Recent research into the behaviour of phosphorus in lake sediments provides a deeper understanding of why some eutrophic lakes struggle to recover, even after nutrient inputs are reduced. Published in Springer Nature, the study led by Emma Polauke, along with Theis Kragh, Jonas Stage Sø, Anna-Marie Klamt, Martin Søndergaard, and Kasper Reitzel (from SDU), examines the mechanisms that control internal phosphorus release in a shallow, hypereutrophic lake in Denmark.

The study, titled “The mobility and release dynamics of sediment phosphorus in a shallow hypereutrophic lake” shows that internal phosphorus loading, where sediments continue to release nutrients into the water column, is a key factor that delays the recovery of lakes affected by eutrophication. The researchers conducted a detailed chemical analysis of surface sediments, combined with whole-lake sonar mapping of basin morphology and sediment hardness, to evaluate how sediments contribute to phosphorus release. Their study focused on changes occurring over a 50-day summer period, comparing the potentially mobile phosphorus in sediments with variations in water column phosphorus concentrations at five sites across the 11-hectare lake.

The study quantified both gross phosphorus release from intact sediment cores and phosphorus settling rates from sediment traps in the epilimnion to calculate net phosphorus flux from sediments into the water. The resulting net flux, estimated at 62 kilograms, closely matched the measured accumulation of phosphorus in the water column, emphasising the reliability of the approach. However, conventional sediment extractions suggested a much higher potential loss of phosphorus (172 kilograms), revealing that much of the phosphorus remains bound in sediment layers and is not immediately mobile.

Interestingly, while 70 percent of the potentially mobile phosphorus pool was redox-sensitive, only 11 percent was lost from the top 10 centimetres of sediment during the study period. Modified chemical extractions further showed that nearly half of the sediment phosphorus was bound in non-oxygen-sensitive iron compounds, such as vivianite. These findings highlight the complex biogeochemistry of sediments and the challenges of accurately predicting internal phosphorus loading.

For lake restoration practitioners and projects like FERRO, these insights emphasise the importance of site-specific assessments of sediment chemistry when planning interventions. Effective restoration strategies must consider not only the total phosphorus present but also the fraction that is truly available for release into the water, in order to design cost-effective and lasting solutions to eutrophication.