Statistical evaluation of differences in phytoplankton richness and abundance as constrained by environmental drivers in transitional waters of the Mediterranean basin

1. This paper is focused on the identification of patterns of variation in phytoplankton abundance and taxonomic composition at the eco-regional scale. In addition, these patterns were evaluated with respect to climatic, hydrological and physiographic drivers.2. A hierarchical sampling design was used to integrate seasonal and spatial variations in taxonomic richness, abundance and taxonomic composition of phytoplankton guilds. Data were collected synoptically during two seasons (autumn and spring) in 12 transitional water bodies in Italy, Albania and Greece.3. The number of taxa and the cell density of phytoplankton guilds varied between seasons and across ecosystems. Overall, physiographic and hydrological components accounted for 61% of the variation in the number of taxa, but just 19% of the variation in cell density.4. At the univariate level, cell density varied significantly with temperature, whereas the number of taxa was found to vary significantly with depth, sinuosity index and outlet structure, lagoon surface area and geographic location.5. The taxonomic composition of phytoplankton guilds varied markedly among lagoons. More than 61% of the 242 taxa identified overall were detected in only one of the lagoons.6. The average similarity of phytoplankton taxonomic structure among ecosystems was 33.15 ± 15.02. Taxonomic similarity varied substantially across both ecosystems and seasons. Overall, more than 70% of variance in taxonomic similarity was explained by physiographic and hydrological forcing factors. Specifically, differences in taxonomic structure were found in transitional ecosystems showing differences in outlet structure, depth and salinity, as well as geographic location. A recurrent pattern of decreasing taxonomic similarity with increasing Euclidean distance was observed for each factor.7. The results obtained in this study suggest that certain large-scale driving forces can explain eco-regional scale patterns of species richness and taxonomic composition but not of cell abundance, which are probably more affected by local forcing factors.