Sediment phosphorus legacy and internal loading delay lake recovery in Azorean volcanic lakes

Phosphorus (P) stored in lake sediments can sustain eutrophication long after reductions in external nutrient inputs, delaying ecosystem recovery. This study investigated sediment P dynamics in four volcanic lakes of the Azores (Portugal) with contrasting trophic status (Fogo, Furnas, Verde-SC and Azul-SC) by integrating historical sediment P fractionation data (2006 and 2010) with 2023 sediment analyses, microcosm experiments and long-term diagenesis modelling. Sediment samples were analyzed for P fractions and subjected to microcosm experiments to assess P release under varying redox conditions. Additionally, a diagenesis model was developed to simulate long-term transformations and redistribution of P fractions. Total sediment P concentrations in the 2023 samples were higher than previously reported values in the historical datasets, with the highest concentrations observed in eutrophic lake Verde-SC and the lowest in oligotrophic lake Fogo. Organic P was the dominant fraction in the lakes Verde-SC and Azul-SC, accounting for up to 61 % and 51 % of total sediment P, respectively. Microcosm experiments demonstrated the strong influence of redox conditions on sediment P release, with maximum P release rates of 3.7 mg·m^-2·d^-1 under anoxic conditions. Model simulations indicated that, even under the hypothetical cessation of external P inputs, reactive sediment P may continue to influence lake trophic status for decades, depending on lake-specific characteristics. These findings demonstrate that sediment P legacy constitutes a major constraint on the lake recovery and highlight the importance of incorporating sediment processes into restoration strategies. By combining historical sediment records, experimental measurements and diagenesis modelling, this study provides one of the first integrated assessments of P legacy in volcanic lakes.