A 4-week laboratory experiment investigated the behaviour (survival and bioirrigation) and impact of the invasive polychaetes Marenzelleria viridis, M. neglecta and M. arctia on sediment-water solutes exchange, porewater chemistry, and Fe and P interactions in high-salinity sandy sediment (HSS) and low-salinity muddy sediment (LSM) from the Baltic Sea. M. viridis showed deep burrowing with efficient bioirrigation (11 L m⁻² day⁻¹) and high survival (71%) in HSS, while M. arctia exhibited shallow burrowing with high bioirrigation (12 L m⁻² day⁻¹) and survival (88%) in LSM. M. neglecta behaved poorly in both ecological settings (bioirrigation, 5–6 L m⁻² day⁻¹; survival, 21–44%). The deep M. viridis bioirrigation enhanced total microbial CO₂ (TCO₂) production in HSS by 175% with a net efflux of NH₄⁺ and PO₄³⁻, at rates 3- to 27-fold higher than for the other species. Although the shallow and intense bioirrigation of M. arctia in LSM stimulated microbial TCO₂ production to some extent (61% enhancement), the nutrient fluxes close to zero indicate that it effectively prevented the P release. Porewater Fe:PO₄³⁻ ratios revealed that the oxidizing effect of M. arctia bioirrigation increased the PO₄³⁻ adsorption capacity of LSM twofold relative to defaunated controls while no buffering of PO₄³⁻ was detected in M. viridis HSS treatment. Therefore, the different behaviour of the three species in various environments and the sharp contrast between M. viridis and M. arctia effects on C, N and P cycling must be considered carefully when the ecological role of Marenzelleria species in the Baltic Sea is evaluated.