Macrofauna can significantly increase benthic nitrogen cycling in estuarine systems. The nature and magnitude of macrofaunal effects on benthic nitrogen cycling vary among species and depend, in part, on animal behavior. Of the species examined, macrofauna that introduce oxygen to subsurface sediment significantly increased the activity of nitrifying bacteria. The nitrification potential of their tubes or burrows was 2- to 200-times greater than that of adjacent sediments and 1- to 61-times greater than that of surface (0-1 cm) sediments. The degree of enhancement of nitrification potential was influenced by sediment ammonium levels and organism irrigation behavior.

Animal-density manipulation experiments demonstrated that, Loimia medusa, a tube- irrigating polychaete, significantly increased benthic oxygen consumption and release of ammonium (NH4+) and nitrate (NO3-) from sediment when concentrations of NO3- in the overlying water were low (3 uM). Increased NO3- fluxes and mass balance and stoichiometric calculations indicate that L. medusa stimulated nitrification and denitrification. Worm respiration and excretion contributed substantially (> 50%) to its effect on sediment-water fluxes of oxygen and regenerated nitrogen (NH4+ plus NO3- plus N2 from coupled nitrification-denitrification). Indirect worm effects (stimulation of microbial activities and increased NH4+ export) accounted for 25% to 50%. Due to its stimulation of the transformation of NH4+ by nitrification and denitrification, more than half of the increased release of regenerated nitrogen exited as NO3- or N2.

A field study examining three benthic communities with different macrofaunal assemblages indicated that the nature and magnitude of macrofaunal effects on benthic nitrogen cycling varied with macrofaunal biomass and community composition, especially the relative abundance of irrigators and bioturbators. At one site, an abundant burrow-irrigating amphipod, Leptocheirus plumulosus, appeared to affect the porewater NH4+ profile and sediment-water fluxes of nitrogen by stimulating nitrification and denitrification. At a second site, a diverse community with bioturbators (23%) and irrigators (16% of total density and 53% of total biomass) caused thorough mixing of the top 9 cm of sediment and appeared to increase NH4+ release from sediment. At a third site with high biomass of Mulinia lateralis, the primary macrofaunal effect was substantial contribution of animal excretion to nitrogen release from sediments.