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.

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.

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.