What factors drive copepod community distribution in the Gulf of Gabes, Eastern Mediterranean Sea?

International audienceThe spatial and temporal variations in copepod communities were investigated during four oceanographic cruises conducted between July 2005 and March 2007 aboard the R/V Hannibal. A close relationship was observed between the temperature, salinity, hydrographic properties and water masses characterising the Gulf of Gabes. Indeed, water thermal stratification began in May–June, and a thermocline was established at a 20-m depth, but ranged from 25 m in July to more than 30 m in September. The zooplankton community is dominated by copepods representing 69 % to 83 % of total zooplankton. Spatial and temporal variation of copepods in relation to environmental factors shows their close relationship with the hydrodynamic features of the water column. Thermal stratification in the column, established in summer, supports copepod development. In fact, copepod abundance increases gradually with rising water temperature and salinity, starting from the beginning of thermal stratification (May–June 2006) and lasting until its completion (July 2005 and September 2006). When the water column is well mixed (March 2007), copepod abundance decreased. Our finding shows that temperature and salinity seem to be the most important physical factors and thus strongly influence the taxonomic diversity and distribution of the copepod population. They are characterised by the dominance of Oithona nana, representing 75–86 % of total cyclopoid abundance. The most abundant species during the stratification period were O. nana, Acartia clausi and Stephos marsalensis in July 2005 and September 2006. However, during the mixing period, Euterpina acutifrons was more abundant, representing 21 % of the total. Unlike the copepod community, which is more abundant during the period of high stratification, phytoplankton proliferates during semi-mixed conditions

Water quality affects the structure of copepod assemblages along the Sfax southern coast (Tunisia, southern Mediterranean Sea)

International audienceThe Sfax southern coast (Gulf of Gabes, Mediterranean Sea) has been under increased anthropogenic pressure for many years. In the present study we investigated the effects of this anthropisation on the spatial distribution of copepod assemblages in relation to the physicochemical features of seawater at 20 stations sampled on 19 March 2013. Copepods represented 73% of total zooplankton abundance. Small planktonic copepods (<1.45 mm), including pollution-tolerant species (e.g. Oithona nana, Paracalanus parvus, Harpacticus littoralis and Tisbe battagliai), proliferated exclusively in stations of ~0.5-m depth characterised by high coastal anthropogenic inputs. The largest copepod species were dominated by Calanus helgolandicus (1.45–2.5 mm) in the offshore zone in depths of ~3 m. Substantial numbers of Oithona plumifera (7.5%) were found at depths between 0.5 and 3 m. Copepod diversity was significantly higher in the southern zone, which is less affected by sewage, than in the northern zone, which was subjected to higher pressure (Shannon–Wiener index H′ = 1.5–2.5 and ≤1.5 bits individual–1). A shift in the planktonic copepod community between the two zones was linked to deterioration of water quality, with higher phosphorus levels, turbidity and chemical oxygen demand (COD) in the northern zone

Diazotrophic cyanobacteria signatures and their relationship to hydrographic conditions in the Gulf of Gabes, Tunisia

International audienceChanges in the planktonic cyanobacteria structure, composition and diversity were followed over three consecutive years (2005–2006–2007) in the Gulf of Gabes (Eastern Mediterranean Sea, Tunisia). Cyanobacteria abundances, biomasses and cell lengths were measured together with selected environmental variables (pH, salinity, temperature and nutrients). The space and time variations of the cyanobacteria in relation to the environmental factors showed a close relationship between these plankton communities and the hydrographic structure of the water column. Cyanobacteria developed over semi-mixed conditions (May–June 2006) and during the thermal stratification (July 2005). The cyanobacterial abundance and biomass was evident between 20 and 35 m in inshore stations and between 20 and 25 m in deeper stations during the semi-mixing conditions and stratification. This thermocline level coincided with the euphotic layer (21.85 ± 3.76 m) allowing access of light radiation. The cyanobacteria bloom occurred during May–June 2006 when the N/P ratio (<10) was clearly below the accepted standard molar ratio of N/P = 16/1. Commonalities among cyanobacterial genera include being highly competitive for low concentrations of inorganic P (DIP) and the ability to acquire organic P compounds. Our study showed that both diazotrophic (N2-fixing) cyanobacteria such as Anabaena sp., Chroococcus sp., Trichodesmium erythraeum, Spirulina sp. and Spirulina subsalsa and non-diazotrophic cyanobacteria such as Pseudoanabaena sp. and Microcystis display a great flexibility in the N sources which allow formation of blooms

Copepod assemblages as a bioindicator of environmental quality in three coastal areas under contrasted anthropogenic inputs (Gulf of Gabes, Tunisia)

International audienceThe study of copepod assemblages indicated the presence of 22 species and 12 families in the southern coast of Sfax, 20 species and 13 families in the northern coast and 14 species and 8 families in the Ghannouch area, with a dominance of Oithonidae (79, 51 and 43% in the southern, northern and Ghannouch coasts, respectively). The relative abundance and the richness diversity of Oithonidae were found to be the most relevant indicators of anthropogenic pollution. Oithona nana, Euterpina acutifrons and Acartia clausi differed significantly in abundance between these three areas under differing degrees of pollution. The study of the structure, composition and density of the copepod fauna showed that the southern coast was a pollution-resistant ecosystem (H ′ ¼ 1.49 + 0.33 bits ind 21 ; 22 species; density ¼ 51.375 + 4.340 × 10 3 ind m 23) followed by Ghannouch area (H ′ ¼ 1.74 + 0.28 bits ind 21 ; 15 species; density ¼ 11.979 + 5.651 × 10 3 ind m 23) and the northern coast, considered as a restored area (H ′ ¼ 1.95 + 0.26 bits ind 21 ; 21 species; density ¼ 6.516 + 4.304 × 10 3 ind m 23). The three ecosystems can thus be classified according to their degree of resistance to the anthropogenic inputs based on the results of the physico-chemical parameters and the species diversity as follows: southern coast. Ghannouch area. northern coast

How could thermal stratification affect horizontal distribution of depth-integrated metazooplankton communities in the Gulf of Gabes (Tunisia)?

