Biogeographical patterns of freshwater micro- and macroorganisms: a comparison between phytoplankton, zooplankton and fish in the eastern Mediterranean

Aim We investigated the biogeographical patterns of phytoplankton, zooplankton and fish in freshwater ecosystems. We tested whether spatial distance or environmental heterogeneity act as potential factors controlling community composition. Location Northern and central Greece, eastern Mediterranean. Method Data on 310 phytoplankton, 72 zooplankton and 37 fish species were collected from seven freshwater systems. Species occurrence data were used to generate similarity matrices describing community composition. We performed Mantel tests to compare spatial patterns in community composition of phytoplankton, zooplankton and fish. Next, we examined the correlation between geographical distance and the degree of similarity in community composition. The analysis was repeated for different taxonomic, trophic and size-based groups of the microorganisms studied. We assessed different environmental variables (topographic and limnological) as predictors of community composition. Results Phytoplankton community composition showed a strong positive correlation with environmental heterogeneity but was not correlated with the geographical distance between systems. Zooplankton community composition was unrelated to geographical distance and was only weakly correlated with environmental variables. In contrast, fish community similarity decayed significantly with distance. We found no relationship along all pairwise comparisons of the compositional matrices of the three groups. The pairwise comparisons of the different taxonomic, trophic and size-based groups of the microorganism communities studied were in accordance with the results for the entire microorganism community. Main conclusions Our results support the proposition that the biogeography of microorganisms does not demonstrate a distance-decay pattern and further suggest that, in reality, the drivers of distribution depend on the specific community examined. In contrast, the biogeography of macroorganisms was affected by geographical distance. These differences reflect the dispersal abilities of the different organisms. The microorganisms exhibit passive dispersal through the air, with local environmental conditions structuring their community composition. On the other hand, for macroorganisms such as fish, the terrestrial environment could pose barriers to their dispersal; with fish structuring distinctive communities over greater distances. Overall, we suggest that the biogeography of freshwater phytoplankton and zooplankton reflects contemporary environmental conditions, while the biogeographical patterns for fish inhabiting the same systems are related to factors affecting their dispersal ability.