Community shifts from eukaryote to cyanobacteria dominated phytoplankton: The role of mixing depth and light quality

Lake stratification strengthens with increasing surface water temperatures, thereby reducing the depth of the mixed layer. Phytoplankton communities are not only exposed to different nutrient availability within a mixed water column, but also to different light quality. We conducted controled laboratory and mesocosm experiments to investigate phytoplankton, especially cyanobacteria, responses to different light quality and mixing depths. Our mesocosm experiment allowed the manipulation of mixing depth in situ by a mesocosm approach and to follow the effects of changing mixing depth on the phytoplankton community composition. Our laboratory experiment allowed the control of temperature and light quantity. To investigate the effect of light quality on phytoplankton, we created a light gradient from full photosynthetic active radiation to a reduced blue spectrum. In both experiments, shifts in phytoplankton community composition from eukaryote to cyanobacteria occurred at shallow mixing depth with higher availability of photosynthetic active radiation. Our results from the mesocosm experiment support the idea that reduced mixing depth can promote cyanobacterial abundance. With our laboratory experiment, we were able to manipulate light quality independent of temperature, available nutrients and light intensity influencing phytoplankton abundance. Results from the laboratory experiments support our hypothesis that a shift in light spectrum alone is a driver, strong enough to enhance cyanobacteria occurrence. Most of the previous studies dealing with cyanobacterial blooms have investigated temperature and eutrophication effects. Certainly, these are major factors for the growth of phytoplankton, but our experiments show that other aspects, such as the quality of light, must be also taken into account to explain cyanobacterial blooms. Such shifts in the phytoplankton community from eukaryote to cyanobacteria dominated communities will have strong consequences for food web dynamics. Several cyanobacteria specific traits, (e.g., toxin production, lack of essential fatty acids, and inedibility through production of large colonies) reduce transfer efficiencies of energy and matter between phyto- and zooplankton and therefore can influence higher trophic levels such as fish