Composition and dynamics of potentially toxic dinoflagellates in a shallow Mediterranean lagoon

The population dynamics of harmful microalgae was investigated in the semi-enclosed shallow lagoon, Tunis northern lagoon (South Mediterranean), during a 1-year period from January to December 2007. A total number of 16 potentially toxic dinoflagellates were identified. Density of each species was analyzed in time and space and in relation to environmental parameters. Some species such as Alexandrium catenella, A. pseudogonyaulax, A. tamarense, Dinophysis sacculus, Prorocentrum lima, P. minimum and Gonyaulax spinifera developed high cell concentrations, particularly during spring and summer seasons. Other species such as A. insuetum, A. lee, A. minutum, Coolia monotis, D. acuminata, D. caudata, D. fortii, Ostreopsis siamensis and P. concavum were detected in this coastal ecosystem, but they were rare or appeared in very low densities.

Densities of the most abundant species were closely associated with spring and summer conditions, being positively correlated with water temperature and salinity. These species performed well in this lagoon, but the specific species-dependent affinity to each of the environment factors determined which organisms proliferated. The spatial distribution of the abundant species exhibited a marked increase towards the western and southern parts of the lagoon represented by sites C, D and E. Examination of the temporal distribution revealed that the species more prone to bloom are those with extensive spatial distribution and frequent presence throughout the year such as D. sacculus.

Statistical analysis of D. sacculus density dynamics showed a significant difference between summer and winter assemblages. It presented a positive correlation with water temperature and salinity, but it seems to be tolerant to all levels of nutrients.