Modelling sediment diagenesis processes in a freshwater ecosystem experiencing transient external phosphorus and iron loading

Phosphorus (P) control is a widely used strategy for the remediation of impaired eutrophic lakes. However, the severity of eutrophication phenomena may persist well after the reduction of external P loading, which is often attributed to internal subsidies associated with the legacy P stores within the sediments. In this study, we investigated a eutrophic system in western Lake Ontario, the Hamilton Harbour, where both external P and iron (Fe) loading rates have been declining over the course of the past two decades. A sediment diagenesis process-based model was developed and calibrated using sediment profiles from 2016 and historical records of hypolimnion P accumulation rates. Modelling results suggest that transient conditions currently prevail in the harbour’s sediments, as the external P and Fe subsidies into the system shifted from the historically elevated discharge rates to the present lower levels. The model corroborated that high deposition fluxes of Fe oxyhydroxides (FeOOH) and oxidation of ferrous Fe could have been conducive to porewater P immobilization in the past, resulting in minimal internal P loading rates between 1987 and the early 2000s - even during periods of high external P loading - while simultaneously storing large amounts of legacy (primarily Fe-bound) P in the sediments. Notwithstanding the reduced external P inputs, our model analysis showed that internal P loading displayed a sharp increase during the 2010s, which was driven by a diminished capacity of Fe to bind P, compared to levels experienced during the 1990s, and the dissolution-driven release of legacy Fe-bound P. We subsequently conducted a local sensitivity analysis to evaluate the key drivers of internal P loading. The three most sensitive parameters during the simulation period (1987–2016) were the organic matter (OM) deposition flux, FeOOH deposition rate, and dissolved oxygen concentration at the sediment-water interface (SWI). These findings reinforce the importance of sedimentary Fe-P cycling in regulating internal P dynamics in the Hamilton Harbour. The model also examined the evolving role of sulfur (S) suggesting that while S had little effect on the P cycle during the earlier years due to the abundance of Fe oxyhydroxides, its potential influence may have increased over time. These identified trends collectively highlight the growing complexity and potential nonlinearity of Fe-P-S interactions in the sediments of a system under transient loading regimes.