Differential Response of Prokaryotic and Eukaryotic Pico‐Sized Phytoplankton to Light/Dark Cycles Simulating Wind‐Induced Mixing in a Turbid Shallow Lake

In eutrophic shallow lakes, high turbidity attenuates light within the first centimetres of the water column. As a result, phytoplankton is exposed to light/dark cycles of variable frequency during wind‐induced vertical mixing. Despite its ecological importance, the impact of this dynamic light on phytoplankton communities remains poorly understood. Here, we analysed two picophytoplankton (< 2 μm) groups from a highly turbid shallow lake: picocyanobacteria and picoeukaryotes, both sharing a similar cell size, trophic position, and a lack of active buoyancy regulation. A long‐term analysis revealed contrasting seasonal patterns, with picocyanobacteria peaking in winter and picoeukaryotes in spring when stronger winds are more frequent. We hypothesized that picocyanobacteria and picoeukaryotes differ in their physiological responses to the light/dark cycles they experience during vertical mixing. To test this, laboratory experiments were conducted using two strains isolated from the lake ( Cyanobium sp. and Choricystis krienitzii) , exposed to different light/dark regimes simulating realistic wind‐induced mixing conditions. Picocyanobacteria exhibited a reduction in photosynthetic rates under higher light/dark cycling frequencies, resulting in a marked decline in cell growth. These conditions also promoted cell aggregation and increased phycocyanin/chlorophyll a ratio. In contrast, the picoeukaryotic strain showed no significant changes in growth, metabolic rates, aggregation, and pigment content across the different treatments. These results demonstrate taxon‐specific responses to light/dark regimes. Light/dark fluctuation associated with vertical mixing therefore represents an additional, previously overlooked selective pressure contributing to niche differentiation and shaping phytoplankton community structure in turbid shallow lakes.