Etude de la qualité des sédiments de la lagune côtière de Kalaât Andalous (golfe de Tunis) : impact sur l’écosystème marin.

Abstract

Lagoon ecosystems have a major component in the balance of coastal wetlands. These systems, which were heavily relied on by port activities, experienced a disruption of their dynamic. The objective of this study is the qualitative characterization of the sediments at Kalaât Andalous harbor lagoon. The investigations were based on the study of the sedimentological facies (Grain size indexes, Folk and ward, 1957), a floristic (EXCLAME) and faunistic identification. Preliminary results showed that sediments are predominantly silty (70%). The mean grain size D50 ranges from 3,395 μm (P6) to 20,975 μm (P8) with a logarithmic facies indicating an excess load deposition zone, following a decrease in transport agent skill (El Arrim, 1996) . The sandy sediments are unimodal (100%) with modes ranging from 160 μm to 200 μm. The sandy distribution is characterized by a fine sand (MZ = 2, 03 Φ to 2.96 Φ; Md 0.15 mm to 0.24 mm), which is predominantly well sorted (σ = 0.28 Φ, 0.39 Φ).KI indexe shows fine sands, asymmetric both towards fine sands (positively skewed) (P 12 – p13) and towards coarse sand (P14).The spatial representation of the lagoon sediments shows that the silty facies predominate the central area (quay) of the harbor. The sandy sediments line both at the inner side of the northern sandy spit of the Foum El Oued (P10, P12) and its southern side (P13 and P14)². The lagoon floristic investigation reveals the presence of 21 species, with the dominance of green algae (52.38 %). In addition, 6 reference species detected with the presence of Cymodocea nodosa at all radials. According to the ratio between reference Species number and RS, the radial 3 located southward of the lido channel, is the least eutrophic radial with a ratio equal to 0.66 (3 reference species, RS = 5). The microorganisms harvested and identified correspond essentially to benthic forminifiera of genus; Ammonia, Elphidium, Uvigirina, Rosalina, Quinqueloculina , Triloculina, Spiriculina, Plantonoc Foraminifera (Globigerina and Orbulina ), ostracods (Smooth and crosslinked) , molluscs (Lamellibranchs and gastropods) , Echinoderm (spicules ) and Annelids . The monitoring of the spatial distribution of taxa at the lagoon level is not random and seems to be governed by hydrodynamic regimes and the lagoon circulation patterns. In fact, in the zones of communication with the sea, the benthic microfauna is more diversified in relation to the rest of the lagoon and is formed by genus such as Ammonia , Elphidium , Rosalina and Miliolidae. This microfaunistic association similar to that currently living in the Gulf of Tunis shows a clear marine influence expressed by frequent exchanges allowing a renewal of the lagoon waters. The quite large presence of tertiary planktonic foraminifera (globigerin, orbulina) is indicative of a marked continental terrigenous influence The geochemical study of the sediments shows that they are not contaminated by metallic trace elements (As, Cd, Cr, Cu, Ni, Pb, Zn) with concentrations below the N1 level (fixed by the GEODE Group of Studies and Observations on Dredging and the Environment). All concentrations of TBT measured at the level of the sediment are low but testify an anthropogenic input and prove the use of this chemical in this region. Assessment of metallic trace elements contamination in macrophyte leaves indicates that the channel area is contaminated by Arsenic and the northeastern area is contaminated by Chrome.