Sediment Denitrification in Two Contrasting Tropical Shallow Lagoons

Abstract

Sediment denitrification was monthly evaluated in two tropical coastal lagoons with different trophic states using the 15N isotope pairing technique. Denitrification rates were very low in both environments, always <5.0 μmol N2 m−2 h−1 and were not significantly different between them. Oxygen consumption varied from 426 to 4248 μmol O2 m−2 h−1 and was generally three times higher in the meso-eutrophic than the oligotrophic lagoon. The low denitrification activity was ascribed to both low water NO3− concentrations (<2.0 μM) and little nitrate supply from nitrification. There was no correlation of denitrification with nitrate or ammonium fluxes. Sediments in temperate environments with similar oxygen consumption rates usually presented a higher proportion of nitrification–denitrification rates. Sediment oxygen consumption was a good predictor of sediment denitrification in both studied lagoons.