This study investigates the potential of bioturbating Tubificidae to alter biogeochemical processes by sediment aeration in order to enhance ecosystem development in eco-engineering projects. We introduced Tubificidae in three different densities (5000, 15,000, and 30,000 individuals m⁻²) in clay-rich sediment from lake Markermeer (The Netherlands). Redox potential, nutrients and major elements were measured from the water column and porewater at different depths. Mineral phase and redox transfers were chemically modelled and oxygen concentrations in bioturbated sediments for each density were mathematically predicted. The measured results of this experiment showed that Tubificidae oxygenated the upper 15 mm of the sediment. This resulted in decomposition of sedimentary organic matter with an associated sixfold increase in NH₄ and NO_x concentrations in the porewater and the water column. However, phosphorus concentrations were declining in the upper 16 mm, likely as a result of immobilization by pyrite oxidation and production of iron oxides. These bioturbation effects were highest in the treatment with an intermediate density of Tubificidae (15,000 worms m⁻²) as aeration effects in the treatment with the highest density of Tubificidae (30,000 worms m⁻²) was impeded by high respiration rates. Furthermore, with a two dimensional diffusion model, simulated effects of respiration and aeration on the oxygen concentration in the sediment suggest that the bioturbation effect is strongest at a density of 12,000 worms m⁻². In ecological engineering projects where fast ecosystem development is important, introducing Tubificidae to aquatic sediments to optimal densities might enhance initial ecosystem development due to improved availability of nitrogen as nutrient.