Nutrient exchange and oxygen demand at the sediment–water interface during dry and wet seasons off the Medjerda River Delta (Tunis Gulf, Tunisia)

In situ benthic flux of oxygen, alkalinity, and nutrients was assessed at the water–sediment interface using benthic chambers at three stations in the Medjerda River Delta (Tunisia) at 10, 20 and 40 m depth during March and August 2012. Simultaneously, three sediment cores were taken at the same locations to determine the diffusive flux of dissolved inorganic nitrogen (DIN = NO₂ ⁻, NO₃ ⁻, NH₄ ⁺) and PO₄ ³⁻ with a Technicon Autoanalyzer III to estimate diagenetic mechanisms occurring below the sediment–water interface. Oxygen consumption at the interface is about 1.7–10 mmol/m²/day essentially controlled by organic matter degradation and oxidation of reduced elements. Nitrate concentration is relatively high in the sediment (above 140 μM for NO₃ ⁻), and its production did not always conform to the general scheme of early diagenesis. NH₄ ⁺ and PO₄ ³⁻ were released at a rate of 96.5 and 1.27 μmol/m²/day, respectively, consistent with those measured in benthic chambers, but amounting to <30 % of benthic fluxes for NH₄ ⁺ and <5 % for PO₄ ³⁻.