters microphytobenthos can provide most of the primary carbon production from which about 50 % were transferred to bacterial biomass.