Increasing diversity of Planktothrix rubescens populations as a consequence of eutrophication and climate change

Planktothrix rubescens forms dense blooms in the metalimnion of oligo- and mesotrophic lakes. This vertical position represents a compromise between the limited light penetrating through the epilimnion and the high nutrient concentrations associated with the hypolimnion. In recent years, phosphorous release from sediments has strongly fertilized Lake Stechlin internally, to a state where light in the metalimnion is extremely limited. Rather than decreasing, Planktothrix blooms have intensified and taken advantage of milder winters, dominating the nutrient enriched upper mixed water column. We sought to understand through metagenome sequencing how Planktothrix populations in Lake Stechlin have developed over the last decade. We compared monthly samples taken from the epilimnion and hypolimnion, commencing in 2011, with fortnightly epilimnic and metalimnic samples during the 2020 Planktothrix bloom. We identified a single Planktothrix species within the lake over the 10-year-period, however, detected temporally variable sub-populations. Interestingly, initial diversity of the Planktothrix populations in the epi- and hypolimnion was low and increased during sustained following bloom events in 2016 and 2020. These findings show a link between environmental change, (increased stratification and reduced ice cover) and the diversity of cyanobacterial populations suggesting that cyanobacteria will continue to expand their spatio-temporal range as global change intensifies.