Hydrocarbon influence on denitrification in bioturbated Mediterranean coastal sediments

An in situ experiment was carried out in bioturbated Mediterranean coastal marine sediments (Gulf of Fos) in order to study the influence of hydrocarbons on denitrification after 1, 4 and 6 months. In the absence of hydrocarbons in the control sediments, the presence of macrofauna stimulated denitrification by 160%. This stimulation is induced by sediment reworking that favours both direct NO3- supply from the water column and the penetration of O-2, which in turn stimulated nitrification, the other source of NO3- in sediment. The presence of hydrocarbons in the experimental sediments either stimulated or inhibited the denitrification. The denitrification response to the presence of hydrocarbon is dependent on the quantity of matter buried by the macrofauna activity. In small quantities, the organic matter related to hydrocarbons 120% enhanced the denitrification compared to the controls. On the other hand, when buried hydrocarbon concentrations were higher (>100 mg saturated hydrocarbon fraction kg(-1) dry sediment), the denitrification was inhibited. On the basis of the results obtained, a descriptive model of the patterns of denitrification in relation to the presence of macrofauna and the distribution of hydrocarbons in sediments is proposed.