Oxygen consumption and nutrient fluxes in coastal marine sediments off the Mejerda River delta (Gulf of Tunis)

The authors studied the benthic flux of oxygen, dissolved iron and manganese, and nutrients in-situ at three points in the Mejerda River Delta, at depths of 10 m, 20 m, and 40 m, in March and August 2012. Three sedimentary cores were simultaneously drilled, at the same locations, to determine the diffusive flux of Fe2+, Mn2+, NO3-, NH4+, and PO43- and to estimate the diagenetic mechanisms occurring below the sediment-water interface. Photosynthesis was not sufficiently high during the day and the oxygen consumption at the sediment-water interface was about 1.7 to 10 mmol/m(2)/day, essentially controlled by the degradation of organic matter and oxidation of reduced elements. Nitrate contents were relatively high in the sediment (above 140 mu M for NO3-) and their production was not always in conformity with the general scheme of early diagenesis, moreover, the benthic flux between the water and sediment was not clearly established. The diffusive flux of NH4+ and PO43- was always directed to the water column, at averages of 1.27 mu mol/m(2)/day for PO43- and 96.5 mu mol/m(2)/day for NH4+, complying with those measured by the benthic chambers, but representing less than 30% of the benthic fluxes for NH4+ and less than 5% for PO43-.