Potential impacts of sebkhet Ariana rainfall discharges on the trophic state of the coastal zone of Raoued (Gulf of Tunis, Tunisia)

The analysis of the hydrobiologic parameters in the coastal zone of Raoued (Gulf of Tunis) was undertaken during 2004 to 2006 period, following the rainfall discharges of sebkhet Ariana. High concentrations of ammonium and orthophosphates, the major elements, characterized the eutrophic state in the coastal zone of Raoued. The maximum concentration of Chlorophyll-a varied between 3.9 to 23.8 μg L−1 in marine environment while in sebkhet Ariana, the maximum concentration reached 102.3 μg L−1 to 151.3 μg L−1 in 2004 and 2005, respectively, with a highest orthophosphate concentration estimated about 2.8 μM. L−1 reaching 3.5 μM. L−1 at the sampling station close to the rainfall discharges. Phytoplankton populations in the sebkhet station were dominated by green algae, cryptomonads and cyanobacteria. Density of the latter taxon was about 3.62 107 cells L−1 in 2004 and 3.4 108 cells L−1 in 2005. In the marine environment, these concentrations have decreased, but remained rather high in the sampling stations situated close to the outflow of the sebkhet. In 2006, the 90th percentile of the dinoflagellates/diatoms ratio was largely favorable to the dinoflagellate species, which could indicate a low water quality of the coastal zone of Raoued. Pseudonitzschia spp (diatoms), Prorocentrum triestinum and Scripsiella spp (dinoflagellates) species were the dominant taxa.