Short-term influence of recolonisation by the polycheate worm Nereis succinea on oxygen and nitrogen fluxes and denitrification: a microcosm simulation

The short-term effects of sediment recolonisation by Nereis succinea on sed iment-water column fluxes of oxygen and dissolved inorganic nitrogen, and r ates of denitrification, were studied in microcosms of homogenised, sieved sediments. The added worms enhanced oxygen uptake by the sediments, due to the increased surface area provided by the burrow walls and the degree of s timulation was stable with time. Similarly, ammonium fluxes to the water co lumn were stimulated by N. succinea, but declined over the 3 day incubation in all microcosms including the controls. Nitrate fluxes were generally gr eater in the faunated microcosms, but highly variable with time. Denitrific ation rates were positively stimulated by N. succinea populations, denitrif ication of water column nitrate was stimulated 10-fold in comparison to den itrification coupled to nitrification in the sediments. Rates of denitrific ation of water column nitrate were not significantly different from rates i n undisturbed sediment cores with similar densities of N. succinea, whereas rates of coupled nitrification-denitrification were 3-fold lower in the ex perimental set-up. These results may reflect the relative growth rates of n itrifying and denitrifying bacteria, which allow more rapid colonisation of new burrow surfaces by denitrifier compared to nitrifier populations. The data indicate that recolonisation by burrowing macrofauna of the highly red uced sediments of the Sacca di Goro, Lagoon, Italy, following the annual dy strophic crisis, may play a significant role in the reoxidation and detoxif ication of the sediments. The increased rates of denitrification associated with the worm burrows, may promote nitrogen losses, but due to the low cap acity of nitrifying bacteria to colonise the new burrow structures, these l osses would be highly dependent upon water column nitrate concentrations.