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

Abstract

Purpose 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. Materials and methods 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 ( 210 Pb) and caesium-137 ( 137 Cs) 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. Results and discussionThe measured excess 210 Pb 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 137 Cs inventory was 3550±120 Bq m −2 , two times higher than the historical 137 Cs atmospheric deposition in the area. The 137 Cs profile displayed a distinct peak, but the 137 Cs 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 137 Cs profile was quantitatively reconstructed from the historical records of atmospheric deposition, using the system-time-averaged (STA) model. Conclusions 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

Anthropogenic, Direct Pressures on Coastal Wetlands

“…The exceptional salinity gradient (3-45) is a unique feature that gives Ichkeul lake and wetlands a rich biodiversity. The area comprises three units: the 89 km 2 brackish water lake, 27 km 2 marshes, and a 13 km 2 isolated and wooded massif; Djebel Ichkeul (Trabelsi et al, 2012).…”

Evidence that excess 210Pb flux varies with sediment accumulation rate and implications for dating recent sediments

“…The restrictive hypothesis of steady-state profiles is common in most 210 Pb models because it enables analytical solutions (Robbins 1978). Thus, the assumption of constant 210 Pb exc flux has been widely applied to a variety of aquatic systems, including lakes (Robbins and Edgington 1975;Appleby et al 1979;Abril 2003b;Trabelsi et al 2012), estuaries (Di Gregorio et al 2007;Díaz-Asencio et al 2009), and coastal areas and marine environments (Emeis et al 2000;Zaborska et al 2008). A variety of models accommodate non-post-depositional redistribution (Stevenson and Battarbee 1991;Appleby 2001), constant diffusion (Laissaoui et al 2008), complete or incomplete mixing (Robbins et al 1977;Abril et al 1992;Abril 2004), translocation (Smith et al 1986), and non-ideal deposition .…”

Tabe'a II Report: Enhancing Regional Capacities for World Heritage

“…High sedimentation rates have been observed for the past 30 years (Trabelsi et al, 2012). Sedimentation is a natural process, but is exacerbated by dams, and could eventually lead to the drying-up of the lake (Conservation Outlook).…”