Résumé: Silicon (Si) is the most abundant element in the earth's crust after oxygen; it is found in aquatic environments as orthosilicic acid. This study concerned for the first time the distribution and flux of dissolved silicates (SiO4) in strategic sites (Charef dam, Bouhamdane dam, and estuary) from the upstream of the Seybouse basin to its mouth to understand silicon biogeochemical transformations under dams and estuarine effects. Surface water sampling was taken and analyzed in the dry (September 2016) and wet (April 2017) seasons. In parallel to sampling dates, physical parameters, and water discharge data were recorded. The highest SiO4 levels were recorded upstream and weakened towards the mouth. It was reduced between 32% and 54% for the Charef and Bouhamdane dams respectively, while the retention rate of the estuary was on average 44%. In terms of flux, the Charef and Bouhamdane dams produced 2 t/yr and 17 t/yr respectively, whereas the estuary produced only 54 t/yr to the coastal zone. Dams and estuarine part play a crucial role in changes in the biogeochemical silicon cycle and are expected to have severe impacts on the river system and the receiving coastline functioning.

Résumé: Dams construction appears to have multi-dimensional effects on the river environment, and it especially affects the thermal condition of rivers. Aims. The objective of this work was to assess downstream thermal responses to the Charef Dam in relation to the water flow and the water residence time. Methods. During the year 2012, the hydrological parameters upstream and downstream of the dam were monitored. Key results. Salinity, electrical conductivity, total dissolved solids (TDS) and water flow all responded similarly, with larger magnitudes in waters upstream of the dam than downstream, whatever the season. In terms of flux, the Charef Dam retains 59 and 56% of the incoming water and TDS fluxes respectively. Furthermore, the upstream water temperature is higher than that of the downstream in 83% of samples. During the dry season, the thermal response of the waters downstream of the Charef Dam can exceed 2°C (downstream cooling). Conclusions. Despite being a small dam, the water residence time (2.3 years) in the Charef Dam has a significant effect on the downstream water temperature. Implications. These temperature changes may cause significant physicochemical and biological issues. Restoration practices such as increasing discharges during dry seasons and riparian cover restoration could mitigate thermal impacts and minimise negative effects.

Résumé: through a fortnightly surface water sampling both at a lower river station and at the estuary outlet. The Seybouse estuary delivered annually 950 × 106 m3 of freshwater yielding 83 kg N km−2 yr−1 of N-NH4 and 12 kg P km−2 yr−1 of P-PO4. More than 2/3 of the annual inputs of freshwater, Si(OH)4 and NO3 entered the sea during the flooding event of late February 2012. Si-Si(OH)4 and N-NO3 yields in the Seybouse estuary represented<1/ 3 those of the Mediterranean rivers. Annaba Bay is subjected to highly polluted waters from the Seybouse estuary, with significant NH4 (72 ± 37 μmol L-1) and PO4 (7 ± 4 μmol L-1) amounts. However it is characterized by low Si(OH)4 (104 ± 43 μmol L-1) amounts. Alteration of Si:N:P ratios at this bay suggest potential risk of eutrophication, except during and weeks after flood episodes.

Résumé: Zooplankton samples were collected in Annaba Bay (Algeria) from January 2009-March 2011 at three coastal sites differently affected by estuarine plumes and external currents. Aim of this survey was to analyze zooplankton composition, abundance and biomass and compare the results with previous studies to reveal possible populations and environmental changes. The mean zooplankton abundance varied between 1,200-6,000 ind. m-3 and biomass 6.70-25.70 mg DW m-3, according to the site. Copepods constituted the main fraction of zooplankton community, and Oithona similis and Paracalanus indicus successively dominated during autumn-winter and spring-summer. The dinoflagellate Noctiluca scintillans was one of the major zooplankton components, and developed high numbers during February-April, becoming common in neritic and coastal regions. The singularity of the zooplankton from Annaba Bay is the prevalence of P. indicus throughout the entire bay and the decrease in Acartia (Acartiura) discaudata and Acartia (Acartiura) clausi (with respect to previous years), possibly replaced by A. negligens. Additionally, Oithona nana abundance markedly decreased with the large development of O. similis. Annaba Bay also differs from other similar Mediterranean coastal areas by the large development of Centropages ponticus populations during the warm period. Among the identified copepod species, the alien species Pseudodiaptomus australiensis and P. arabicus are reported for the first time in the Mediterranean Sea. The occurrence of copepodid V stages of P. australiensis suggests that this species survives and reproduces in Annaba Bay, but so far without developing an abundant population.

Résumé: Phosphate (PO4) and silicates (SiO4) levels and loads were assessed from 217 rain event samples collected in Annaba region during forty‐four months monitoring (October 2011–May 2015). Seasonal average levels of PO4 and SiO4 ranged from 0.98‐1.57 µmol l ‐1 and 7.9‐16.9 µmol l ‐1 respectively, with maximum values encountered during spring and autumn when Saharan and tropical (El Niño Southern Oscillation: ENSO) influences dominated the area. PO4 wet atmospheric loads varied with seasons (0.22‐0.32 mmol m‐2 yr‐1 ) and years (0.65‐0.94 mmol m‐2 yr‐1 ). These amounts may be considered as the highest values ever occurred in the Mediterranean area. PO4 inputs increased during winter rainy period (0.34 mmol m‐2 yr‐1 ), and during autumn coinciding with large scale meteorological event impacts, in particular ENSO warm events which may increase autumn rainfall. Annual SiO4 fluxes ranged from 6.74 to 8.96 mmol m‐2 yr‐1 , and followed the same seasonal pattern of PO4.

Résumé: Zooplankton samples were collected in Annaba Bay from January 2009-March 2011 at three coastal sites differ¬ently affected by estuarine plumes and external currents. Aim of this survey was to analyze zooplankton composition, abundance and biomass and compare the results with previous studies to reveal possible populations and environmental changes. The mean zooplankton abundance varied between 1,200-6,000 ind. m-3 and biomass 6.70-25.70 mg DW m-3, according to the site. Copepods constituted the main fraction of zooplankton community, and Oithona similis and Paracalanus indicus successively dominated during autumn-winter and spring-summer. The dinoflagellate Noctiluca scintillans was one of the major zooplankton components, and developed high numbers during February-April, becoming common in neritic and coastal regions. The singularity of the zooplankton from Annaba Bay is the prevalence of Paracalanus indicus throughout the entire bay and the decrease in Acartia (Acartiura) discaudata and Acartia (Acartiura) clausi (with respect to previous years), possibly replaced by Acartia negligens. Additionally, Oi¬thona nana abundance markedly decreased with the large development of Oi¬thona similis. Annaba Bay also differs from other similar Mediterranean coastal areas by the large development of Centropages ponticus populations during the warm period. Among the identified copepod species, the alien species Pseudodiaptomus australiensis and Pseudodiaptomus arabicus are reported for the first time in the Mediterranean Sea. The occurrence of young stages of Pseudodiaptomus australiensis suggests that this species survives and reproduces in Annaba Bay, but so far without developing an abundant population.