The potential for using benthic algae to identify areas around the lake littoral that are potentially influenced by diffuse inputs of nutrients is considered through research based around Ennerdale Water in NW England. This is an oligotrophic lake with a highly sensitive population of Margalifera margalifera in the outflow river. Water chemistry, algal biomass, diatoms and macroalgae were studied at six points around the perimeter of the lake. Evidence of higher concentrations of dissolved nutrients along with alterations in the diatom and, to a lesser extent, macroalgae assemblages was found in the north-west corner of the lake. Algal biomass gave a more ambiguous signal, with local variations in biomass masked by stronger seasonal signals that determined the quantity of algae present at a site on any given date. These results demonstrate the potential for using benthic algae to generate more focussed data on the condition of a lake than is possible with the whole-lake assessments on which classifications are based.