The mobility and release dynamics of sediment phosphorus in a shallow hypereutrophic lake

Internal phosphorus (P) loading often delays or prevents the recovery of eutrophic lakes. Since the biogeochemical composition of surface sediments plays a key role in sediment P release dynamics, detailed sediment chemical analyses are essential for estimating internal loading potential and implementing cost-effective lake management programs. To identify reliable methods for quantifying surface sediments’ contribution to internal P loading, sediment-chemical screening and whole-lake sonar analysis of basin morphology and sediment hardness were conducted in a shallow, 11-ha hypereutrophic lake in Denmark. Over a 50-day summer period, changes in lake water P concentrations were compared with changes in the potentially mobile P content in surface sediments at five sites. Gross sediment P release rates from intact sediment cores and P settling rates from sediment traps placed in the epilimnion were used to calculate the net P flux from the sediments to the lake water. The period’s net P flux was estimated at 62 kg (95% CI: 42.8–77.5), closely matching the calculated P accumulation in the lake water body (55 kg). Conventional sequential sediment P extractions indicated a total loss of 172 kg P. Although 70% of the potentially mobile P pool was redox-sensitive, only 11% was lost from the 0–10 cm sediment layer over the 50 days. Modified extraction procedures revealed that 46% of the sediment P was bound in non-oxygen-sensitive iron-hydroxides (e.g., vivianite), highlighting the complexity of sediment biogeochemistry and the challenges of accurately assessing changes in the potentially mobile P pool for estimating internal loading.