Nitrogen cycling in sediments of the Lagoon of Venice, Italy

Abstract

In order to identify the importance of different areas for nitrogen retention in the Lagoon of Venice, Italy, undisturbed sediment cores of different origin were collected in late April 1998. The cores were transferred to the laboratory and incubated for denitrification activity using the method of 'nitrogen isotope pairing'. The sampled sediments were collected in a freshwater stream (Stn A), an intertidal area (Stn B), in an area characterised by Ulva sp. (Stn C) and in an area vegetated by Zostera sp. (Stn D). Stn D was close to the lagoon outlet (mouth) and highly affected by tidal actions. The ambient concentrations of water column nitrate ranged from 132 to 4.4 μM. The highest denitrification rate was found in the area with the highest concentration of water nitrate, i.e. at Stns A and B, 292 ± 75 and 204 ± 52 μmol m-2 h-1, respectively. Sediment denitrification in the areas closer the lagoon outlet was significantly lower, 16 ± 2 μmol m-2 h-1 at Stn C and 11 ± 2 μmol m-2 h-1 at Stn D. Oxygen and nitrate consumption, release of ammonium and denitrification in the sediment at Stn B was correlated to the biomass of benthic infauna. About 30% of the denitrification in the B sediment was explained by the presence of benthic infauna. More than 50% of the denitrification in the sediment at Stn D was due to denitrification of nitrate produced by nitrification.