RATES OF NITRIFICATION AND CARBON UPTAKE IN THE RHONE RIVER PLUME (NORTHWESTERN MEDITERRANEAN-SEA)

Nitrification rates were measured along a salinity gradient in the Rhone River estuary, using specific inhibitors (allylthiourea and chlorate) coupled with the measurement of change in nitrite concentration and inorganic carbon uptake by nitrifiers. Rates of ammonium and nitrite oxidation were similar up to 15 practical salinity units (from 1 to 2 mumol N oxidized liter-1 day-1). For higher salinities, nitrite and ammonium oxidation rates were 0.14 and 0.23 mumol N oxidized liter-1 day-1, respectively. Ammonium oxidizers assimilated 19-150 X 10(-3) mumol C liter-1 day-1, while nitrite oxidizers fixed 4.8-72.6 X 10(-3) mumol C liter-1 day-1. The amounts of nitrogen oxidized and C incorporated demonstrated a linear correlation (r2 > 0.99). The ratio of N oxidized to C incorporated ranged between 14.3 to 12.3 for ammonium oxidizers, and between 31.6 and 29 for nitrite oxidizers, the lower values being measured in seawater.