Studying oxygen isotopes in sediments from Rutland Water Nature Reserve

This project is investigating how the phosphorus content and phosphate oxygen isotope (δ¹⁸O-PO₄) signatures in sediment cores change over time, to establish the value of this proxy for environmental reconstruction research. The research builds on a fellowship project between BGS and Loughborough University with Dr Savannah Worne, and is part of an ENVISION DTP PhD project at Lancaster University. 

The importance of phosphate oxygen isotopes

Normally, the bonds between phosphorus and oxygen in phosphate (PO₄^3-) are very stable and don’t break down easily under typical conditions on Earth. This means that oxygen isotopes within PO₄^3- remain unchanged, unless biological processes are involved. However, certain enzyme-driven reactions, both inside and outside cells, can break these bonds and allow oxygen isotopes to exchange with the surrounding water. This has led to the discovery of a temperature-dependent balance between water and PO₄^3- cycling, which can help scientists better understand how PO₄^3- is processed by living organisms.

Recent advances in analysing δ¹⁸O-PO₄ have made it easier to use them as indicators of biological cycling of inorganic PO₄^3-. Using modern water oxygen isotope (δ¹⁸O-H₂O) data, we can calculate the temperature-dependent equilibrium value for δ¹⁸O-PO₄, which reflects the complete biological turnover of phosphate.   

Applying this method to lake sediments is a new and innovative technique that builds on current soil methodologies and allows for past studies of phosphorus cycling. We expect that the δ¹⁸O-PO₄ value in the sediments will reflect the level of biological processing at the time of deposition, with values moving closer to equilibrium when PO₄^3- is utilised more. To date, there have only been rare applications of δ¹⁸O-PO₄ to lake sediments, with no prior applications to a lake sediment core. In part, this reflects the unknown preservation of the δ¹⁸O-PO₄ signature within the core over time.

Rutland Water

Rutland Water is one of the largest artificial reservoirs in Europe, located in the East Midlands. Spanning approximately 4200 acres, it was constructed in the 1970s to ensure a reliable water supply for the surrounding region. Over the years, the reservoir has evolved into a vital site for drinking water supply, wildlife conservation and recreational activities, drawing nature enthusiasts and visitors alike.  

A key part of the site is the Rutland Water Nature Reserve, which is composed of woods, grassland and meadows as well as eight shallow water lagoons, covering around 1000 hectares. Managed by Anglian Water and the Leicestershire and Rutland Wildlife Trust, this area of Rutland is internationally renowned for its rich biodiversity, with wetlands, woodlands and open waters providing habitats for a variety of wildlife species, including the famous ospreys. Our research aligns directly with the water quality management goals of the site, to ensure the ongoing sustainability of this unique environment.

Sampling and research activities

In collaboration with the Leicestershire and Rutland Wildlife Trust, we collected three sediment cores from a nutrient-rich lagoon in the Rutland Water Nature Reserve to study how phosphorus levels and the PO₄^3- oxygen values in lake sediments change over time.

The first core was cut into thin layers and analysed immediately to give us a baseline of current conditions. The other two cores were stored under different conditions for six months to see how much the phosphorus concentrations and isotope values might change over time. One core was sliced into layers before storage (exposing it to air), while the other was kept intact in its tube, mimicking in-lake preservation conditions. These two cores were treated with isotopically enriched water before storage, with the intention that the isotope label would appear in future data sets if biological activity persisted, even at depth. 

Preliminary discoveries

So far, the analysis of the first core has provided useful baseline results, by identifying four different pools that phosphorus is bound to: bioavailable, microbial, metal-bound and non-labile. The results hint at the varying stability of these phosphorus forms within the sediments.  This analysis also gives us an opportunity to improve our analytical methods.

Findings from the stored cores will be key to our understanding of how phosphorus in sediments behaves and changes over time, offering insights into nutrient cycling at Rutland Water. All of this data will be part of my ongoing PhD thesis.

About the author

Christopher Bengt is a PhD student enrolled at Lancaster University. His PhD is funded through the Envision Doctoral Training Partnership and the BGS University Funding Initiative.