Recent sedimentation rates in Garaet El Ichkeul Lake, NW Tunisia, as affected by the construction of dams and a regulatory sluice

Ichkeul National Park, NW Tunisia, is a UNESCO Biosphere Reserve. Garaet El Ichkeul Lake is known for its seasonal variability in water level and salinity. In recent decades, the waterbody has been affected by the construction of new hydraulic structures. To reduce the impacts of dams and to maintain the sustainability of the ecosystem, a sluice was built at the outlet of the lake, and it operated for the first time in 1996. This paper describes an investigation of recent sedimentation dynamics in Ichkeul Lake, determined by radiometric dating of sediment cores.A sediment core was collected with a UWITEC gravity corer at the deepest, central part of the lake in August 2009. Specific activities of unsupported lead-210 (Pb-210) and caesium-137 (Cs-137) were measured in the core, enabling calculation of recent sediment accumulation rates (SAR). Published radiometric data from nearby sediment cores, collected in 1997 and 1982, provide a comparison.The measured excess Pb-210 inventory was 5300 +/- 500 Bq m(-2), leading to an estimation of constant flux of 165 +/- 16 Bq m(-2) yr(-1), a value higher than the best estimate for local atmospheric fluxes (123 +/- 12 Bq m(-2) yr(-1)) and the flux estimated from the core collected in 1982 (48 Bq m(-2) yr(-1)). The Cs-137 inventory was 3550 +/- 120 Bq m(-2), two times higher than the historical Cs-137 atmospheric deposition in the area. The Cs-137 profile displayed a distinct peak, but the Cs-137 depth-distribution did not follow the pattern expected from atmospheric deposition. Application of the constant rate of supply (CRS) model, with the reference point method, produced a chronology and SAR values comparable to those found in previous work. The whole Cs-137 profile was quantitatively reconstructed from the historical records of atmospheric deposition, using the system-time-averaged (STA) model.The CRS and STA models provide consistent sediment accumulation results for the whole data set, considering the time resolution of the chronology ( 6 years) and analytical uncertainties. Results from cores sampled in 1982, 1997 and 2009 reveal an increasing SAR trend, from 0.25 g cm(-2) yr(-1) in the early 1940s to 0.67 g cm(-2) yr(-1) at present. In the 13 years since installation of sluice gates at Tinja, SAR in the central Ichkeul Lake has not declined. Thus, if siltation continues at the present rate, shallowing of the lake will seriously affect the hydromorphology and ecology of this important lake.