Regeneration of sedimentary phosphorus and iron in a large seasonally hypoxic estuary deciphered by 224Ra/228Th disequilibria

Using a ²²⁴Ra/²²⁸Th disequilibrium approach, we demonstrate in this study that benthic fluxes of dissolved inorganic phosphorus (DIP) in the seasonally hypoxic Yangtze River Estuary were largely manipulated by two counteracting processes: the decomposition of sedimentary organic matter and adsorption of DIP onto iron (Fe) oxides. The decomposition rate of sedimentary organic matter rose exponentially with bottom water dissolved oxygen (DO) concentration multiplied by the amplification factor of sediment surface area (ξ), a variable used to describe the intensity of bio-irrigation and physical reworking in the sediment deposit. In the summer of 2020, the Yangtze River catchment encountered the largest flood event in the past 20 years. As a result of enhanced physical reworking of the seabed, dissolved inorganic carbon (DIC) flux increased by approximately 4-fold as compared to the summer of 2019 (657 vs. 154 mmol m⁻² d⁻¹). DIP flux exhibited similar inter-annual variations to DIC flux, indicating that changes in the decomposition rate of sedimentary organic matter were the main cause of the inter-annual differences in DIP flux (1.5 mmol m⁻² d⁻¹ in 2020 vs. 0.3 mmol m⁻² d⁻¹ in 2019). On the other hand, benthic dissolved Fe fluxes declined exponentially with rising DO concentration because of re-oxidation of Fe²⁺ in porewater into Fe oxides. As a consequence, approximately 90% of DIP sourced from the decomposition of sedimentary organic matter was retained within the sediment when the bottom water was well oxygenated (DO >125 μmol l⁻¹). Our results imply that regeneration of sedimentary P and Fe may be efficient only within a very narrow redox window where DO concentrations are below a threshold value of ∼60 μmol l⁻¹, but bio-irrigation or physical reworking is still active.