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

Abstract

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.