Using functional approaches to study phytoplankton communities in a temperate region exposed to tropical species dispersal

Functional characteristics play an important role in shaping phytoplankton distributions. This approach can help to explain the success of tropical taxa in temperate ecosystems. We used functional groups (FG) and functional traits (FT) to assess the phytoplankton distribution in five temperate lakes exposed to tropical taxa dispersal. We ran redundancy analyses to assess the relationship between each functional classification and the environment. Both approaches showed similar variance of the phytoplankton biomass and responded to similar environmental variables. The FG approach separated lakes by trophic status (total phosphorus: TP), while the FT approach reflected the lakes' hydromorphology (conductivity, depth, mixing regime). The traits of motility and mixotrophy coincided with low mixing and high light. Tropical taxa were dominant in two lakes, rare in one and absent in two lakes. When dominant, tropical taxa exerted an overwhelming effect on phytoplankton biomass and community composition, and contributed to foam occurrences. The success of tropical taxa in only two lakes seems related to the interplay of their traits (accessory pigments, morphology) and the prevailing environmental filters (low light, deep mixing, and intermediate TP). Their rarity or absence in the other lakes evidences how traits can increase fitness in one environment but not in other.