Bio-irrigation Promotes Reactive Phosphorus Recycling in an Oxidized Sedimentary Environment

Bio-irrigation by burrowing macrofauna regulates benthic functioning via direct and indirect effects on sediment properties, microbial activities, oxygen dynamics, and organic matter and nutrient turnover. The effects of macrofauna bio-irrigation on benthic nitrogen cycling have been thoroughly investigated, whereas those on phosphorus (P) are comparatively understudied. This is surprising as such effects contribute to sediment oxidation and have a large potential to regulate P mobility and increase P retention. Dissolved oxygen (O₂) and inorganic phosphorus (DIP) fluxes, pore water chemistry (DIP_pw, Fe[II]_pw, Mn[II]_pw, pH_pw, and oxidation–reduction potential (ORP_pw)), and solid-phase Fe(III) pools were measured in reconstructed sediments without or with surface (the amphipod Corophium volutator) and deep (the polychaete Alitta succinea) burrowing macrofauna. Sediments and burrowing macrofauna were collected from the Goro Lagoon (Po River Delta, Italy) in April 2022. Measurements were carried out after a 2-week preincubation to allow sediment conditioning by bioturbators (e.g., burrow construction, bio-irrigation, burrow wall oxidation, steady chemical gradients within sediments and between pore and bottom waters). ORP_pw analysis suggested that bio-irrigated sediments were less reduced, and Fe solid-phase analysis suggested a tendency towards an increase in the Fe(III) pool in deep bio-irrigated sediments. Both bioturbators stimulated O₂ fluxes and DIP recycling (by a factor of ~ 2), and halved DIP_pw, Fe(II)_pw, and Mn(II)_pw concentrations. The amphipod contributed to DIP fluxes via direct excretion, whereas polychaete excretion was negligible. Polychaetes contributed to DIP fluxes by ventilation of deep burrows within DIP-rich pore water. Bio-irrigation by both burrowers simultaneously promoted higher DIP recycling and sediment oxidation, ensuring the mobilization of a limiting nutrient and preventing the accumulation of reduced chemical species in the surface sediment.