Spatial and Vertical Distribution of Phytoplankton During a Winter Cruise in the South of the Kerkennah Islands in the Gulf of Gabes, Tunisia

Spatial and vertical distribution of phytoplankton community were examined in the coast of the south of the Kerkennah Islands at three different depths 0, 2 and 6 m during the winter 2009 at 6 sampling stations. A total of 49 taxa belonging to five phytoplankton groups were identified （H＇=1.7 to 3.5 bits＇cell^-1）. Dinoflagellates and Diatoms were the most abundant phytoplankton groups which reached 27 and 71% of total abundance, respectively. The spatial distribution of the phytoplankton abundance showed a significant variation between the stations （P 〈 0.001）. While the vertical distribution of total phytoplankton showed a clear pattem, ANOVA test showed only an insignificant variation between the different depths at each station （P 〉 0.01）. On the whole, the maximum abundance of phytoplankton was recorded at the depths of 2 and 6 m （6 ×10^3 ind..Ll）. This might be due to the decrease of the nutrient concentrations in deeper water specially those of phosphate.

Factor driving heterotrophic dinoflagellate in relation to environment conditions in Kerkennah Islands(eastern coast of Tunisa)

Objective:To study the seasonal variability of heterotrophic dinoflagellate in the station of Cercina(southern coast of Tunisia).Methods:Sampling was done in 2007 in Cercina station located in the western coast of Kerkennah(34°41'27''N;11°07'45''E)(Southern Tunisia).Three replicates of water samples were taken during 10 days of each month.Environmental variables and nutrients were measured in situ.Results:A significant seasonal difference was observed for temperature and water salinity.The highest values were observed in spring and summer.No significant seasonal difference was,however,detected for nitrite,nitrate,ammonia,silica and phosphate.Sixty-five species of dinoflagellate were identified in the station of Cercina.Abundance of dinoflagellates fluctuated between seasons with values showing a significant seasonal and monthly difference.The highest mean abundance was recorded in spring in April,while the lowest abundance was detected in December in winter.Protoperidinium granii was the main species contributing to the dissimilarity between spring and winter with 13.98%followed by Peridinium sp.with 12.5%of dissimilarity and by Polykrikos sp.with 10.58%.Conclusions:Heterotrophic dinoflagellates proliferate in spring and summer.This increase was justified by the nutrient availability.Protoperidinium granii and Polykrikos kofoidii were the main heterotrophic dinoflagellate making difference between seasons and their densities were positively correlated with both temperature and salinity.

Occurrences of the toxic dinoflagellate Ostreopsis ovata in relation with environmental factors in Kerkennah Island(Southern coast of Tunisia)

Objective:To study the seasonal and monthly variability of the toxic dinoflagellate Ostreopsis ovata(O.ovata)in relation to environmental parameters in Kerkennah Island.Methods:Three water samples replicate of one-litter were taken daily for ten consecutive days on 12 months.All sampling water was kept in the dark at ambient temperature until their microscopic observation.Environmental variables such as salinity and temperature were measured in the field concomitantly as phytoplankton sampling.Nutrients(ammonium,nitrite,nitrate,phosphate and silicate)were analysed in laboratory with Auto-analyser Luebbe type.Cell identification and enumeration in water samples were performed with an inverted microscope after the sedimentation.Results:The highest abundance of O.ovata was recorded in summer.Analysis of variance showed significant difference of abundance between seasons,whereas no significant difference for month was detected.Factorial analysis ordination showed a positive correlation of Ostreopsis mainly with temperature and low correlation with nitrite and nitrate whereas the second axis(with 26.30%of variance)showed that Ostreopsis was correlated with temperature and salinity.Conclusions:The maximum abundance of Ostreopsis was reached in summer when temperature was high and a low relationship between O.ovata and nutrient was detected.

Heavy metal concentrations inLiza aurata(Risso,1810)captured from the Kerkennah Islands(Gulf of Gabes)and associated health risks

Objective:To determine the concentration of heavy metals(Cd,Cu,Fe,Ni,Pb and Zn)in the liver,gills and muscle of Liza aurata(L.aurata)collected from the Kerkennah Islands.The target hazard quotient was also used as an indicator of human health risks associated with fish consumption.Methods:The sample was collected from Kerkennah Islands(Gulf of Gabes,Tunisia).Heavy metals in L.aurata tissues(gills,liver and muscle)were analysed with an atomic absorption spectrometer(PerkinElmer).Results:As expected,muscle always possessed the lowest concentrations of all metals.The maximum concentrations of Cd(0.52 mg/kg wet weight),Cu(5.43 mg/kg wet weight),Fe(115.27mg/kg wet weight)and Zn(112.20 mg/kg wet weight)were measured in the liver,while the highest Ni(1.54 mg/kg wet weight)and Pb(1.43 mg/kg wet weight)were recorded in gills.The target hazard quotient through consumption of fish and calculated by adding the inividual target hazard quotient was below 1,indicating that there was no significant potential health risk associated with consumption of L.aurata.Conclusions:L.aurata from the Kerkennah Islands may be suitable for human consumption.

Seasonal and daily lfuctuation of diatoms during spring tide periods in Kerkennah Islands

Objective:To study seasonal and the daily distribution of diatoms in the three tidal periods(flood,slack and ebb period)during the spring tide.Methods:Water samples were taken and environmental variables were measured three times in each tidal period during 10 days of spring tide.Sampling was done in 2007 in Cercina station located in the western coast of Kerkennah(34°41'27''N;11°07'45''E)(Southern Tunisia).Results:Nutrients showed significant variation between seasons,increasing in spring and decreasing noticeably in autumn and winter.About 36 diatom species were found.Results revealed a remarkable abundance increase in spring and summer.Irregular differences in diatom abundances were revealed over the tidal periods,with the highest rates being detected during the flood and the ebb period,while the abundance rate was lowest during the slack period.This could presumably be attributed to the increase of nutrient supply of suspended particulate matter during water motion.The results revealed a correlation between diatom abundance and temperature,NO_(2)^(-),NO_(3)^(-),Si(OH)_(4) and PO_(4)^(3).Temperature seemed to be the most important factors which may influence the distribution and diatom abundance.Conclusions:Tide has various effects on the nutrients status and diatoms community(in terms of species composition,succession and abundance)between different tidal periods.Fluctuation of diatoms was correlated with changes in the circulation of water bodies and changes in nutrient regime.