Eutrophication is first evidenced as a shift in the range of variability inherent to the receiving ecosystem. Situations judged as extreme in the past tend to become more common, eventually leading to a more drastic or qualitative change in the structure and behavior of the perturbed system. Depending on the properties, buffering capacity, and resilience of the system, its response can be in the form of a gradual trend, but it can also react with unpredictable oscillations. It is difficult to diagnose to which degree the changes recorded in recent years in shelf systems are a result of excessive nutrient input, changing climatic cycles, or simply an effect of improvement in observational coverage. The feedback loops that can buffer or amplify anthropogenic impact are discussed in the context of seasonal cycles. The authors examine some recent trends and reports of unusual phenomena from the Baltic and North Seas that have been attributed to eutrophication. The detrimental effects of eutrophication far outweigh whatever beneficial effects it might have, eg increasing productivity or removal of atmospheric CO2 via burial of organic matter. -from Authors