This dataset comprises an annually interpolated, multi-proxy geochemical and biological record from a sediment core sequence collected from Lagoon 3 (LG3), Rutland Water Nature Reserve, Rutland, UK, spanning 1976 to 2023 CE. LG3 is a shallow (3-4 m) constructed lagoon that has received water exclusively from the final treated effluent of Oakham Water Recycling Centre since 1979, with no other surface inflows or storm overflow inputs. The site therefore constitutes a controlled natural experiment for attributing sedimentary and ecological change to documented changes in wastewater tertiary treatment, specifically the operation of a Natural Filtration System (1991-2015) and the introduction of Chemical Phosphorus Removal (ferric sulphate dosing) in 2001. Stable isotope ratios are given in conventional delta notation, in units of per mil (parts per thousand). The dataset contains 28 variables: bulk organic carbon and nitrogen concentrations (C %, N %) and C/N ratio; bulk stable isotope ratios (δ15N per mil vs AIR, δ13C per mil vs VPDB); four sequentially extracted sedimentary phosphorus fractions (labile-P, microbial-P, NaOH-P, HCl-P; micrograms P per gram dry sediment) and their sum as total P; phosphate oxygen isotope ratios measured on the HCl-extractable (Ca-bound) phosphorus fraction (δ18O-PO4, per mil vs VSMOW); fifteen sedimentary photosynthetic pigments quantified by HPLC (including chlorophyll-a and -b, phaeophorbide-a, phaeophytin-a and -b, canthaxanthin, myxoxanthophyll, lutein and zeaxanthin (co-eluted), fucoxanthin, diatoxanthin, diadinoxanthin, violaxanthin, alloxanthin and crocoxanthin), expressed as nmol per microgram organic matter; and sedimentary microcystin-LR flux (ng per square centimetre per year).
