Biogeochemical cycles of Org-C, Tot-N and Tot-S in the sediment of the Ghar El Melh Lagoon (north of Tunisia)

Abstract

Pore water analysis of chloride (Cl −), alkalinity, ammonium (NH 4 +), and sulfate (SO 4 2−) in the Lagoon of Ghar El Melh and the evolution of their relative concentrations indicate that these concentrations had changed over time reflecting the non-steady state conditions prevailing in this lagoon. Cl −, which is considered in this study as a non-conservative tracer element, was used to determine the effective diffusion rates, including bioirrigation or wind action in lagoon sediment, indicating that the effective diffusion coefficient ( D eff) varied between 10 and 20 dm 2 year −1 in summer and was equal to 50 dm 2 year −1 in autumn. The so-calculated diffusion rates clearly expressed the particular role of bioirrigation and wind action as major factors which control the biogeochemical processes in this lagoon. The examination of the 210Pb profile along the sediment column indicated that bioturbation was low in this lagoon, and did play only a very limited role. On the other hand, based on the excess of 210Pb, the sedimentation rate ( w) had been estimated to be less than 0.1 cm year −1 in this lagoon. The alkalinity and NH 4 + modeling combined with calculations for Cl − diffusion have led to the estimation of the following production rates ( R p) at the surface of the lagoon sediment: alkalinity=52–60 mmol l −1 year −1 and NH 4 +=8 mmol l −1 year −1. In spring, D eff and R p were fitted simultaneously showing the following results: D eff alk=10–50 dm 2 year −1; R p alk=57 mmol l −1 year −1; D eff NH 4 + =150 dm 2 year −1 and R p NH 4 + =8 mmol l −1 year −1. Such particularly high values of D eff reflect one more time the crucial impact of bioirrigation or wind action on the biogeochemical processes developing within the pore-water sediment. This result is however supported by the increase of micro-organisms in the sediment and by the typical windy periods occurring in spring. Fluxes calculated using the concentration gradients in pore water within the undisturbed sediment, ranged from 4.48 to 11.13 mmol m −2 day −1 for alkalinity, 0.011–0.351 mmol m −2 day −1 for NH 4 + and 4.64–11.92 mmol m −2 day −1 for T–CO 2. Nevertheless, the calculated NO 3 − fluxes were very low. Seasonal fluxes of SO 4 2−, estimated by the alkalinity budget, are found to range from 1.18 and 3.43 mmol m −2 day −1 in the undisturbed sediment. Combination of fluxes determined using pore water concentration gradients and solid phases have led to a better understanding of the biogeochemical cycles of organic carbon (Org-C), total nitrogen (Tot-N) and total sulfur (Tot-S) developing in the undisturbed sediment of the lagoon. The results showed that Org-C regeneration remained important in the deep sediment, while most of the Tot-N regeneration occurred mainly near the sediment–water interface. As regards the Tot-S, this element appeared to be mainly governed either by the SO 4 2− reduction within the undisturbed sediment and/or by the FeS oxidation/reduction processes occurring at the upper part of the sediment.