Coupling sedimentary cycling with eutrophication severity under transient external phosphorus and iron loading regimes

Internal phosphorus (P) loading can profoundly modulate the success of eutrophication mitigation efforts, and its magnitude is strongly influenced by a multitude of external stressors and internal ecosystem mechanisms. Building upon a sediment diagenesis mechanistic model developed for the Hamilton Harbour, Lake Ontario, Canada (1987–2016), the present study investigates the mechanisms underlying the evolution of internal P loading in response to concurrent reductions in external iron (Fe) and P inputs. Model projections suggest that sediment P release will likely increase and remain relatively high (>4 mg P m-2 day-1) for over a decade following the recent external P loading reductions. Specifically, as Fe inputs are also expected to decline, the sedimentary P cycle is projected to gradually decouple from the Fe cycle, owing to a sharp decline in Fe oxyhydroxide (FeOOH) content, which in turn can moderate the sediment retention capacity and may mark a transition from slow to fast diagenetic regime accompanied by enhanced P recycling. To evaluate how projected changes in sedimentary P cycling could affect the water quality in Hamilton Harbour, we examined the joint influence of internal P loading and water exchanges with Lake Ontario on hypolimnetic P accumulation rates. Sustained internal P loading at 4 mg P m-2 day-1, under a 30-day residence time, could elevate the late-summer hypolimnetic P concentrations by over 25–30 μg P l-1. In terms of other facets of water quality, these elevated concentrations will likely have an impact on the epilimnetic phytoplankton biomass levels and may lead to frequent violations of existing delisting targets for the Beneficial Use Impairment (BUI) “Eutrophication or Undesirable Algae”. Alongside the predictions regarding the evolution of internal nutrient loading, our study concludes by discussing the potential role of other external stressors and internal “ecological unknowns” that may determine the degree and timing of restoration of the Hamilton Harbour.