The data consist of a spreadsheet containing data for three lake sediment cores collected from Lagoon 3 at Rutland Water. The data were used to support the understanding of phosphate oxygen isotopes in lake sediments with focus on stability and application for assessing palaeo nutrient dynamics. Core 1 was used as a baseline core and analysed immediately after collection, and core 2 and 3 were treated with oxygen enriched water and incubated for 6 months under controlled conditions to then be analysed and track changes in phosphate oxygen isotope over time.