<p>The oxygen isotope composition of phosphate (δ¹⁸O-PO₄) has been successfully used to study modern biological cycling of phosphorus (P) from the upper few centimetres of lake sediments. However, there is a lack of knowledge regarding the stability and preservation of δ¹⁸O-PO₄ over longer time scales in deeper lake sediments. Three sediment cores were collected from a nutrient-rich lagoon at Rutland Water Nature Reserve to explore P dynamics under controlled conditions, including a baseline (untreated) core and cores stored with oxygen-enriched water at 4 to 7&#xa0;°C for six months. Results of the baseline core suggests that P in the sediment has undergone biological turnover. Additionally, results of the two treated cores using δ¹⁸O-PO₄ from HCl extractable inorganic P pool remained stable, even under altered water oxygen isotope conditions. These finding offer proof of concept for using δ¹⁸O-PO₄ as a tracer of past nutrient inputs and cycling with a range of potential applications in the area of past ecosystem reconstruction.</p>