<p>This study investigated the impacts of eutrophication on a marl lake (Sunbiggin Tarn, United Kingdom) using the geochemical and molecular composition of a 33.5&#xa0;cm radiometrically dated sediment core. The core revealed that there was a period of rapid increase in sedimentation rates with associated enrichment in δ¹⁵N, δ¹³C_org, carbon/nitrogen (C/N) and the flux of photosynthetic pigments. This represented a period of enhanced eutrophication which shifted the ecological composition of algal taxa towards greater abundances of chlorophytes and cryptophytes indicating higher nutrient availability. The higher sedimentation rates and nutrient availability was likely to be driven by the increase in Black-headed Gull (BHGU) population around the lake during the 1980s which resulted in extensive loss of fenland surrounding the lake and increase in open water area. From 1996 to 2005 the gull population declined and the lake showed signs of recovery with overall abundance of photosynthetic pigments declining and water clarity increasing, indicative of reduced nutrient loading. However, it is likely the lake was still in a period of recovery decades after the peak of the gull populations as indicators of nutrient loading continued to decline. This study highlights that even typically resilient marl lakes can be sensitive to eutrophication.</p>