According to the EC Water Framework Directive (WFD), the quality of the coastal water type N5 at Helgoland (German Bight, North Sea) was assessed based on several years of quantitative monitoring and revision of historical qualitative data. The British Reduced Species List (RSL)-Index originally introduced by Emma Wells in 2007 was adapted to the local situation and extended by three quantitative parameters linked to water quality: (1) the sublittoral depth limit of selected macroalgae, (2) the spatial extension of dense Fucus beds and (3) the abundance of Ulva lactuca in a selected area. As a completely new approach, we propose to implement a sigmoid population model, describing the temporal development of a population, as the basis for setting boundaries within the classification matrix calculating the ecological quality ratio (EQR). Species richness, the basic ecological parameter in the RSL, was measured once during the main growingseason in 2006 and 2007. Calculation of the RSL-index and the EQR was adapted to the situation at Helgoland by using a local species list derived from historical references and by classifying macroalgae into the required ecological status groups (ESGs) and their state of opportunism proposing clear definitions for these categories. Results indicate a prominent deviation from the historical reference situation. The three additional parameters represent different trends in the reaction pattern to water quality: (1) Thesublittoral depth limits of Laminaria hyperborea and associated red algae increased between 1970 and 2007 and thereby mirrored the increase in water clarity since the period of intensive eutrophication in the 1970s. (2) As a major structuring part of the intertidal, the spatial extent of the main Fucus cover represents an important indicator but it is susceptible to perturbation. (3) Ephemeral algae like Ulva are typical nutrient indicators, but also appear as early successioners. The consistent appearance of tubular Ulva species (Enteromorpha) in areas with regular physical disturbance was thus notregarded as an indicator and omitted from the index. Abundance of Ulva lactuca, however, was incorporated, but has no quantitative historical reference. In the course of the final calculations, the metric EQRs of each parameter were weighted according to their relevance in sufficiently detecting changes in water quality. The median of all parameter-EQRs represented the final EQR for WFD purposes.