The diel vertical migration of the nuisance alga Gonyostomum semen is controlled by temperature and by a circadian clock

Gonyostomum semen is a bloom-forming freshwater raphidophyte that is currently on the increase, which concerns stakeholders and ecologists alike. Much indicates that the success of G. semen is linked to its diel vertical migration (DVM). The DVM was therefore studied in its natural setting using environmental sensors with data logging. DVM happened partly in the absence of light detectable by sensors, suggesting that the DVM involved gravitaxis, although an involvement of phototaxis could not be fully excluded. G. semen migrated downwards the earlier the warmer the water was. The upwards migration showed features of endogenous rhythms, implying that the temporal control of the DVM also has a circadian component that ensures synchronization with the day-night cycle. It is hypothesized that by causing an early descent, exposure to warm water may give G. semen more time to exploit the nutrient-rich lower part of the water column before the circadian control initiates upwards migration. The warming of lakes, caused by global warming or by the more effective conversion of light energy into heat due to the brownification of lakes, may therefore be of advantage to G. semen. In addition, the steep temperature gradient, which is typical for humic lakes during summer stratification, may help avoid the desynchronizing effect that turbulence can have on gravitactic DVMs. Above features of the DVM of G. semen therefore provide putative explanations for why the alga is particularly successful in humic lakes and why it profits from lake brownification and global warming.