Functional dynamics of phytoplankton assemblages in hypertrophic lakes: Functional- and species diversity is highly resistant to cyanobacterial blooms

Under increasing pressure of climatic change and anthropogenic eutrophication, water blooms, i.e. the formation of high phytoplankton biomass of a single or a few species, have become more and more frequent in lake ecosystems that is caused mostly by Cyanobacteria. The dynamics of phytoplankton under a cyanobacterial pressure may provide important information about what a competitive advantage of Cyanobacteria have over eukaryotic microalgae. The research, therefore focused on the relationship between cyanobacterial blooms and the accompanying species in hypertrophic shallow lakes where nutrients are in excess. The following hypotheses were selected for testing: i) Cyanobacterial blooms negatively affect the species richness and diversity of phytoplankton assemblages in the studied hypertrophic lakes. ii) Cyanobacterial blooms negatively affect the multi-trait functional diversity of phytoplankton assemblages in the studied hypertrophic lakes. iii) The formation of a cyanobacterial bloom causes a shift in the species and trait composition of phytoplankton assemblages. It was found that the species diversity of non-cyanobacterial assemblages was not affected negatively by increasing density of Cyanobacteria. Cyanobacterial biomass negatively affected the functional richness and functional dispersion of the assemblages, but both relationships were only marginally significant. Some temporal shift was detected in the community weighted mean values of width, colonial ability of accompanying microalgae, and also in flagellatedness and mixotrophic ability. We conclude that Cyanobacterial blooms basically alter dominance relations in the phytoplankton and reduce availability of light in waters. However, it does not necessarily coincide with the elimination of other taxa and reduction their species numbers or diversity. The formerly published data on the decrease in species richness can be likely explained by methodological deficiencies, which are responsible for reduced detectability of species. To better understand the seasonal dynamics and resilience of phytoplankton assemblages in hypertrophic water bodies the development of new methodologies by which diversity and succession of subordinate species can be evaluated is required.

Automated summary

The study suggests that cyanobacterial blooms do not have a negative impact on the species diversity of phytoplankton assemblages, but they do affect the functional diversity. Cyanobacterial blooms alter dominance relations within the phytoplankton and reduce light availability, but it does not necessarily result in the elimination of other taxa or reduction in species numbers or diversity.