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Impacts of an uncontrolled phosphogypsum dumpsite on summer distribution of phytoplankton, copepods and ciliates in relation to abiotic variables along the near-shore of the southwestern

International audienceIn connection with the Taparura Project, studies of spatial distribution of the crustacean zooplankton community, nutrients, phytoplankton and ciliates were conducted in July 2007 at 45 stations spread over fifteen transects along the coast north of Sfax. The results showed that the N/P ratio was lower than the Redfield ratio, suggesting potential N limitation. Phytoplankton was characterised by the proliferation of several diatoms, while ciliates were largely dominated by spirotrichs. Copepods were the most abundant zooplankton present during the entire study period, comprising 61% of the total zooplankton community. Twelve copepod families were identified at every station, with a high percentage of Oithonidae (77% of copepods) dominated by Oithona nana. The abundance of this species was correlated with that of diatoms, Cocoolithophorideae and ciliated Colpodea, suggesting that O. nana may feed on a wide range of prey. Despite human pressure and industrial activities, the coastal waters north of Sfax showed a wide diversity of phytoplankton, ciliates and zooplankton

Coupling of phytoplankton community structure to nutrients, ciliates and copepods in the Gulf of Gabès (south Ionian Sea, Tunisia)

14 pagesInternational audienceThe summer spatial distribution of the phytoplankton community in the Gulf of Gabe`s (Tunisia, Eastern Mediterranean Sea), together with environmental factors, were studied during a preliminary study conducted on July 2005 aboard the RV ‘Hannibal'. The phytoplankton community, which showed a decrease in concentration along a coastal–open sea gradient, was dominated by Dictyochophyceae (41%) followed by Dinophyceae (25%), Bacillariophyceae (16%), Cyanobacteriae (17%) and Euglenophyceae (1%). The phytoplankton found along the coast was dominated by opportunistic species (e.g. Dictyocha fibula) associated with high nutrient availability. In the open sea, phytoplankton development seemed influenced by Atlantic hydrodynamics. In addition, the Gulf of Gabe`s is characterized by an oligotrophic status with a summer stratification that impacted on species composition especially in off-shore areas. The coupling of phytoplankton dynamics to nutrients, ciliates and copepods showed the potential role played by ciliates not only as predators of phytoplankton but also as prey for filter-feeding copepods accounting for the increased fisheries productivity of the Gulf of Gabe`s

Assessment of anthropogenic inputs in the surface waters of the southern coastal area of Sfax during spring (Tunisia, Southern Mediterranean Sea).

International audienceThe coastal marine area of Sfax (Tunisia), which is well-known for its high productivity and fisheries, is also subjected to anthropogenic inputs from diverse industrial, urban and agriculture activities. We investigated the spatial distribution of physical, chemical and biogeochemical parameters in the surface waters of the southern coastal area of Sfax. Pertinent tracers of anthropogenic inputs were identified. Twenty stations were sampled during March 2013 in the vicinity of the coastal areas reserved for waste discharge. Phosphogypsum wastes dumped close to the beaches were the main source of PO4(3-), Cl(-) and SO4(2-) in seawater. The high content in total polyphenolic compounds was due to the olive oil treatment waste water released from margins. These inorganic and organic inputs in the surface waters were associated with elevated COD. The BOD5/COD (3) ratios highlighted a chemical pollution with organic load of a low biodegradability

Dynamics of dinoflagellates and environmental factors during the summer in the Gulf of Gabes (Tunisia, Eastern Mediterranean Sea)

13 pagesInternational audienceThe summer spatial distribution of the dinoflagellate community along an open coastal sea gradient in the Gulf of Gabes (Tunisia, Eastern Mediterranean Sea), together with environmental factors, were studied. The most dominant families were represented by Gymnodiniaceae (32%), Peridiniaceae (20%), Prorocentraceae (15%), Ceratiaceae (13%) and Ebriaceae (10%). The dinoflagellate community was spatially more concentrated along the coast of the gulf than in the open sea. Eight toxic dinoflagellates were recorded, including Karenia cf. selliformis (37% of total toxic dinoflagellates) which was evenly distributed in both the neritic and open sea areas. Dinocysts contributed 33% of the total motile cells and were more abundant along the coast than in the open sea. This high concentration may be ascribed to nitrogen inputs in the coastal waters of Gabes. The Modified Atlantic Water governed dinoflagellate development in the open sea. The degradation of the water quality due to eutrophication in the Gulf of Gabes may have significant socioeconomic consequences. We suggest that a management framework, similar to that used in freshwater ecosystems, should be developed for the Gulf coast in order to drastically reduce urban interferences