The biodegradation of urea by phototrophic and heterotrophic micro organisms and the urea sea water concentration were investigated in relation to the microbial population, inorganic sources of nitrogen and the hydrography of the Elbe estuary and Southern German Bight during three cruises (HGL Cruises) from the end of July to the end of September 1976. During the HGL cruises and a preliminary investigation in Beaumaris Bay and the Menai Strait, no evidence was found to suggest that land derived urea was an important contributor to sea water urea concentration. In situ production seems likely to be a major source of urea. Urea contributes an ecologically significant amount to the ttotal nitrogen' (ammonium, nitrate, nitrite and urea-N) in the Elbe estuary and S. German Bight. Urea-N concentrations in the latter water, although showing greater spacial variation than the inorganic nitrogen, often exceeded the concentrations of the inorganic nitrogen. Urea concentrations were highest during periods of greatest biological activity (Cruise HGL I:July-August) and decreased as the year advanced. Both phototrophic and heterotrophic micro organisms are important in the biodegradation of urea and rates of degradation were highest during the time of highest biological activity (Cruise HGL I: July-August), phototrophic urea degradation predominating in the Elbe estuary while heterotrophic degradation generally predominated in offshore waters. A complex relationship may exist between the urea producers (possibly zooplankton) and urea degraders as well as between the phototrophic and heterotrophic micro organisms. Distinct water regimes (the Elbe, Coastal and North Sea regimes) of different physical characteristics, often separated by frontal regions were encountered on the HGL cruises. The distribution of the micro organisms and sea water urea concentrations were related to the hydrography and these in turn affected the distribution of rates of urea biodegradation. Urea assimilation indices indicated that in the stratified North Sea regime, containing low inorganic nitrogen, bacteria were more effective urea degraders than phytoplankton. Cellular ATP: cellular C ratios may be a useful indicator of physiological activity and environmental stress. Urea assimilation indices and cellular ATP: cellular C ratios may indicate that the micro organisms in the Elbe estuary are under greater environmental stress than those in the more offshore waters of the North sea regime.