Environmental policy and legislation pursuing the protection of marine ecosystems have commonly considered pollutant content in abiotic matrices. Nevertheless, this approach does not describe the eventual effects on living organisms. The aim of this study was to couple two monitoring tools for environmental diagnostics: the chemical tool, metal concentrations in hard-bottom sediments; and the ecological tool, observations of invertebrate communities inhabiting Lessonia trabeculata holdfasts. The study was conducted during austral summer, in central Chile, in a subtidal ecosystem threatened by a sewage outfall, and was compared with a nearby control zone. Environmental assessment was conducted at three different sites with distances ranging from the outfall to 60 m (there was similar site disposal in control zone). Metal levels in sediments were highest in the site closest to the outfall, decreasing with distance, and were also low in control sites. Changes found in community taxonomic diversity were consistent with the presence of the outfall, especially at 30 m away. The subtidal ecosystem was polluted. It is important to incorporate these multi- and interdisciplinary approaches to environmental research and legislation in order to obtain information that represents effects at different timescales, improves field logistics and truthfully characterises environmental conditions.