Diversification of phytoplankton community structure and related parameters along a large-scale longitudinal east-west transect of the Mediterranean Sea

Environmental and phytoplankton parameters were explored at nine stations located along a large scale (3188 km) east-west longitudinal transect (4 degrees 95'E-32 degrees 67'E) of the Mediterranean Sea. The trend analysis confirmed a statistically significant east-west decrease of mean temperatures and salinities and an increase of nutrients (P-PO4 and N-NO3), chlorophyll alpha, primary productivity and phytoplankton cell concentrations. A total of 216 species were identified: 48 diatoms, 112 dinoflagellates and 56 coccolithophores. On the basis of this analysis, the structure of the microphytoplankton community (Shannon's diversity, species richness, taxa dominance and community dissimilarities) along the transect was investigated. Many species (102) of this community were rare, having a frequency of occurrence <= 2.0 in all samples, but they were very important because they controlled the levels of species diversity. A highly significant power relationship between species number-sample number was also recorded along the studied transect. The analysis showed that the prevailing east-west gradients of the physical and chemical conditions determined phytoplankton abundance and community diversification.