Anqi Liang’s research while affiliated with University of Toronto and other places

Phosphorus (P) levels in eutrophic lakes with restricted external P inputs often show hysteresis resulting from sediment P diagenetic recycling. Hard water oxygenated lakes are often thought to be less susceptible to P release from sediment due to precipitation of P with calcium carbonate and P sorption on iron (Fe) oxyhydroxides in oxidized surface sediments. However, perpetuated eutrophic conditions in many hard water oxygenated lakes persist despite drastic reduction of external P inputs. What limits P sediment binding capacity and drives its release from sediments in such lakes is often less understood. Here we characterize sediment P diagenetic processes driving persistent internal loading in the Bay of Quinte, a hard water, polymictic embayment of Lake Ontario, Canada. We used a multifaceted approach, combining a comprehensive three-year study of sediment P binding forms, recent sediment stratigraphy and accumulation, high spatial resolution measurement of dissolved oxygen, redox potential and pH across the sediment-water interface and depth profiles of nutrient and metals concentrations in pore water. Despite oxygenated bottom water conditions P diffusive fluxes during summer are substantial (1–6.5 mg P/m ² /day), representing as much as 50% of external P inputs. Estimated rates of benthic organic carbon degradation indicate that anaerobic organic P mineralization in anoxic sediments is the main driver of P release, which is strongly influenced by Fe reduction. Estimated rates of benthic organic carbon degradation indicate that anaerobic organic P mineralization in anoxic sediments is the main driver of P release. In surface sediment the redox sensitive P is the prevailing diagenetically reactive P phase, emphasizing strong coupling with ferric Fe. While this indicates substantial P sorption on Fe oxyhydroxides, their spare binding capacity is limited, as reducible Fe: P ratios are close to sorption limit. The calcium carbonate bound P increased with sediment depth at the expense of redox sensitive P, suggesting diagenetic sequestration into apatite with sediment burial. However, only 40% of diagenetically reactive P forms, which account for ~2/3 of the total P in surface sediment, are permanently retained during burial. Importantly, our data show that in fact significant P-flux occurs in shallow areas, with an apparent discordance with concurrent measures of long-term P release from the sediments. These results also demonstrate that short-term measures cannot be extrapolated to long-term estimates and vice versa.

Internal P flux contributes significantly to P budget in the Bay of Quinte. • Dynamics of sediment P transformations were studied using diagenetic modelling. • Summer sediment P diffusive fluxes varied between 1.5 and 3.6 mg P m −2 d −1. • Diagenesis of redox sensitive and organic P forms drives substantial P diffu-sive flux. • Sediment P retention was dominated by apatite formation and varied between 71 and 75%. Internal phosphorus (P) loading significantly contributes to hysteresis in ecosystem response to nutrient reme-diation, but the dynamics of sediment P transformations are often poorly characterized. Here, we applied a reaction-transport diagenetic model to investigate sediment P dynamics in the Bay of Quinte, a polymictic, spatially complex embayment of Lake Ontario, (Canada). We quantified spatial and temporal variability of sediment P binding forms and estimated P diffusive fluxes and sediment P retention in different parts of the bay. Our model supports the notion that diagenetic recycling of redox sensitive and organic bound P forms drive sediment P release. In the recent years, summer sediment P diffusive fluxes varied in the range of 3.2-3.6 mg P m −2 d −1 in the upper bay compared to 1.5 mg P m −2 d −1 in the middle-lower bay. Meanwhile sediment P retention ranged between 71% and 75% in the upper and middle-lower bay, respectively. The reconstruction of temporal trends of internal P loading in the past century, suggests that against the backdrop of reduced external P inputs, sediment P exerts growing control over the lake nutrient budget. Higher sediment P diffusive fluxes since mid-20th century with particular increase in the past 20 years in the shallower upper basins, emphasize limited sediment P retention potential and suggest prolonged ecosystem recovery, highlighting the importance of ongoing P control measures.